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The effect of depth on net primary production in aquatic ecosystems

The effect of depth on net primary production in aquatic ecosystems



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The figure shows the relationship between the water depth and net primary production (=P-R). I want to know why the production (P) initially increases with water depth near the surface? I have seen similar relationships from other sources but never seen a clear explanation.


After a quick glance at the book "Light and Photosynthesis in Aquatic Ecosystems" by Kirk (2010), I think that the cause for the productivity dip towards the surface partially lies in photoinhibition, due to high light intensities at the surface. Here are a couple of relevant quotes from the book (Google books: p. 371):

In this light saturated state, the electron transport and/or CO2 fixing enzymes (most likely, the latter) are working as fast as they are capable, and so any additional absorbed quanta are not used for photosynthesis at all. From the end of the linear region through to the light-saturated region ([i.e. close to the surface, my addition]), since photosynthetic rate does not increase in proportion to irradiance, (P/Ed steadily falls, see Fig. 10.3) the quantum yield and conversion efficiency necessarily undergoes a progressive fall in value. This is accentuated further if, at even higher light intensities, photoinhibition sets in. If the cells contain photoprotective carotenoids, in which absorbed light energy is dissipated as heat rather than being transferred to the reaction centre…

However, the suspended bottle methods often used to estimate these depth gradients might be part of the problem, by overestimating the effect of photoinhibition, by forcing plancton to stay at the same depth (p 358):

Depth profiles of phytoplancton photosynthesis, such as those in Fig. 10.4, determined by the suspended bottle method, tend to overestimate the extent to which photoinhibition diminished primary production. In nature, the phytoplancton are not forced to remain at the same depth for prolonged periods. Some, such as dinoflagellates and blue-green algae, can migrate to a depth where light intensity is more suitable. Even non-motile algae will only remain at the same depth for extended periods under rather still conditions.

I hope I got the quotes right (quick manual retyping). There are many more relevant sections in the book as well, which seems to cover all kinds of aspects of aquatic photosynthetic efficiency and how this can be a function of depth.


I think it has to do with what wavelength of light is absorbed by photosynthetic organisms at what depth.

Ultraviolet light with short wavelength is absorbed closest to the surface. Red light (which is responsible for photosynthesis) is absorbed at a deeper point in aquatic systems by the primary producers like phytoplankton and metaphyta which increase the productivity of that particular depth.

[I'll include citations, references and diagrams as soon as I find enough time]


Compared to the surface, slightly below the surface in the water column the availability of nutrients increases as winds and ocean currents cause increased mixing of nutrient rich deep water. The photic zone of the water column is quick to use up the macronutrients necessary to sustain primary production, however upwelling of nutrient rich deep water becomes a significant factor in determining an area's production.

[ I will provide citation later. ]


Net Primary Productivity

Plants capture and store solar energy through photosynthesis. During photosynthesis, living plants convert carbon dioxide in the air into sugar molecules they use for food. In the process of making their own food, plants also provide the oxygen we need to breathe. Thus, plants provide the energy and air required by most life forms on Earth. Plant productivity also plays a major role in the global carbon cycle by absorbing some of the carbon dioxide released when people burn coal, oil, and other fossil fuels. The carbon plants absorb becomes part of leaves, roots, stalks or tree trunks, and ultimately, the soil.

The maps above show one way to monitor the carbon “metabolism” of Earth’s vegetation. They show net primary productivity, which is how much carbon dioxide vegetation takes in during photosynthesis minus how much carbon dioxide the plants release during respiration (metabolizing sugars and starches for energy). The data come from the Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA’s Terra satellite. Values range from near 0 grams of carbon per square meter per day (tan) to 6.5 grams per square meter per day (dark green). A negative value means decomposition or respiration overpowered carbon absorption more carbon was released to the atmosphere than the plants took in.

In mid-latitudes, productivity is obviously tied to seasonal change, with productivity peaking in each hemisphere’s summer. The boreal forests of Canada and Russia experience high productivity in June and July and then a slow decline through fall and winter. Year-round, tropical forests in South America, Africa, Southeast Asia, and Indonesia have high productivity, not surprising with the abundant sunlight, warmth, and rainfall. However, even the tropics, there are variations in productivity over the course of the year. For example, in the Amazon, productivity is especially high from roughly August through October, which is the area’s dry season. Because the trees have access to a plentiful supply of ground water that builds up in the rainy season, they actually grow better when the rainy skies clear and allow more sunlight to reach the forest.

View, download, or analyze more of these data from NASA Earth Observations (NEO):
Net Primary Productivity


The global carbon cycle ( f . ian woodward )

The pool of carbon in the atmosphere and its monthly, annual and decadal dynamics is the best-quantified component of the global carbon cycle ( Keeling & Whorf, 2000 ). The terrestrial and oceanic carbon pools exchange primarily with the atmosphere, but none of the individual pools or fluxes are known with great precision, due to their marked spatial variability and large sizes. However, the net effect of large terrestrial and oceanic source and sink fluxes on the atmospheric pool of carbon can be determined from the trends in atmospheric CO2 recorded since continuous monitoring was instituted in 1958 ( Keeling & Whorf, 2000 ). Between 1991 and 1997 only about 45% of industrial CO2 emissions accumulated in the atmosphere ( Battle et al. 2000 ), indicating that the terrestrial and oceanic sinks must influence the atmospheric accumulation. There is also now an improving capacity to differentiate between terrestrial and oceanic fluxes (e.g. Battle et al. 2000 ). These measurements show that the carbon cycle is out of equilibrium as a result of human activities. Releases of carbon through fossil fuel burning are quite well quantified but the impacts of deforestation on carbon release are less well characterized ( Nepstad et al. 1999 ) and are not readily distinguishable, by atmospheric measurements, from fluxes to and from vegetation. The current situation is that a poorly quantified pre-industrial global carbon cycle is being subjected to human forcing, directly through changes in carbon fluxes and pools and indirectly through changes in climate. In terms of anthropogenic concern there are two major questions regarding the global carbon cycle. How will the cycle respond in this non-equilibrium mode, in particular how will increasing concentrations of atmospheric CO2 influence terrestrial and oceanic photosynthesis and chemistry? In an era when mitigation strategies are on the international agenda, how effective and for how long will natural carbon sinks absorb significant fractions of anthropogenic carbon releases?

The oceanic sink

Most of the early ocean work was concerned with defining environmental impacts on the solubility of CO2 in water − the so-called solubility pump. The effectiveness of the solubility pump at sequestering anthropogenic releases of CO2 depends on ocean temperature, vertical mixing and global circulation patterns ( Falkowski et al. 2000 ). More recent work has also considered biological uptake of CO2 in the oceans − the biological pump. The biological pump describes the processes by which phytoplankton absorb CO2 from the surface waters by photosynthesis. Following respiratory losses, dead organic matter and commonly associated calcium carbonate descend from the photic ocean surface to the ocean interior, effectively locking carbon away from the atmosphere for extended periods. This pump has not generally been considered important in absorbing further increases in anthropogenic CO2 because CO2 uptake by phytoplankton is primarily limited by the supply of nutrients such as nitrogen, phosphorus and iron, and increasing CO2 supply should have little impact ( Heimann 1997 ). Future changes in ocean circulation patterns and stratification, in response to global warming, will exert significant impacts on the availability of nutrients and the effectiveness of the biological pump. Sarmiento et al. (1998) suggest that changes in the biology of the pump may be the most critical component of the oceanic responses to future changes in climate and CO2. Unfortunately, for future projections, Sarmiento et al. (1998) conclude that the response of the biological oceanic community to the climate change is difficult to predict on present understanding. Perhaps the future approach may need to be closer to that taken for terrestrial ecosystems, with a greater emphasis on carbon flux physiology, nutrient exchange capacity and community dynamics.

The terrestrial sink

The turnover of marine phytoplankton is very rapid, on the order of a week, and so any increases in productivity, through CO2 and nutrient enrichment, will have rather little impact on standing stocks. This contrasts with the decadal-scale turnover for trees, the dominant terrestrial sinks and for which even small increases in productivity could lead to substantial increases in carbon storage. The longevity and dynamics of trees, particularly through natural and anthropogenic disturbances, are critical for defining the terrestrial part of the global carbon cycle. There is abundant evidence that plants can increase their photosynthetic capacity with CO2 enrichment. However this response slows with increasing CO2 and, like the phytoplankton, is also influenced by the supply of other nutrients, in particular nitrogen and phosphorus. Modelling ( Cao & Woodward 1998 ) and experiment ( DeLucia et al. 1999 ) now indicate clearly that ecosystems can increase their carbon sequestering capacity with CO2 enrichment, but that the oft-vaunted impacts of pollutant N deposition are rather small ( Nadelhoffer et al. 1999 ).

