Stoichiometric Theory Research Articles

Ecological responses to simulated benthic‐derived nutrient subsidies mediated by omnivorous fish

Summary1. Fish can play an important role in coupling benthic and pelagic habitats by consuming benthic prey and providing essential nutrients to algae in dissolved form. However, little is known about the factors affecting the magnitude of this nutrient subsidy.2. Using laboratory and mesocosm experiments we evaluated how varying ingestion rates of bluegill sunfish (Lepomis macrochirus) affects fish excretion rates of both nitrogen (N) and phosphorus (P). During the 10‐week mesocosm experiment, we also evaluated how varying ingestion rates may affect plankton community dynamics, and nutrient flux between pelagic and benthic habitats. Lastly, bioenergetic/mass balance models were used to examine the nutrient stoichiometry of fish body composition and excretion products.3. Under laboratory conditions, both N and P excretion rates increased with increased ingestion of benthic prey surrogates (earthworms). This effect was more pronounced for N than P. Furthermore, under the more realistic conditions of the mesocosm experiment ingestion rate had no significant effect on P excretion rate.4. Increased fish ingestion rate in the mesocosm experiment increased total algal biomass and the flux of nutrients from the water column to sediments. Effects of variable ingestion were much stronger on periphyton biomass and algal sedimentation rates than on phytoplankton or zooplankton biomass or composition.5. Fish body nutrient composition was greatly affected by ingestion rate. N content increased and P content decreased with ingestion rate. As a result, the N : P ratio of fish bodies also increased with ingestion rate. The N : P ratio of nutrients excreted by fish also increased with ingestion rate, counter to predictions of stoichiometric theory, which predicts that excreted N : P ratio is negatively correlated to body N : P. However, this finding can be explained by relaxing the assumption of constant nutrient assimilation rates, and our mass balance data suggest that assimilation rates vary indeed with ingestion rate.6. Our study provides experimental evidence that translocation of benthic‐derived nutrients by fish can affect the flux of nutrients among habitats, while also suggesting that stoichiometry models need to better incorporate how variable ingestion rates affect nutrient assimilation and excretion rates.

The Convergence Coordinates of the Affinity of a Component to the Stationary Phase vs. the Composition of Mobile Phase in HPLC

By plotting of the affinity of component to adsorbent (βa) vs. the logarithm of the solvent activity in the mobile phase in a reversed phase HPLC system, it is shown that when the concentration of organic solvent in the mobile phase increases, βa of a series of aromatic alcohol homologues tends to converge to a point. A theoretical model for evaluating the convergent coordinates was presented by using the linear relationship between βa and log aD and the stoichiometric displacement theory for retention (SDT-R). The physical meaning of the coordinates of convergence point was also elucidated. Convergence occurs when the activity of the displacing solvent approaches the value when in a pure state. The experimental results fit the equations and the coordinate of the convergence point well. At the same time, a unified equation to express the quantitative relation between parameter βa describing the adsorption process in a liquid-solid system and k describing the retention process in a liquid chromatographic system is also derived.

Growth rates of Antarctic krill, Euphausia superba: Comparison of the instantaneous growth rate method with nitrogen and phosphorus stoichiometry

Zooplankton growth rates are hard to measure directly, and proxy measurements are desirable to encompass the variety of species and scales of interest. The growth rate hypothesis of stoichiometric theory states that a negative relationship exists between nitrogen : phosphorus (N : P) stoichiometry and growth rate, driven by cellular ribosomal ribonucleic acid (rRNA) content. Despite the wealth of literature on the growth rate hypothesis, there exist no practical demonstrations of its use in the marine literature. We thus investigated whether this hypothesis could be the basis of a technique to estimate growth rates of Euphausia superba by comparing, for the same individual krill, elemental stoichiometry and growth rates derived from the instantaneous growth rate (IGR) method. These growth rates were the first IGR measurements from South Georgia; from within a restricted area over the course of just 1 month, these rates were highly variable, from negative to near maximum rates recorded for the species. Although there were significant differences in N: P ratio and phosphorus content between individuals and schools, there was no relationship between N: P ratio and growth rate when data were grouped by school. Thus, our data do not support the predictions of the growth rate hypothesis at an intraspecific scale. However, when all data were pooled, the mean values of growth rate and N: P ratio did fit the interspecific relationship established previously for freshwater zooplankton. We suggest that krill maintain the biochemical machinery for high growth potential and maintain high growth in summer to take advantage of short‐term fluctuations in food.

