Large Scale Implications of Ecological Stoichiometry

In this and other entries, ecological stoichiometry has been presented as a theoretical and empirical approach for linking organism biochemical properties to larger-scale ecological patterns and processes. A logical framework for ecological stoichiometry (though the term was not used) was initially formalized by W. A. Reiners in the mid-1980s. Up to that time, ecosystem science had been largely organized around energetics. Reiners offered his framework as a complementary view of ecosystems in which the flow of matter, rather than energy, was the organizing feature.

Using a limited number of logical steps, Reiners combined biochemical and ecological axioms (true statements) to derive a series of theorems that describe how biological processes control biogeochemical cycling from local to global scales (Figure 5). One of Reiners' principal axioms

(axiom 2) was that protoplasmic life is consistent across organisms. We now know that this axiom is not strictly true and that cellular stoichiometry can vary, with important ecological implications. Interestingly, this shortcoming has not diminished the utility of this logical sequence because

Theorem 5: Global biogeochemical cycles have been altered by life over time

Theorem 4: World biota regulates global biogeochemical cycles

Figure 5 A logic flowchart of stoichiometric theory illustrating how a small number of axioms from biochemistry (clear boxes) lead to theorems describing global biogeochemical cycles (shaded boxes). Adapted from Reiners WA (1986) Complementary models for ecosystems. American Naturalist 127: 59-73.

it still remains useful for understanding how stoichiometric variation at basal levels of biological organization can cascade to have ecosystem and even global scale implications.

Prior to Reiners' theoretical work, A. C. Redfield found a surprising congruence in C:N:P ratios in plankton from widespread regions of the world's oceans. In this seminal work, Redfield determined that ocean seston had a consistent C:N:P stochiometry equal to 106:16:1. The 'Redfield ratio', as it has become known, is perhaps the most famous result in ecological stoichiometry and many have come to revere it as a rare ecological constant, analogous to better-known constants in chemistry and physics. Perhaps more surprising, and more interesting in the context of this entry, seston C:N:P mirrors the ratio of these elements in dissolved pools. Redfield interpreted the equivalence between N:P ratios in seston and dissolved pools as evidence that N and P had balanced flow into and out of biotic pools. Furthermore, regressions of ocean N and P concentrations pass through the origin, indicating that these elements are depleted from ocean water simultaneously. Given the myriad of geological and meteorological factors that could influence nutrient availability in the world's ocean, there is no a priori expectation for this equivalence. Redfield's explanation for this finding was that the biota controlled the relative availability of these elements in the ocean. He went on to suggest that P, rather than N availability limited ocean production because biological processes (e.g., N-fixation) are capable of adjusting N availability to match P constraints. Contemporary support for Redfield's contention is found in recently published models of N-fixation in the open ocean. In addition, extensive datasets on N-fixation in lakes clearly indicates that overall N:P ratio constrains when and where N-fixation is active (restricted to conditions where molar N:P < 20). Interestingly, similar mechanisms have been invoked to explain the development of P-limitation in terrestrial soils as they age (as discussed earlier in this article). Redfield's findings have proved important beyond the realm of oceanography because they suggest that biotic processes control element cycles at global scales, a world view that Reiners and others would further develop.

Global scale coupling of C, N, and P cycles involve a highly complex set of interactions between Earth's subsystems (biosphere, atmosphere, and geosphere). Feedbacks among these systems operate over a suite of spatial and temporal scales and include interactions between terrestrial, freshwater, and ocean ecosystems that remain poorly understood. However, this complexity has not hindered researchers from exploring the role of biological processes in structuring biogeochemical cycles and their evolution through geological time. Perhaps the best example of such work relates to maintenance of global atmospheric oxygen concentration. Oxygen has remained between 15% and 35% of the atmosphere for the last several hundred million years. Under current biogeochemical conditions, atmospheric oxygen turns over every 4000 years. Combined, these observations suggest that a dynamic system of feedbacks may exist to stabilize oxygen content. Biotic activity is a principle driver of modern oxygen cycling. The production and breakdown of organic matter produces and consumes equal amounts of oxygen and as a result does not perturb the existing oxygen levels. However, biomass standing stock has not been constant through time. In addition, slow cycling of oxygen through nonbiotic processes such as carbonate precipitation, oxidation of uplifted iron, photolysis of water, and oxidation of ammonium also influence long-term patterns in oxygen concentration. Combined, it is not obvious why these processes should combine to create stability in the atmospheric pool. One mechanism for the maintenance of this stability is variation in the efficiency of P burial in the world's oceans. A thorough discussion of this mechanism is beyond the scope of this text. Suffice it to say that the crucial feature of this mechanism is the positive relationship between oxygen content and the efficiency of P burial via precipitation with iron hydroxides. Efficient P burial ultimately leads to reduced delivery to the photic zone, reduced autotrophic activity, and consequent oxygen production. Other examples of biologically driven feedback mechanism have been identified by several authors. Feedbacks, and their role in autoregulation of Earth system properties, require constraints and ecological stoichiometry suggests that the elemental composition of core biomolecules and the processes that create them provide these constraints.

Given the pervasiveness of human activity and its impact on element abundances, efforts to understand the complexity associated with biological and physical controls on biogeochemical cycling are of paramount importance if society is to forecast ecological conditions and design strategies for responsible stewardship. Though a vigorous debate continues over the relative importance of biological and geological processes in controlling global scale elemental cycles, it is nevertheless critical to continue to explore the stoichiometric underpinnings of biological processes and their consequences for ecosystem and biosphere structure and function.

See also: Biogeochemical Models; Denitrification; Detritus; Ecological Efficiency; Ecological Stoichiomety: Overview; Ecosystems; Evolution of Oceans; Evolutionary and Biochemical Aspects; Excretion; Freshwater Lakes; Grazing Models; Grazing; Growth Constraints: Michaelis-Menten Equation and Liebig's Law; Homeostasis; Lake Models; Microbial Models; Organismal Ecophysiology; Population and Community Interactions; Temperate Forest; Trace Elements.

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