Absorbing anthropogenic releases of CO2 with climatic change

Increased oceanic sequestration of atmospheric CO2 as organic matter causes a transfer to the ocean interior. Unfortunately, this also locks away the nutrients that limit carbon sequestration. Projections of the future climate indicate warming and an increase in precipitation, both of which will tend to increase stratification and reduce upwelling of nutrients. In addition, the supply of wind-blown iron, a limiting marine nutrient, from the dry continents may be reduced with a wetter climate. In combination, these features should decrease the capacity of the biological pump ( Falkowski et al. 1998 ) to sequester anthropogenic carbon. However, changes in oceanic circulation patterns and, in areas with increased precipitation, increased estuarine runoff with high concentrations of nutrients, may partially compensate for this reduced oceanic activity.

Experimental observations on plants suggest that CO2 enrichment can stimulate the carbon sequestering capacity but warming, with no change in water supply will tend to reduce this capacity. Models at the global scale (e.g. Cao & Woodward 1998 ) indicate that global climate model simulations of future climatic warming alone would cause a global decrease in the terrestrial sink capacity for sequestering carbon, with vegetation and soils adding to the atmospheric pool of carbon. The inclusion of the direct effects of increasing atmospheric CO2 with this warming reverses this trend, with vegetation and soils increasing their carbon sequestration capacity. However, there is evidence for a decline in this capacity as the CO2 stimulation of productivity reaches saturation.

Uncertainty

Estimates of oceanic and terrestrial sink capacities for carbon are currently quite uncertain but the best that can be achieved to date comes from three major and largely independent methods of estimation. The three methods are, broadly, the inversion of time series of atmospheric composition (e.g. CO2, O2 and δ 13 C), in situ observations and model simulations. No single technique is currently adequate for a full and accurate global picture of the spatial and temporal activities of the global carbon sinks. The measurements of atmospheric composition are sparse, particularly over the terrestrial biosphere, and sinks can only be estimated from measurements after the use of atmospheric transport models. In some cases maps of vegetation distribution are also required. This is particularly so for the interpretation of δ 13 C data, where the distribution of species with the C4 pathway of photosynthesis is required. In situ observations of CO2 fluxes, or temporal changes in the sizes of carbon pools, are also sparse, particularly over the oceans and between the tropics. In addition, observations on land need to track impacts on CO2 fluxes of processes such as disturbance, harvesting and changes in land use. Finally, models have the problems of insufficient understanding of processes, of oversimplification and of severe limitations to adequate testing. Reducing these uncertainties will require improved interactions between these three approaches. This will involve the assimilation of observations, such as from remote sensing, into models and the wider use of statistical techniques for investigating model and data uncertainties. There is still some way to go before the uncertainties of the carbon cycle can be minimized so that, for example, continental-scale sinks can be identified and quantified with precision and small-scale observations can converge with global-scale model simulations.


Materials and methods

Climate data

A spinup simulation was performed to create steady state conditions for the ecosystems from which future projections began. Biome-BGC requires daily estimates of maximum and minimum temperature, precipitation, daylength, solar radiation and vapor pressure deficit. Temperature and precipitation data for the spinup period were obtained from the PRISM (Parameter-elevation Regressions on Independent Slopes Model) project (Daly et al. 2000). Vapor pressure deficit, solar radiation and day length are not produced by PRISM. These variables were created from PRISM data using the MT-CLIM algorithms (Kimball et al. 1997). The resulting climatological data for the spinup were developed at a daily temporal resolution and matched the spatial resolution of the GCM data. The delta method (Mote and Salathé, 2009) was used to temporally downscale the monthly PRISM Data, from 1940–2000, to a daily temporal resolution which were used for the spinup period (Supplementary Material (SM) 1).

The projection covered three emissions scenarios from 2001–2100 derived from the Third Intergovernmental Panel on Climate Change (IPCC) Assessment Special Report on Emission Scenarios (Nakićenović et al. 2000). These emissions scenarios encompass different assumptions regarding socioeconomic drivers such as gross domestic product, population growth, technological innovation and greenhouse gas emissions (SM 2). For the present study, the IPCC scenarios corresponded to the A1B, A2, and B2 scenarios. Climate data processed by Coulson et al. (2010a, b) emanate from a series of GCM’s including the Climate Centre for Modelling and Analysis (GCGM2), Australia’s Commonwealth Scientific and Industrial Research Organisation (CSIRO MK2), Hadley Centre for Climate Prediction and Research UK (HadCM3) and Model for Interdisciplinary Research on Climate (MIROC 3.2). Coulson et al. (2010a, b) spatially downscaled data from a selected suite of GCM’s used in the Fourth IPCC Assessment resulting in estimates of monthly averaged maximum and minimum temperature and precipitation at the 5 arc minute spatial resolution. The GCM data were spatially downscaled by Coulson et al. (2010a, b) using the ANUSPLIN software to fit a two-dimensional spline function at each month’s for each climate variable. These procedures are described at length in Joyce et al. (2011, 2014).

As with the PRISM data monthly estimates of maximum and minimum temperature and precipitation from the GCM’s were temporally downscaled to a daily time-step to accommodate the needs of Biome-BGC using the delta method (SM 1). From these daily data, estimates of daylength, solar radiation and vapor pressure deficit were derived using the MT-CLIM algorithms.

Spatial domain of analysis

Defining the extent and composition of the area for estimating potential future rangeland NPP required two steps (SM 1). For the first step, the spatial extent of rangelands in the coterminous U.S. was identified from Reeves and Mitchell (2011) (SM 3). In the second step, the relative abundance of C3, C4, and shrub plant functional types were quantified at each pixel in the rangeland domain from Step 1. Cross-hatched regions depicted in SM 3 were used for validation and comparison purposes but not for describing NPP response to climate change, because, although shrub or herb dominated, these regions are outside the rangelands study area as defined from Reeves and Mitchell (2011). All modeling of NPP projections was conducted at the 5 arc minute pixel level. Results, however, were often aggregated to larger spatial extents including the six assessment regions depicted in SM 3.

Nitrogen deposition, carbon dioxide concentrations, and soils

The emission scenarios included decadal, aspatial estimates of NOx emissions (SM4). Decadal values were linearly interpolated to estimate annual values. Further, nitrogen deposition was assumed to increase at the same rate as nitrogen emissions, therefore, permitting the rate of increase of nitrogen emissions as a proxy for the rate of increase of nitrogen deposition. Since these data were tabular, data from Holland et al. (2005), describing current patterns of nitrogen deposition, were used to apportion projections of deposition across the landscape for each scenario. This ensured that the spatial pattern and relative magnitude of nitrogen deposition was held constant throughout the projection, although total deposition was based on the temporal trends found in estimated nitrogen emissions for each scenario (SM 4).

As with nitrogen emissions, decadal estimates of CO2 concentrations accompanied each IPCC emissions scenario and are represented in SM 4, which shows the trajectory of CO2 concentrations across the projection. Estimates of decadal CO2 concentration were aspatial and applied symmetrically to every pixel as CO2 is assumed to be distributed homogenously across the study area.

To provide linkages with the forthcoming Fifth IPCC Assessment Report (AR5) (Moss et al. 2010 Rogelj et al. 2012) (http://www.ipcc.ch/activities/activities.shtml#.Ug6Z0UG1Hh4), suggested linkages between the CO2 concentrations found in the Representative Concentration Pathways and the socioeconomic scenarios used in the present study is provided in SM 4. Our linkages are based on similarities of CO2 concentration by 2100, and differ, for the B2 scenario, from the linkages conducted by Rogelj et al. (2012).

The final parameters needed to simulate future NPP with Biome-BGC included soil data, which were derived from the State Soil Geographic database (STATSGO) (U.S. Department of Agriculture and Natural Resources Conservation Service NRCS 1994). The percentage of sand, silt and clay and depth to bedrock were quantified for the study area. These data were gridded to the 5 arc minute resolution to match all other spatially explicit simulation parameters. These soils data are used by Biome-BGC to control belowground processes such as decomposition and water balance.

Simulating vegetation productivity

Using the present vegetation distribution, climate, CO2, NOx, and soils data as described above, estimates of annual NPP on U.S. rangelands were produced by simulating ecosystem dynamics through application of Biome-BGC across the entire projection for every GCM and scenario combination (SM 5). The contemporary distribution of plant functional types and photosynthetic pathways were held constant throughout the projection. This was done because it isn’t feasible to make spatially-explicit, realistic assumptions about vegetation change over a century time span over the study area. Therefore, results only reflect changes in CO2 concentration, nitrogen deposition, and climate variables, which was the focus of the study. Change in NPP at each pixel was characterized by the slope of the linear trend with respect to time (2001–2100).