Inducible defences prevent strong population fluctuations in bi‐ and tritrophic food chains

AbstractRecent theoretical work (Vos et al. 2004) predicts that inducible defences prevent strong population fluctuations under high levels of nutrient enrichment. Here we evaluate this model prediction and show that inducible defences in algae stabilize the dynamics of experimentally assembled bi‐ and tritrophic planktonic food chains. At high phosphorus levels, we observed strong population fluctuations in all food chains with undefended algae. These fluctuations did not occur when algae had inducible defences. At low phosphorus levels, we observed deterministic consumer extinctions, as predicted by stoichiometric theory. Our study thus shows that both biotically and abiotically induced changes in algal food quality affect the stability and persistence of planktonic food chains.

Stoichiometry and population dynamics

AbstractPopulation dynamics theory forms the quantitative core from which most ecologists have developed their intuition about how species interactions, heterogeneity, and biodiversity play out in time. Throughout its development, theoretical population biology has built on variants of the Lotka–Volterra equations and in nearly all cases has taken a single‐currency approach to understanding population change, abstracting populations as aggregations of individuals or biomass. In this review, we explore how depicting organisms as built of more than one thing (for example, C and an important nutrient, such as P) in stoichiometrically explicit models results in qualitatively different predictions about the resulting dynamics. Fundamentally, stoichiometric models incorporate both food quantity and food quality effects in a single framework, allow key feedbacks such as consumer‐driven nutrient recycling to occur, and generally appear to stabilize predator–prey systems while simultaneously producing rich dynamics with alternative domains of attraction and occasionally counterintuitive outcomes, such as coexistence of more than one predator species on a single‐prey item and decreased herbivore performance in response to increased light intensity experienced by the autotrophs. In addition to the theoretical background, we also review recent laboratory and field studies considering stoichiometric effects on autotroph–herbivore systems, emphasizing algae–Daphnia interactions. These studies support the predictions of stoichiometric theory, providing empirical evidence for alternative stable states under stoichiometric constraints, for negative effects of solar radiation on herbivores via stoichiometric food quality, and for diversity‐enhancing effects of poor food quality. Stoichiometric theory has strong potential for both quantitative and qualitative improvements in the predictive power of population ecology, a major priority in light of the multivariate anthropogenic and natural perturbations experienced by populations. However, full development and testing of stoichiometric population dynamics theory will require greater intellectual tolerance and exchange between researchers working in ecosystem and population ecology.

A General Thermodynamic and Stoichiometric Theory of Stability of Chemical Species

A new approach to the evaluation of stability of chemical species is proposed. The main idea is to allow the species to conceptually interact via a special type of chemical reaction so that an equilibrium state is achieved. Additionally, the energies of the species are allowed to vary from their initial (standard) to their equilibrium values so that the energy change of any conceivable reaction at equilibrium is equal to zero. The equilibrium energies of the species are further evaluated using an optimization procedure similar to that of the least-squares method. A species is considered unstable if the difference between its initial and equilibrium energy is positive (the species releases energy). On the contrary, if the difference between the initial and the equilibrium energy of a species is negative, the species is stable (the species absorbs energy). This type of stability of species is referred to as the overall stability. Employing the response reactions (RERs) formalism, one shows further that the overall stabilities of the species may be partitioned into different types of contributions. In particular, it is shown that the conventional definition of stability of the species is just an appropriately normalized part of the overall stability.

Studies on the Thermodynamics During the Biopolymer Retention Process in Reversed‐Phase Liquid Chromatography. I. The Free Energy Change and Its Fractions, Adsorption, and Desorption Free Energy Changes

