Large Scale Stoichiometry

Ecological stoichiometry also offers insights into factors regulating the multiple pathways by which energy and multiple chemical elements move or are stored at the scale of ecosystems and above. For example, whole-ecosystem manipulations of food web structure have shown how ecosystem nutrient levels and stoichiometric

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Figure 8 Algae and grazers in light gradients. As light increases, algal biomass increases and algal P:C decreases, worsening food quality for grazers. Maximal grazer growth (highest mass) occurs at intermediate light levels.

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Figure 9 Algal (food) and grazer (consumer) dynamics over time (>90 days) at three light levels. At low light, algal biomass remained low, P:C in algae was high, and grazers were variable but often abundant. At high light, algal abundance was initially high and P:C was low, but eventually the grazer became abundant and the system resembled the low-light condition. Finally, at very high light, algal abundance remained high, P:C remained low, and grazer abundance remained low.

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Figure 9 Algal (food) and grazer (consumer) dynamics over time (>90 days) at three light levels. At low light, algal biomass remained low, P:C in algae was high, and grazers were variable but often abundant. At high light, algal abundance was initially high and P:C was low, but eventually the grazer became abundant and the system resembled the low-light condition. Finally, at very high light, algal abundance remained high, P:C remained low, and grazer abundance remained low.

constraints influence the operation of cascading trophic interactions in which alterations in top trophic levels impinge on ecosystem productivity and nutrient cycling. In an experimentally fertilized, P-rich lake, introduction of a fourth trophic level (piscivorous northern pike, Esox lucius) led to strong reductions in planktivorous minnows and thus a major increase in zooplankton biomass and especially Daphnia. Consistent with stoichiometric nutrient recycling theory, increased Daphnia abundance was associated with increased N:P ratios in nutrient pools and a major reduction in the previously dominant N-fixing cyanobacteria. In a parallel experiment in which pike were introduced to a similar minnow-dominated but unfertilized lake, reduced minnow abundances did not result in increased Daphnia abundance. Instead, Daphnia abundances declined in parallel with changes in an unma-nipulated control lake. Both Daphnia declines were associated with major increases in seston C:P ratios, suggesting that the Daphnia success in these lakes was strongly controlled by 'bottom-up' food-quality constraints and relatively unaffected by 'top-down' effects of food web interactions. These observations are consistent with an emerging view that strong trophic cascades may be confined to ecosystems having nutrient-rich auto-trophic production at their base, as ecosystems with poor quality, nutrient-limited autotroph biomass may be unable to support high biomasses of herbivores capable of exerting significant grazing pressure.

Comparative studies across multiple ecosystems have suggested that stoichiometric constraints play an important role in regulating the fate of organic matter and the cycling of nutrients at the ecosystem scale. As mentioned earlier (Figure 5), when considered across multiple studies in diverse terrestrial ecosystems, the rate constant of detrital breakdown correlates negatively with detritus C:nutrient ratio, although considerable variation can exist within particular studies. That is, nutrient-rich detritus (low C:nutrient ratio) breaks down rapidly, returning nutrients to available pools for reuptake by plants, while low-nutrient detritus (high C:nutrient ratio) breaks down slowly and may enhance immobilization of soil nutrients by microbiota, slowing the reuse by plants. Other large-scale patterns in the fate of C in ecosystems appear to be tied to autotroph nutrient content. When data for numerous ecosystems across diverse habitats (oceanic, limnetic, terrestrial) were compiled, strong positive correlations are seen between the percentage of primary production consumed by herbivores and plant nutrient content and turnover rate: ecosystems with fast-growing, nutrient-rich plant production support significant populations for herbivores which consume important quantities of that production. In contrast, in ecosystems with slow-growing, low-nutrient plant biomass, significant quantities of primary production escape consumption in the grazing food chain and enter detrital pathways and long-term C storage. Likewise, the release or retention of nitrogen or organic C appears to be a function of stoichiometric balance in watersheds. Several studies have shown that stream NO3 concentrations or export rates are negatively correlated with average watershed soil C:N ratios, consistent with stoichiometric theory. Conversely, other studies have shown that concentrations or export rates of dissolved organic C from watersheds are positively correlated with watershed soil C:N ratios. Thus, ecophysiological limitations on the processing of organic C and nutrients by soil microorganisms ramify to affect the fluxes of materials at watershed and regional scales.

Stoichiometric constraints also play a role in the regulation of biosphere-scale processes governing oceanic and terrestrial C-cycling, atmospheric CO2 concentrations, and thus global climate. In the open ocean, biogeochemical processes appear to be closely linked to multiple limitations associated with light intensity (as mediated by water column mixing processes), macronu-trients (N, P), and micronutrients (especially iron). In the enormous central Pacific gyre, long-term studies have shown climatic variations associated with El Nino that reduce the intensity of vertical mixing processes and cross-thermocline nutrient transfers that enhance the success of light-limited, N-fixing cyanobacteria. Dominance of these algae, in turn, increases the C:P ratio of primary production and thus increases net carbon sequestration in deep waters. In other parts of the ocean, low iron supplies rather than light may limit primary production and N-fixation and thus stoichiometric coupling of C- and macronutrient cycles can ultimately depend on iron supply, itself regulated by long-term processes associated with delivery of continental dust. Since the production of continental dust is itself closely regulated by climatic conditions (rainfall patterns), a set of complex feedbacks that plays out over tens of thousands of years is established.

These feedbacks have been incorporated into global biogeochemical models that investigate the autoregula-tory capability of the biosphere ('Gaia'). In these studies, it has been shown that a self-regulating system at the scale of the biosphere emerges if the organisms driving the Earth's biogeochemical systems operate under functional constraints on their processing of energy and matter. In other words, no higher-level selection at the scale of the biosphere ('Gaia' as organism) is required for planetary self-regulation to operate. Instead, 'Gaia' is a complex interactive system with dynamics determined as an emergent property of selection operating primarily on the ecophysiological traits and constraints of individual organisms.

See also: Abundance; Allometric Principles; Aquatic Organisms; Bifurcation; Biogeochemical Models; Coexistence; Denitrification; Detritus; Ecological Efficiency; Ecosystems; Ecosystem Patterns and Processes; Evolution of Oceans; Evolutionary and Biochemical Aspects; Excretion; Freshwater Lakes; Gause's Competitive Exclusion Principle; Grazing; Grazing Models; Growth Constraints: MichaelisMenten Equation and Liebig's Law; Growth Models; History of Ecology; Homeostasis; Lake Models; Lake Restoration Methods; Mass Production of Marine Macroalgae; Microbial Models; Multitrophic Integration for Sustainable Marine Aquaculture; Optimal Foraging Theory; Organismal Ecophysiology; Pelagic Predators; Plant Competition; Plant Growth Models; Population and Community Interactions; Predation; Temperate Forest; Terrestrial Arthropods; Trace Elements; r-Strategist/K-Strategists.

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