Analysis of NPP and drivers of change

Though the Biome-BGC simulations were conducted across the entire projection for every GCM and scenario combination, NPP data were averaged across each GCM to produce scenario-level results. Biome-BGC was run for each emission scenario using climate projections from each of several GCMs. NPP derived from different GCMs was then averaged for each emission scenario. The spatial differences between the various GCM’s are intensively explored in Joyce et al. (2011) and are therefore omitted here. Spearman correlation coefficients were computed at each pixel to identify which of the aforementioned drivers was most closely related to patterns of NPP through time for each scenario. The bioclimatic factor with the highest correlation to estimated NPP was retained and considered the dominant driver of change in productivity. In a similar fashion, temporal trends in NPP were described using a linear trend to determine the direction and magnitude of change. Precipitation and temperature were also evaluated using a linear trend enabling the direction and magnitude of change relative to present day conditions to be quantified. For NPP and climate data, the response of the three emissions scenarios were averaged together and differences among them were represented as standard deviations about the mean.

Quantifying model agreement

Direct validation of projected estimates of NPP was not possible because no spatially extensive field-referenced estimates of NPP currently exist for coterminous U.S. rangelands. There are data, however, from remote sensing instruments that can be used to characterize agreement among NPP estimates. Net primary productivity derived from the Moderate Resolution Imaging Spectroradiometer (MODIS) satellite has been used for evaluating NPP and biomass dynamics of rangeland biomes (Heinsch et al. 2006 Reeves et al. 2006 Zhang et al. 2010). The MODIS-derived NPP estimates are available globally at 1 km 2 spatial resolution and were compared with NPP derived using Biome-BGC within the regions identified in SM 3. This analysis does not constitute an accuracy assessment but provides insight into locations where model results may be questionable or, conversely, where they are more reliable.


Ecological Efficiency: The Transfer of Energy between Trophic Levels

As illustrated in Figure (PageIndex<3>), large amounts of energy are lost from the ecosystem from one trophic level to the next level as energy flows from the primary producers through the various trophic levels of consumers and decomposers. The main reason for this loss is the second law of thermodynamics, which states that whenever energy is converted from one form to another, there is a tendency toward disorder (entropy) in the system. In biologic systems, this means a great deal of energy is lost as metabolic heat when the organisms from one trophic level consume the next level. In the Silver Springs ecosystem example (Figure 46.1.1), we see that the primary consumers produced 1103 kcal/m 2 /yr from the 7618 kcal/m 2 /yr of energy available to them from the primary producers. The measurement of energy transfer efficiency between two successive trophic levels is termed the trophic level transfer efficiency (TLTE) and is defined by the formula:

In Silver Springs, the TLTE between the first two trophic levels was approximately 14.8 percent. The low efficiency of energy transfer between trophic levels is usually the major factor that limits the length of food chains observed in a food web. The fact is, after four to six energy transfers, there is not enough energy left to support another trophic level. In the Lake Ontario example shown in Figure (PageIndex<3>), only three energy transfers occurred between the primary producer, (green algae), and the apex consumer (Chinook salmon).

Ecologists have many different methods of measuring energy transfers within ecosystems. Some transfers are easier or more difficult to measure depending on the complexity of the ecosystem and how much access scientists have to observe the ecosystem. In other words, some ecosystems are more difficult to study than others, and sometimes the quantification of energy transfers has to be estimated.

Another main parameter that is important in characterizing energy flow within an ecosystem is the net production efficiency. Net production efficiency (NPE) allows ecologists to quantify how efficiently organisms of a particular trophic level incorporate the energy they receive into biomass it is calculated using the following formula:

Net consumer productivity is the energy content available to the organisms of the next trophic level. Assimilation is the biomass (energy content generated per unit area) of the present trophic level after accounting for the energy lost due to incomplete ingestion of food, energy used for respiration, and energy lost as waste. Incomplete ingestion refers to the fact that some consumers eat only a part of their food. For example, when a lion kills an antelope, it will eat everything except the hide and bones. The lion is missing the energy-rich bone marrow inside the bone, so the lion does not make use of all the calories its prey could provide.

Thus, NPE measures how efficiently each trophic level uses and incorporates the energy from its food into biomass to fuel the next trophic level. In general, cold-blooded animals (ectotherms), such as invertebrates, fish, amphibians, and reptiles, use less of the energy they obtain for respiration and heat than warm-blooded animals (endotherms), such as birds and mammals. The extra heat generated in endotherms, although an advantage in terms of the activity of these organisms in colder environments, is a major disadvantage in terms of NPE. Therefore, many endotherms have to eat more often than ectotherms to get the energy they need for survival. In general, NPE for ectotherms is an order of magnitude (10x) higher than for endotherms. For example, the NPE for a caterpillar eating leaves has been measured at 18 percent, whereas the NPE for a squirrel eating acorns may be as low as 1.6 percent.

The inefficiency of energy use by warm-blooded animals has broad implications for the world's food supply. It is widely accepted that the meat industry uses large amounts of crops to feed livestock, and because the NPE is low, much of the energy from animal feed is lost. For example, it costs about 1¢ to produce 1000 dietary calories (kcal) of corn or soybeans, but approximately

Consequences of Food Webs: Biological Magnification

One of the most important environmental consequences of ecosystem dynamics is biomagnification. Biomagnification is the increasing concentration of persistent, toxic substances in organisms at each trophic level, from the primary producers to the apex consumers. Many substances have been shown to bioaccumulate, including classical studies with the pesticide dichlorodiphenyltrichloroethane (DDT), which was published in the 1960s bestseller, Silent Spring, by Rachel Carson. DDT was a commonly used pesticide before its dangers became known. In some aquatic ecosystems, organisms from each trophic level consumed many organisms of the lower level, which caused DDT to increase in birds (apex consumers) that ate fish. Thus, the birds accumulated sufficient amounts of DDT to cause fragility in their eggshells. This effect increased egg breakage during nesting and was shown to have adverse effects on these bird populations. The use of DDT was banned in the United States in the 1970s.

Other substances that biomagnify are polychlorinated biphenyls (PCBs), which were used in coolant liquids in the United States until their use was banned in 1979, and heavy metals, such as mercury, lead, and cadmium. These substances were best studied in aquatic ecosystems, where fish species at different trophic levels accumulate toxic substances brought through the ecosystem by the primary producers. As illustrated in a study performed by the National Oceanic and Atmospheric Administration (NOAA) in the Saginaw Bay of Lake Huron (Figure (PageIndex<3>)), PCB concentrations increased from the ecosystem&rsquos primary producers (phytoplankton) through the different trophic levels of fish species. The apex consumer (walleye) has more than four times the amount of PCBs compared to phytoplankton. Also, based on results from other studies, birds that eat these fish may have PCB levels at least one order of magnitude higher than those found in the lake fish.

Figure (PageIndex<3>): This chart shows the PCB concentrations found at the various trophic levels in the Saginaw Bay ecosystem of Lake Huron. Numbers on the x-axis reflect enrichment with heavy isotopes of nitrogen ( 15 N), which is a marker for increasing trophic level. Notice that the fish in the higher trophic levels accumulate more PCBs than those in lower trophic levels. (credit: Patricia Van Hoof, NOAA, GLERL)

Other concerns have been raised by the accumulation of heavy metals, such as mercury and cadmium, in certain types of seafood. The United States Environmental Protection Agency (EPA) recommends that pregnant women and young children should not consume any swordfish, shark, king mackerel, or tilefish because of their high mercury content. These individuals are advised to eat fish low in mercury: salmon, tilapia, shrimp, pollock, and catfish. Biomagnification is a good example of how ecosystem dynamics can affect our everyday lives, even influencing the food we eat.

.19 to produce a similar number of calories growing cattle for beef consumption. The same energy content of milk from cattle is also costly, at approximately

Art Connections

[link] Pyramids depicting the number of organisms or biomass may be inverted, upright, or even diamond-shaped. Energy pyramids, however, are always upright. Why?

[link] Pyramids of organisms may be inverted or diamond-shaped because a large organism, such as a tree, can sustain many smaller organisms. Likewise, a low biomass of organisms can sustain a larger biomass at the next trophic level because the organisms reproduce rapidly and thus supply continuous nourishment. Energy pyramids, however, must always be upright because of the laws of thermodynamics. The first law of thermodynamics states that energy can neither be created nor destroyed thus, each trophic level must acquire energy from the trophic level below. The second law of thermodynamics states that, during the transfer of energy, some energy is always lost as heat thus, less energy is available at each higher trophic level.

.16 per 1000 kcal. Much of this difference is due to the low NPE of cattle. Thus, there has been a growing movement worldwide to promote the consumption of non-meat and non-dairy foods so that less energy is wasted feeding animals for the meat industry.


Autochthonous and allochthonous production

All biotic communities depend on a supply of energy for their activities. In most terrestrial systems this is contributed in situ by the photosynthesis of green plants - this is autochthonous production. Exceptions exist, however, particularly where colonial animals deposit feces derived from food consumed at a distance from the colony (e.g. bat colonies in caves, seabirds on coastland) - guano is an example of allochthonous organic matter (dead organic material formed outside the ecosystem).

In aquatic communities, the autochthonous input is provided by the photosynthesis of large plants and attached algae in shallow waters (littoral zone) and by microscopic phytoplankton

Figure 17.2 Interannual variation in net primary productivity (NPP) in a grassland in Queensland, Australia (above-ground NPP), a cropland in Iowa, USA (total above- and below-ground NPP) and a tropical savanna in Senegal (above-ground NPP). Black horizontal lines show the mean NPP for the whole study period. (After Zheng et al., 2003.)

in the open water. However, a substantial proportion of the organic matter in aquatic communities comes from allochthon-ous material that arrives in rivers, via groundwater or is blown in by the wind. The relative importance of the two autochthonous sources (littoral and planktonic) and the allochthonous source of organic material in an aquatic system depends on the dimensions of the body of water and the types of terrestrial community that deposit organic material into it.

A small stream running through a wooded catchment derives most of its energy input from litter shed by surrounding vegetation (Figure 17.4). Shading from the trees prevents any significant growth of planktonic or attached algae or aquatic higher plants. As the stream widens further downstream, shading by trees is restricted to the margins and autochthonous primary production increases. Still further downstream, in deeper and more turbid waters, rooted higher plants contribute much less, and the role of the microscopic phytoplankton becomes more important. Where large river channels are characterized by a flood plain, with associated oxbow lakes, swamps and marshes, allochthonous dissolved and particulate organic may be carried to the river channel from its flood plain during episodes of flooding (Junk et al., 1989 Townsend 1996).

The sequence from small, shallow lakes to large, deep ones shares some of the characteristics of the river continuum just discussed (Figure 17.5). A small lake is likely to derive quite a large proportion of its energy from the land because its periphery is large in relation to its area. Small lakes are also usually shallow, so internal littoral production is more important than that by phytoplankton. In contrast, a large, deep lake will derive only limited organic matter from outside (small periphery relative to lake surface area) and littoral production, limited to the shallow margins, may also be low. The organic inputs to the community may then be due almost entirely to photosynthesis by the phytoplankton.

. vary in systematic ways in lakes, rivers and estuaries

Figure 17.3 Seasonal development of maximum daily gross primary productivity (GPP) for deciduous and coniferous forests in temperate (Europe and North America) and boreal locations (Canada, Scandinavia and Iceland). The different symbols in each panel relate to different forests. Daily GPP is expressed as the percentage of the maximum achieved in each forest during 365 days of the year. (After Falge et al., 2002.)

Figure 17.3 Seasonal development of maximum daily gross primary productivity (GPP) for deciduous and coniferous forests in temperate (Europe and North America) and boreal locations (Canada, Scandinavia and Iceland). The different symbols in each panel relate to different forests. Daily GPP is expressed as the percentage of the maximum achieved in each forest during 365 days of the year. (After Falge et al., 2002.)

Figure 17.4 Longitudinal variation in the nature of the energy base in stream communities.

Figure 17.4 Longitudinal variation in the nature of the energy base in stream communities.

Figure 17.5 Variation in the importance of terrestrial input of organic matter and littoral and planktonic primary production in contrasting aquatic communities.

Estuaries are often highly productive systems, receiving allochthonous material and a rich supply of nutrients from the rivers that feed them. The most important autochthonous contribution to their energy base varies. In large estuarine basins, with restricted interchange with the open ocean and with small marsh peripheries relative to basin area, phytoplankton tend to dominate. By contrast, seaweeds dominate in some open basins with extensive connections to the sea. In turn, continental shelf communities derive a proportion of their energy from terrestrial sources (particularly via estuaries) and their shallowness often provides for significant production by littoral seaweed communities. Indeed, some of the most productive systems of all are to be found among seaweed beds and reefs.

Finally, the open ocean can be described in one sense as the largest, deepest 'lake' of all. The input of organic material from terrestrial communities is negligible, and the great depth precludes photosynthesis in the darkness of the sea bed. The phytoplank-ton are then all-important as primary producers.


Biological productivity

an ecological and general biological concept signifying the reproduction of the biomass of plants, microorganisms, and animals in an ecosystem. In the narrower sense, it means the reproduction of animals and plants used by man. Biological productivity is realized in each individual case through the reproduction of species populations of plants and animals that takes place at a certain rate and that can be expressed by a definite quantity&mdashproduction per year (or some other unit of time) per unit of area (for terrestrial and bottom-dwelling organisms) or per unit of volume (for organisms living in open water or soil). The production of a particular species population can also be related to its abundance or biomass. The biological productivity of different terrestrial and aquatic ecosystems is manifested in many forms. The products produced in natural communities and used by man (for example, wood, fish, and fur) are correspondingly varied. Man is usually concerned with increasing the biological productivity of an ecosystem because this enhances the possibilities for exploiting nature&rsquos biological resources. However, in some cases a high biological productivity can be harmful (for example, excessive development of a particular phytoplankton species in highly productive waters&mdashblue-green algae in fresh water and toxic peridinean species in the sea).

The concept of biological productivity is similar in many respects to that of soil fertility, but it is broader in content and scope because it can be applied to any biogeocenosis or ecosystem. The term &ldquobiological productivity&rdquo is also applied from time to time to cultivated communities whose productivity is largely the result of social labor. However, both natural terrestrial and natural aquatic ecosystems are under man&rsquos direct or indirect influence. Therefore, as the population grows and mankind&rsquos scientific and technological resources increase, the biological productivity of increasingly diverse ecosystems reflects not only their original natural characteristics but the result of human influences.

Production, but not the biomass of a community or of its components, is a general and adequate measure of biological productivity. The biomass of an individual species or of the population as a whole can be used to estimate production and productivity only when comparing ecosystems of the same or similar structure and species composition, but it is completely unsuitable as a general measure of biological productivity. For example, as a result of the high intensity of photosynthesis of unicellular planktonic algae, approximately the same amount of organic matter per unit of area is synthesized a year in the most productive parts of the ocean as in highly productive forests, although the forests&rsquo biomass is hundreds of thousands of times greater than the phytoplankton biomass.

The production of each population during a particular period of time is the total increase in all the individuals, including the increase in structures separated from the organisms and the increase in individuals removed (eliminated) for one reason or another from the population during the period in question. In the extreme case, if there is no elimination and all the individuals survive to the end of the period under study, production is equal to the increase in the biomass. If, however, the initial (B1) and final (B2) biomasses are equal, it means that the increase is compensated by elimination, that is, under this condition production (P) is equal to elimination (E). In the general case,

Production thus defined is sometimes called &ldquonet production&rdquo in contrast to &ldquototal production,&rdquo which includes not only the increase but also expenditures on energy metabolism. The terms &ldquonet production&rdquo and &ldquototal production&rdquo have become established as far as plants are concerned. As applied to animals, &ldquototal production&rdquo is assimilated food or &ldquoassimilation,&rdquo while &ldquoproduction&rdquo is used in the sense of net production.

The production of autotrophic organisms capable of photosynthesis or chemosynthesis is called primary production, and the organisms themselves are called producers. Green plants&mdashhigher plants on land and lower plants in the water&mdashare a leading factor in the creation of primary production. The production of heterotrophic organisms is usually considered part of secondary production, and the organisms themselves are called consumers. All types of secondary production originate from the utilization of the matter and energy of primary production. However, energy can be used only once to do work, unlike matter which returns again and again to the cycle. The complex trophic relations can be expressed schematically in the form of &ldquoenergy flow&rdquo through an ecosystem, that is, a step-by-step process of utilizing the energy of solar radiation and the matter of primary production. The first trophic level of utilization of solar energy consists of photosynthesizing organisms that create primary production the second level includes the herbivorous animals that consume them the third is constituted by carnivorous animals and the fourth embraces second-order predators. Each succeeding trophic level consumes the production of the preceding one. Moreover, part of the energy of the food consumed and assimilated is expended on energy metabolism and is dissipated. Hence the production of each succeeding trophic level is smaller than the production of the preceding one (for example, the yield based on the same primary production of herbivorous animals is always larger than the predators living at their expense). Biomass as well as production frequently decreases from the lower to the higher trophic levels. However, unlike production, the biomass of the following level may also be greater than the biomass of the preceding one (for example, the phytoplankton biomass is smaller than the total biomass of the entire animal population of the ocean living at its expense). Heterotrophic microorganisms are a prominent factor in the mechanism of biological productivity. They utilize the dead organic matter that reaches them from all the trophic levels, partly mineralizing it and partly converting it into the substance of microbial bodies. The latter are an important source of nutrition for many aquatic (benthic and planktonic filter-feeding and detritus-eating) and terrestrial (soil fauna) animals.

Production is divided according to another principle into intermediate and final. Intermediate production includes the production consumed by other members of an ecosystem, and the substance again returns to the cycle occurring within the ecosystem. Final production is that which is removed in some form or other from an ecosystem, that is, it goes beyond the limits of the ecosystem. Final production also includes the types of production used by man that may belong to any trophic level, even the first level occupied by plants.

The mounting demands and growing technical might of mankind are swiftly increasing mankind&rsquos capability of influencing living nature. It is becoming necessary to control ecosystems. All the means of influencing the biological productivity of ecosystems and controlling it are aimed either at boosting useful primary production (different forms of fertilizer, reclamation, regulation of the abundance and composition of consumers of primary production, and so on) or at raising the efficiency with which primary production is used at the succeeding trophic levels in the direction essential for man. This requires a good knowledge of the species composition and structure of ecosystems and of the ecology of individual species. The forms of economic exploitation and control of living nature that are based on a knowledge of the characteristics of local ecosystems and the form of biological productivity characteristic of them are the most promising.


Discussion

This study aimed to determine the relative importance of direct and indirect effects of eCO2 on ecosystem NPP and ET. Our results suggest that, in xeric environments, indirect effects can be comparable to or even larger than the direct, photosynthetic effect of eCO2 on NPP. On average, indirect effects accounted for 28% of the total stimulation of NPP. The indirect/direct effect ratio ranged from less than 0.1 for tropical and moist sites to more than 1 for semiarid C4 grasslands. The hypothesized decrease of effect size with extremely dry conditions (28) was not supported by our simulations, which represent integrated responses across multiple years. However, our results should be regarded as potential responses, in the absence of nutrient limitations. Suppression of eCO2 effects on NPP by severe water deficit remains a possibility for explaining interannual variation in response within sites. Note that, for arid or semiarid sites characterized by herbaceous species and relatively fast biomass turnover rates, eCO2 stimulation of NPP does not necessarily translate to an increase in standing biomass, even after several years (64, 65), but may be detected in an increase in soil organic carbon (66). This result is only partially captured in model simulations that still show a positive effect on biomass also in the most arid ecosystems, even though the effect is considerably smaller than for NPP. Limitations in nutrient uptake exacerbated by water stress can also dampen the biomass response of the most arid sites. Additionally, eCO2 may stimulate rhizodeposition, potentially explaining the discrepancy between NPP and biomass responses.

We found that changes in ET due to eCO2 were smaller than what a pure, direct response of stomatal conductance would suggest, even at the ecosystem level (i.e., direct effect of −5 to −15%), because indirect effects tend to compensate partially or totally for the direct effect. Further, water “saved” via reduced stomatal conductance is likely to be consumed in water-limited systems, either immediately via increased LAI or by extension of the growing period if LAI is unaffected by eCO2 (Fig. 2). Some of the effects might be due to changes in root biomass, which were included in the model however, changes in rooting depth in response to eCO2 were not considered, and it is therefore possible that indirect effects of eCO2 may increase beyond those simulated here, if development of deeper roots were able to access water not otherwise available. The overall difference between the two CO2 scenarios (375 ppm vs. 550 ppm) in terms of water fluxes (ET) was typically less than 8% and mostly constrained between −5% and 0. Changes in water use of this magnitude would rarely be observable, due to a combination of measurement uncertainty (e.g., ref. 67) and interannual variability (e.g., ref. 39).

Over the large number of sites we simulated, the total change in NPP with the increase in CO2 concentration was mostly in the order of 20 to 35%. These values are very similar or slightly larger than observations in FACE experiments when nutrient limitations do not play a role (28, 68). In fact, our results should be considered as the potential response of NPP to eCO2 in the absence of sink limitations (e.g., ref. 58). The variation in the NPP response as a function of the wetness index is quite impressive, because these are numerical simulations from a mechanistic model rather than observations from real experiments. The large scatter in intermediate wetness conditions suggests that differences in phenology, temperature, short-term meteorological variability, biome, and soil type, all of which were accounted for in the simulations, play a significant role in the NPP response to eCO2. Contrary to the situation with ET, the sum of indirect effects tends to enhance the response of NPP to eCO2 because it adds to the direct physiological response. This is especially evident in semiarid sites, which are responsive to eCO2 even when C4 species are predominant, as supported from observations (25).

Our results demonstrate that mechanistic models of terrestrial ecosystems, despite known limitations (e.g., refs. 46, 58, 69, and 70), do provide substantial insights on ecosystem response to eCO2 that are impossible to obtain with field experiments alone. Model limitations and structure may affect the magnitude of some of the estimates but are unlikely to change the prevailing patterns, with the important exception of nutrient limitation. Furthermore, T&C generated total responses to eCO2 that closely matched observations. For instance, the average modeled eCO2 effect size of NPP, ET, and EWUE is consistent for the Duke-FACE and for the first 7 y of the ORNL-FACE experiments.

Regardless of inherent shortcomings of simulation models, ecosystems at the dry end of the climate spectrum, which experience repeated water stress, are expected to be the most responsive to eCO2 in terms of productivity. When indirect LAI effects are removed, mimicking a lack of stimulation in LAI increase (Fig. 2), productivity in these sites responds more strongly to eCO2. Further, the significant positive relationships between VPD, a measure of atmospheric dryness, and total NPP response to eCO2 (SI Appendix, Fig. S4B) supports the idea that the drier sites are where the most significant effects of eCO2 on NPP should be expected. This finding agrees with modeling studies based on optimality principles (71, 72) and is supported by global patterns of positive response of semiarid ecosystems to CO2 fertilization (73, 74), forcing the reevaluation of the role of semiarid ecosystems in the land carbon sink (75, 76). All this evidence corroborates our results and suggests that projections of eCO2 effects at local and global scales are substantially affected by mechanisms and feedbacks contributing to indirect effects, which are inherently more challenging to model than the direct effect on carbon assimilation. Information on indirect effects derivable from conventional field experiments is necessarily limited. This issue demands both novel experiments specifically designed to target indirect effects and mechanistic solutions in models that do not strongly depend on empirical results. In this context, particular focus should be devoted to addressing the representation of water stress effects on the response of ecosystem productivity.


Human Consumption of Net Primary Production

In an effort to gauge human impact on ecosystems, scientists at NASA and the World Wildlife Fund recently published estimates of how much of Earth’s plant life humans consume for food, fiber, wood, and fuel. By understanding patterns of consumption, and how the planetary supply of plant life relates to the demand for it, these results may enable better management of Earth’s rich biological heritage. Understanding the patterns of supply and demand is critical for identifying areas of severe human impact on ecosystems and planning for sustainable future growth. The details of this study appear in the June 24, 2004, issue of Nature magazine.

Using data collected between 1982-98 by the NOAA Advanced Very High Resolution Radiometer, the researchers calculated the total amount of carbon absorbed by land plants each year and fixed in plant structures—a measure referred to as “Net Primary Production,” or NPP. Then the researchers used computer models to estimate how much of Earth’s land-based net primary productivity is consumed by humans. They found that humans require 20 percent of the NPP generated on land every year. Of course, consumption varies greatly by region and is influenced by three factors: population, per capita consumption, and technology. For more details, please see the NASA press release, entitled NASA Scientists Get Global Fix on Food, Wood, & Fiber Use.

The maps above show human appropriation of land-based net primary production. The shades in the top map represent billions of grams of carbon consumed each year for a given location on Earth. Tan shows low values while dark brown shows high values. The bottom map represents the percentage of NPP consumed by humans each year for a given location. The map reveals that in certain places—such as the northeastern United States, much of Europe, the Middle East, as well as Southern and Eastern Asia—humans consume far more of plants’ net primary productivity than is locally produced. Therefore, people living in these areas must import food, fiber, wood, and fuel in order to meet their demands for products derived from plants.

NASA images courtesy Marc Imhoff and Lahouari Bounoua at Goddard Space Flight Center


The effect of depth on net primary production in aquatic ecosystems - Biology

Campbell Biology Chapter 55 (powell_h)

1) How do the Taylor Glacier bacteria produce their energy?
A) photosynthesis
B) heterotrophism
C) chemoautotrophism
D) thermophobism
E) chemosynthesis

2) In ecosystems, why is the term cycling used to describe material transfer, whereas the term flow is used for energy exchange?
A) Materials are repeatedly used, but energy flows through and out of ecosystems.
B) Both material and energy are recycled and are then transferred to other ecosystems as in a flow.
C) Materials are cycled into ecosystems from other ecosystems, but energy constantly flows within the ecosystem.
D) Both material and energy flow in a never-ending stream within an ecosystem.
E) None of the choices is correct.

3) Which statement most accurately describes how matter and energy are used in ecosystems?
A) Matter is cycled through ecosystems energy is not.
B) Energy is cycled through ecosystems matter is not.
C) Energy can be converted into matter matter cannot be converted into energy.
D) Matter can be converted into energy energy cannot be converted into matter.
E) Matter is used in ecosystems energy is not.

4) The law of conservation of matter states that matter cannot be created, yet matter is sometimes gained or lost to an ecosystem. What is the reason for this seeming contradiction?
A) Chemoautotrophic organisms can convert matter to energy.
B) Matter can be moved in/out of an ecosystem from/to another ecosystem.
C) Photosynthetic organisms convert solar energy to sugars.
D) Detrivores convert matter to energy.
E) Heterotrophs convert heat to energy.

5) Photosynthetic organisms are unique to most ecosystems because they
A) synthesize organic compounds they obtain from decaying heterotrophs.
B) synthesize inorganic compounds from organic compounds.
C) use light energy to synthesize organic compounds.
D) use chemical energy to synthesize organic compounds.
E) convert light energy into matter.

6) A cow's herbivorous diet indicates that it is a(n)
A) primary consumer.
B) secondary consumer.
C) decomposer.
D) autotroph.
E) producer.

7) To recycle nutrients, an ecosystem must have, at a minimum,
A) producers.
B) producers and decomposers.
C) producers, primary consumers, and decomposers.
D) producers, primary consumers, secondary consumers, and decomposers.
E) producers, primary consumers, secondary consumers, top carnivores, and decomposers.

8) Which of the following terms encompasses all of the others?
A) heterotrophs
B) herbivores
C) carnivores
D) primary consumers
E) secondary consumers

9) Many homeowners mow their lawns during the summer and collect the clippings, which are then hauled to the local landfill. Which of the following actions would most benefit the suburban ecosystem?
A) Allow sheep to graze the lawn and then collect the sheep's feces to be delivered to the landfill.
B) Collect the lawn clippings and burn them.
C) Collect the lawn clippings and add them to a compost pile, don't collect the clippings and let them decompose into the lawn, or apply composted clippings to the lawn.
D) Collect the clippings and wash them into the nearest storm sewer that feeds into the local lake.
E) Dig up the lawn and cover the yard with asphalt.

10) Which of the following is an example of an ecosystem?
A) All of the brook trout in a 500 hectare² river drainage system.
B) The plants, animals, and decomposers that inhabit an alpine meadow.
C) A pond and all of the plant and animal species that live in it.
D) The intricate interactions of the various plant and animal species on a savanna during a drought.
E) Interactions between all of the organisms and their physical environment in a tropical rain forest.

11) If the sun were to suddenly stop providing energy to Earth, most ecosystems would vanish. Which of the following ecosystems would likely survive the longest after this hypothetical disaster?
A) tropical rain forest
B) tundra
C) benthic ocean
D) grassland
E) desert

12) Which of the following is true of detrivores?
A) They recycle chemical elements directly back to primary consumers.
B) They synthesize organic molecules that are used by primary producers.
C) They convert organic materials from all trophic levels to inorganic compounds usable by primary producers.
D) They secrete enzymes that convert the organic molecules of detritus into CO₂ and H₂O.
E) Some species are autotrophic, while others are heterotrophic.

13) The major role of detrivores in ecosystems is to
A) provide a nutritional resource for heterotrophs.
B) recycle chemical nutrients to a form capable of being used by autotrophs.
C) prevent the buildup of the organic remains of organisms, feces, and so on.
D) return energy lost to the ecosystem by other organisms.

14) Approximately 1% of the solar radiation that strikes a plant is converted into the chemical bond energy of sugars. Why is this amount so low?
A) Approximately 99% of the solar radiation is converted to heat energy.
B) Only 1% of the wavelengths of visible light are absorbed by photosynthetic pigments.
C) Most solar energy strikes water and land surfaces.
D) Approximately 99% of the solar radiation is reflected.
E) Only the green wavelengths are absorbed by plants for photosynthesis.

15) What percentage of solar radiation striking a plant is converted into chemical energy?
A) 1%
B) 10%
C) 25%
D) 50%
E) 100%

16) Subtraction of which of the following will convert gross primary productivity into net primary productivity?
A) the energy contained in the standing crop
B) the energy used by heterotrophs in respiration
C) the energy used by autotrophs in respiration
D) the energy fixed by photosynthesis
E) all solar energy

17) Which of these ecosystems accounts for the largest amount of Earth's net primary productivity?
A) tundra
B) savanna
C) salt marsh
D) open ocean
E) tropical rain forest

18) Which of these ecosystems has the highest net primary productivity per square meter?
A) savanna
B) open ocean
C) boreal forest
D) tropical rain forest
E) temperate forest

19) Which data is most useful to measure primary productivity in a terrestrial ecosystem?
A) temperature readings
B) potential evapotranspiration
C) intensity of solar radiation
D) annual precipitation
E) amount of carbon fixed

20) Which of the following is a true statement regarding mineral nutrients in soils and their implication for primary productivity?
A) Globally, phosphorous availability is most limiting to primary productivity.
B) Adding a nonlimiting nutrient will stimulate primary productivity.
C) Adding more of a limiting nutrient will increase primary productivity, indefinitely.
D) Phosphorous is sometimes unavailable to producers due to leaching.
E) Alkaline soils are more productive than acidic soils.

21) The total biomass of photosynthetic autotrophs present in an ecosystem is known as
A) gross primary productivity.
B) standing crop.
C) net primary productivity.
D) secondary productivity.
E) trophic efficiency.

22) How is it that the open ocean produces the highest net primary productivity of Earth's ecosystems, yet net primary productivity per square meter is relatively low?
A) Oceans contain greater concentrations of nutrients compared to other ecosystems.
B) Oceans receive a lesser amount of solar energy per unit area.
C) Oceans have the largest area of all the ecosystems on Earth.
D) Ocean ecosystems have less species diversity.
E) Oceanic producers are generally much smaller than oceanic consumers.

23) Why is net primary production (NPP) a more useful measurement to an ecosystem ecologist than gross primary production (GPP)?
A) NPP can be expressed in energy/unit of area/unit of time.
B) NPP can be expressed in terms of carbon fixed by photosynthesis for an entire ecosystem.
C) NPP represents the stored chemical energy that is available to consumers in the ecosystem.
D) NPP is the same as the standing crop.
E) NPP shows the rate at which the standing crop is utilized by consumers.

24) How is net ecosystem production (NEP) typically estimated in ecosystems?
A) the ratio of producers to consumers
B) the amount of heat energy released by the ecosystem
C) the net flux of CO₂ or O₂ in or out of an ecosystem
D) the rate of decomposition by detrivores
E) the annual total of incoming solar radiation per unit of area

25) Aquatic primary productivity is most limited by which of the following?
A) light and nutrient availability
B) predation by primary consumers
C) increased pressure with depth
D) pollution
E) temperature

26) Aquatic ecosystems are least likely to be limited by which of the following nutrients?
A) nitrogen
B) carbon
C) phosphorus
D) iron
E) zinc

27) What is the primary limiting factor for aquatic productivity?
A) pressure
B) lack of nutrients
C) light availability
D) herbivores
E) competition

28) Which of the following ecosystems would likely have a larger net primary productivity/hectare and why?
A) open ocean because of the total biomass of photosynthetic autotrophs
B) grassland because of the small standing crop biomass that results from consumption by herbivores and rapid decomposition
C) tropical rain forest because of the massive standing crop biomass and species diversity
D) cave due to the lack of photosynthetic autotrophs
E) tundra because of the incredibly rapid period of growth during the summer season

29) How is it that satellites can detect differences in primary productivity on Earth?
A) Photosynthetic organisms absorb more visible light in the 350—750 wavelengths.
B) Satellite instruments can detect reflectance patterns of the photosynthetic organisms of different ecosystems.
C) Sensitive satellite instruments can measure the amount of NADPH produced in the summative light reactions of different ecosystems.
D) Satellites detect differences by comparing the wavelengths of light captured and reflected by photoautotrophs to the amount of light reaching different ecosystems.
E) Satellites detect differences by measuring the amount of water vapor emitted by transpiring producers.

30) Which of the following lists of organisms is ranked in correct order from lowest to highest percent in production efficiency?
A) mammals, fish, insects
B) insects, fish, mammals
C) fish, insects, mammals
D) insects, mammals, fish
E) mammals, insects, fish

31) A 3-hectare lake in the American Midwest suddenly has succumbed to an algal bloom. What is the likely cause of eutrophication in freshwater ecosystems, such as this one?
A) increased solar radiation
B) introduction of non-native tertiary consumer fish
C) nutrient runoff
D) accidental introduction of a prolific culture of algae
E) iron dust blowing into the lake

32) The amount of chemical energy in consumers' food that is converted to their own new biomass during a given time period is known as which of the following?
A) biomass
B) standing crop
C) biomagnification
D) primary production
E) secondary production

33) What is secondary production?
A) energy converted by secondary consumers from primary consumers
B) solar energy that is converted to chemical energy by photosynthesis
C) food that is converted to new biomass by consumers
D) energy that is not used by consumers for growth and reproduction
E) growth that takes place during the second year of life in consumers

34) How does inefficient transfer of energy among trophic levels result in the typically high endangerment status of many top-level predators?
A) Top-level predators are destined to have small populations that are sparsely distributed.
B) Predators have relatively large population sizes.
C) Predators are more disease-prone than animals at lower trophic levels.
D) Predators have short life spans and short reproductive periods.
E) Top-level predators are more likely to be stricken with parasites.

35) Trophic efficiency is
A) the ratio of net secondary production to assimilation of primary production.
B) the percentage of production transferred from one trophic level to the next.
C) a measure of how nutrients are cycled from one trophic level to the next.
D) usually greater than production efficiencies.
E) about 90% in most ecosystems.

36) Owls eat rats, mice, shrews, and small birds. Assume that, over a period of time, an owl consumes 5,000 J of animal material. The owl loses 2,300 J in feces and owl pellets and uses 2,500 J for cellular respiration. What is the primary efficiency of this owl?
A) 0.02%
B) 1%
C) 4%
D) 10%
E) 40%

37) Why does a vegetarian leave a smaller ecological footprint than an omnivore?
A) Fewer animals are slaughtered for human consumption.
B) There is an excess of plant biomass in all terrestrial ecosystems.
C) Vegetarians need to ingest less chemical energy than omnivores.
D) Vegetarians require less protein than do omnivores.
E) Eating meat is an inefficient way of acquiring photosynthetic productivity.

38) For most terrestrial ecosystems, pyramids of numbers, biomass, and energy are essentially the samethey have a broad base and a narrow top. The primary reason for this pattern is that
A) secondary consumers and top carnivores require less energy than producers.
B) at each step, energy is lost from the system as a result of keeping the organisms alive.
C) as matter passes through ecosystems, some of it is lost to the environment.
D) biomagnification of toxic materials limits the secondary consumers and top carnivores.
E) top carnivores and secondary consumers have a more general diet than primary producers.

39) Which of the following is primarily responsible for limiting the number of trophic levels in most ecosystems?
A) Many primary and higher-order consumers are opportunistic feeders.
B) Decomposers compete with higher-order consumers for nutrients and energy.
C) Nutrient cycles involve both abiotic and biotic components of ecosystems.
D) Nutrient cycling rates tend to be limited by decomposition.
E) Energy transfer between tropic levels is in almost all cases less than 20% efficient.

40) Which trophic level is most vulnerable to extinction?
A) producer level
B) primary consumer level
C) secondary consumer level
D) tertiary consumer level
E) decomposer level

41) Secondary consumers that can eat only primary consumers receive what percent of the energy fixed by primary producers in a typical field ecosystem?
A) 0.1%
B) 1%
C) 10%
D) 20%
E) 80%

42) Which statement best describes what ultimately happens to the chemical energy that is not converted to new biomass in the process of energy transfer between trophic levels in an ecosystem?
A) It is undigested and winds up in the feces and is not passed on to higher trophic levels.
B) It is used by organisms to maintain their life processes through the reactions of cellular respiration.
C) Heat produced by cellular respiration is used by heterotrophs to thermoregulate.
D) It is eliminated as feces or is dissipated into space as heat in accordance with the second law of thermodynamics.
E) It is recycled by decomposers to a form that is once again usable by primary producers.

43) Consider the food chain grass → grasshopper → mouse → snake → hawk. How much of the chemical energy fixed by photosynthesis of the grass (100%) is available to the hawk?
A) 0.01%
B) 0.1%
C) 1%
D) 10%
E) 60%

44) If the flow of energy in an arctic ecosystem goes through a simple food chain, perhaps involving humans, starting from phytoplankton to zooplankton to fish to seals to polar bears, then which of the following could be true?
A) Polar bears can provide more food for humans than seals can.
B) The total biomass of the fish is lower than that of the seals.
C) Seal meat probably contains the highest concentrations of fat-soluble toxins.
D) Seal populations are larger than fish populations.
E) The fish can potentially provide more food for humans than the seal meat can.

45) Nitrogen is available to plants only in the form of
A) N2 in the atmosphere.
B) nitrite ions in the soil.
C) uric acid from animal excretions.
D) amino acids from decomposing plant and animal proteins.
E) nitrate ions in the soil.

46) Which of the following locations is the reservoir for nitrogen in the nitrogen cycle?
A) atmosphere
B) sedimentary bedrock
C) fossilized plant and animal remains (coal, oil, and natural gas)
D) plant and animal biomass
E) soil

47) Which of the following locations is the reservoir for carbon for the carbon cycle?
A) atmosphere
B) sediments and sedimentary rocks
C) fossilized plant and animal remains (coal, oil, and natural gas)
D) plant and animal biomass
E) all of the above

48) In the nitrogen cycle, the bacteria that replenish the atmosphere with N2 are
A) Rhizobium bacteria.
B) nitrifying bacteria.
C) denitrifying bacteria.
D) methanogenic protozoans.
E) nitrogen-fixing bacteria.

49) How does phosphorus normally enter ecosystems?
A) cellular respiration
B) photosynthesis
C) rock weathering
D) vulcanism
E) atmospheric phosphorous gas

50) Which of the following is an example of a local biogeochemical cycle?
A) O₂ released by oak trees in a forest
B) CO₂ absorbed by phytoplankton in the open ocean
C) excess NO₃- converted to N₂ by denitrifying soil bacteria
D) phosphorous being absorbed from the soil by a corn plant
E) organic carbon remains of a leaf being converted to CO₂ by a fungus

51) Which of the following statements is correct about biogeochemical cycling?
A) The phosphorus cycle involves the recycling of atmospheric phosphorus.
B) The phosphorus cycle involves the weathering of rocks.
C) The carbon cycle is a localized cycle that primarily involves the burning of fossil fuels.
D) The carbon cycle has maintained a constant atmospheric concentration of CO₂ for the past million years.
E) The nitrogen cycle involves movement of diatomic nitrogen between the biotic and abiotic components of the ecosystem.

52) Which of the following properly links the nutrient to its reservoir?
A) nitrogenionic nitrogen in the soil
B) wateratmospheric water vapor
C) carbondissolved CO₂ in aquatic ecosystems
D) phosphoroussedimentary rocks
E) All of the options are correct.

53) In terms of nutrient cycling, why does timber harvesting in a temperate forest cause less ecological devastation than timber harvesting in tropical rain forests?
A) Trees are generally less numerous in temperate forests, so fewer nutrients will be removed from the temperate forest ecosystem during a harvest.
B) Temperate forest tree species require fewer nutrients to survive than their tropical counterpart species, so a harvest removes fewer nutrients from the temperate ecosystem.
C) The warmer temperatures in the tropics influence rain forest species to assimilate nutrients more slowly, so tropical nutrient absorption is much slower than in temperate forests.
D) There are far fewer decomposers in tropical rain forests, so turning organic matter into usable nutrients is a slower process than in temperate forest ecosystems.
E) Typical harvests remove up to 75% of the nutrients in the woody trunks of tropical rain forest trees, leaving nutrient-impoverished soils behind.

54) Why do logged tropical rain forest soils typically have nutrient-poor soils?
A) Tropical bedrock contains little phosphorous.
B) Logging results in soil temperatures that are lethal to nitrogen-fixing bacteria.
C) Most of the nutrients in the ecosystem are removed in the harvested timber.
D) The cation exchange capacity of the soil is reversed as a result of logging.
E) Nutrients evaporate easily into the atmosphere in the post-logged forest.

55) What is the first step in ecosystem restoration?
A) to restore the physical structure
B) to restore native species that have been extirpated due to disturbance
C) to remove competitive invasive species
D) to identify the limiting factors of the producers
E) to remove toxic pollutants

56) What is the goal of restoration ecology?
A) to replace a ruined ecosystem with a more suitable ecosystem for that area
B) to speed up the restoration of a degraded ecosystem
C) to completely restore a disturbed ecosystem to its former undisturbed state
D) to prevent further degradation by protecting an area with park status
E) to manage competition between species in human-altered ecosystems

57) Which of the following statements is true?
A) An ecosystem's trophic structure determines the rate at which energy cycles within the system.
B) At any point in time, it is impossible for consumers to outnumber producers in an ecosystem.
C) Chemoautotrophic prokaryotes near deep-sea vents are primary producers.
D) There has been a well-documented increase in atmospheric nitrogen over the past several decades.
E) The reservoir of ecosystem phosphorous is the atmosphere.

58) In a typical grassland community, which of the following has the smallest biomass?
A) hawk
B) snake
C) shrew
D) grasshopper
E) grass

59) In a typical grassland community, which of the following is the primary consumer?
A) hawk
B) snake
C) shrew
D) grasshopper
E) grass

60) When levels of CO₂ are experimentally increased in a typical grassland community, C₃ plants generally respond with a greater increase in productivity than C₄ plants. This is because
A) C₃ plants are more efficient in their use of CO₂.
B) C₃ plants are able to obtain the same amount of CO₂ by keeping their stomata open for shorter periods of time.
C) C₄ plants don't use CO₂ as their source of carbon.
D) C₃ plants are more limited by CO₂ availability because they lack mechanisms to prevent transpirational water loss.
E) C₃ plants have special adaptations for CO₂ uptake, such as larger stomata.

Food web for a particular terrestrial ecosystem (arrows represent energy flow and letters represent species)

61) Examine this food web for a particular terrestrial ecosystem. Which species is autotrophic?
A) A
B) B
C) C
D) D
E) E

Food web for a particular terrestrial ecosystem (arrows represent energy flow and letters represent species)

62) Examine this food web for a particular terrestrial ecosystem. Which species is most likely a decomposer on this food web?
A) A
B) B
C) C
D) D
E) E

Food web for a particular terrestrial ecosystem (arrows represent energy flow and letters represent species)

63) Examine this food web for a particular terrestrial ecosystem. Species C is toxic to predators. Which species is most likely to benefit from being a mimic of C?
A) A
B) B
C) C
D) D
E) E

Food web for a particular terrestrial ecosystem (arrows represent energy flow and letters represent species)

64) Examine this food web for a particular terrestrial ecosystem. Which pair of species could be omnivores?
A) A and B
B) A and D
C) B and C
D) C and D
E) C and E

Diagram of a food web (arrows represent energy flow and letters represent species)

65) If the figure above represents a terrestrial food web, the combined biomass of C + D would probably be
A) greater than the biomass of A.
B) less than the biomass of H.
C) greater than the biomass of B.
D) less than the biomass of A + B.
E) less than the biomass of E.

Diagram of a food web (arrows represent energy flow and letters represent species)

66) If the figure above represents a marine food web, the smallest organism might be
A) A.
B) F.
C) C.
D) I.
E) E.

67) On the diagram of the nitrogen cycle, which number represents nitrite (NO₂)?
A) 1
B) 2
C) 3
D) 4

68) On the diagram of the nitrogen cycle, which number represents ammonia (NH₄+)?
A) 1
B) 2
C) 3
D) 4

69) On the diagram of the nitrogen cycle, which number represents nitrogen-fixing bacteria?
A) 5
B) 6
C) 7

70) On the diagram of the nitrogen cycle, which number represents nitrifying bacteria?
A) 5
B) 6
C) 7

71) Suppose you are studying the nitrogen cycling in a pond ecosystem over the course of a month. While you are collecting data, a flock of 100 Canada geese lands and spends the night during a fall migration. What could you do to eliminate error in your study as a result of this event?
A) Find out how much nitrogen is consumed in plant material by a Canada goose over about a 12-hour period, multiply this number by 100, and add that amount to the total nitrogen in the ecosystem.
B) Find out how much nitrogen is eliminated by a Canada goose over about a 12-hour period, multiply this number by 100, and subtract that amount from the total nitrogen in the ecosystem.
C) Find out how much nitrogen is consumed and eliminated by a Canada goose over about a 12-hour period and multiply this number by 100 enter this +/- value into the nitrogen budget of the ecosystem.
D) Do nothing. The Canada geese visitation to the lake would have negligible impact on the nitrogen budget of the pond.
E) Put a net over the pond so that no more migrating flocks can land on the pond and alter the nitrogen balance of the pond.

72) As big as it is, the ocean is nutrient-limited. If you wanted to investigate this, one reasonable approach would be to
A) follow whale migrations in order to determine where most nutrients are located.
B) observe Antarctic Ocean productivity from year to year to see if it changes.
C) experimentally enrich some areas of the ocean and compare their productivity to that of untreated areas.
D) compare nutrient concentrations between the photic zone and the benthic zone in various marine locations.
E) contrast nutrient uptake by autotrophs in marine locations that are different temperatures.

73) A porcupine eats 3,000 J of plant material. Of this, 2,100 J is indigestible and is eliminated as feces, 800 J are used in cellular respiration, and 100 J are used for growth and reproduction. What is the approximate production efficiency of this animal?
A) 0.03%
B) 3%
C) 10%
D) 27%
E) 33%

74) Which of the following organisms is incorrectly paired with its trophic level?
A) cyanobacteriumprimary producer
B) grasshopperprimary consumer
C) zooplanktonprimary producer
D) eagletertiary consumer
E) fungusdetritivore

75) Which of these ecosystems has the lowest net primary production per square meter?
A) a salt marsh
B) an open ocean
C) a coral reef
D) a grassland
E) a tropical rain forest

76) The discipline that applies ecological principles to returning degraded ecosystems to a more natural state is known as
A) population viability analysis.
B) landscape ecology.
C) conservation ecology.
D) restoration ecology.
E) resource conservation.

77) Nitrifying bacteria participate in the nitrogen cycle mainly by
A) converting nitrogen gas to ammonia.
B) releasing ammonium from organic compounds, thus returning it to the soil.
C) converting ammonia to nitrogen gas, which returns to the atmosphere.
D) converting ammonium to nitrate, which plants absorb.
E) incorporating nitrogen into amino acids and organic compounds.

78) Which of the following has the greatest effect on the rate of chemical cycling in an
ecosystem?
A) the ecosystem's rate of primary production
B) the production efficiency of the ecosystem's consumers
C) the rate of decomposition in the ecosystem
D) the trophic efficiency of the ecosystem
E) the location of the nutrient reservoirs in the ecosystem

79) The Hubbard Brook watershed deforestation experiment yielded all of the following results except:
A) Most minerals were recycled within a forest ecosystem.
B) The flow of minerals out of a natural watershed was offset by minerals flowing in.
C) Deforestation increased water runoff.
D) The nitrate concentration in waters draining the deforested area became dangerously high.
E) Calcium levels remained high in the soil of deforested areas.

80) Which of the following would be considered an example of bioremediation?
A) adding nitrogen-fixing microorganisms to a degraded ecosystem to increase nitrogen availability
B) using a bulldozer to regrade a strip mine
C) dredging a river bottom to remove contaminated sediments
D) reconfiguring the channel of a river
E) adding seeds of a chromium-accumulating plant to soil contaminated by chromium

81) If you applied a fungicide to a cornfield, what would you expect to happen to the rate of decomposition and net ecosystem production (NEP)?
A) Both decomposition rate and NEP would decrease.
B) Both decomposition rate and NEP would increase.
C) Neither would change.
D) Decomposition rate would increase and NEP would decrease.
E) Decomposition rate would decrease and NEP would increase.


Watch the video: 2 Measuring Primary Productivity (August 2022).