Based on the stoichiometric displacement theory for retention of solute (SDT‐R), the changes of free energy of biopolymers during retention process in reversed‐phase high performance liquid chromatography (RPLC) can be divided into two independent fractions, the adsorption and desorption free energy changes. The totally net free energy change and the two fractions of seven kinds of proteins in RPLC were accurately measured. The standard deviation between experimental values and theoretical expectation was less than ±2%. In addition, from the point of view of thermodynamics, with the comparisons of the adsorption and desorption free energy changes between small solutes and biopolymers in RPLC, the following experimental facts were first explained quantitatively: [Robbat, A., Jr.; Liu, T.Y. J. Chromatagr. 1990, 513, 117] The elution range of organic solvent for small solutes in the mobile phase in RPLC is usually broad, while that for biopolymers is very narrow. [Tan, L.C.; Carr, P.W. J. Chromatogr. 1993, 656, 521] Small solutes in RPLC can be eluted with isocratic elution, but biopolymers only with gradient elution. [Tchapla, A.; Heron, S.; Colin, H.; Guiochon, G. Anal. Chem. 1988, 60, 1443] The resolution of small solutes in RPLC depends on column length, while that of biopolymers is almost independent of it.

Consumer‐resource stoichiometry in detritus‐based streams

AbstractStoichiometric relationships between consumers and resources in detritus‐based ecosystems have received little attention, despite the importance of detritus in most food webs. We analysed carbon (C), nitrogen (N), and phosphorus (P) content of invertebrate consumers, and basal food resources in two forested headwater streams (one reference and the other nutrient‐enriched). We found large elemental imbalances between consumers and food resources compared with living plant‐based systems, particularly in regard to P content, which were reduced with enrichment. Enrichment significantly increased nutrient content of food resources (consistent with uptake of N and P by detritus‐associated microbes). P content of some invertebrates also increased in the enriched vs. reference stream, suggesting deviation from strict homeostasis. Nutrient content varied significantly among invertebrate functional feeding groups, orders and, to some extent, size classes. Future application of stoichiometric theory to detritus‐based systems should consider the potential for relatively large consumer‐resource elemental imbalances and P storage by insect consumers.

Biological stoichiometry: a theoretical framework connecting ecosystem ecology, evolution, and biochemistry for application in astrobiology

Astrobiology is an extremely wide-ranging field and thus is in special need of conceptual and theoretical frameworks that can integrate its various arenas of study. In this paper I review recent work associated with a conceptual framework known as ‘ecological stoichiometry’ and even more recent extensions in the development of ‘biological stoichiometry’. Ecological stoichiometry is the study of the balance of energy and multiple chemical elements in ecological interactions and has developed rapidly in the study of nutrient cycling and energy flow in aquatic food webs. It identifies the elemental composition of interacting biota as central in understanding the nature of their interactions and dynamics, including key feedbacks via nutrient recycling. Biological stoichiometry extends this mode of thinking to all types of biological systems. It especially seeks to better understand, at the biochemical and genetic levels, the factors influencing the elemental composition of living things and the evolutionary forces that drive and constrain that elemental composition. By connecting key concepts of ecosystem ecology, evolutionary biology and biochemistry, stoichiometric theory integrates biological information into a more coherent whole that holds considerable promise for application in astrobiology. Several examples of potential astrobiological applications of stoichiometric analysis are offered, including ones related to pre-biotic evolution, the Cambrian explosion, biosignatures and biological feedbacks on planetary carbon cycling.

STOICHIOMETRIC CONSTRAINTS VS. ALGAL DEFENSES: TESTING MECHANISMS OF ZOOPLANKTON FOOD LIMITATION

The nature of resource limitation for zooplankton grazers is strongly debated. We provide a test of stoichiometric constraints and three alternative mechanisms of resource limitation in six lakes that represent a natural gradient in resource quality. Growth assays measured the responses of juvenile Daphnia to concentrated natural resources and to sup- plements of phosphate (P), essential fatty acids (EFA), and P-deficient and P-rich algae. Assimilation experiments quantified the digestibility of 32 P-labeled resources. Daphnia growth was much lower on resources from deep compared with shallow lakes (0.13 6 0.01 d 21 and 0.30 6 0.03 d 21 , respectively) but was independent of resource concentration. Growth was negatively correlated with the resource C:P ratio, but the relationship was not causal as responses to the P supplement were very weak. Adding EFA also produced weak responses. In contrast, supplements of readily digested algae stimulated moderate (P-de- ficient) and strong (P-rich) increases in growth that accounted for growth variation among the lakes. Changes in Daphnia P balance were also contrary to P limitation. In all, our growth assays implicate energy limitation unrelated to food concentration and, therefore, support the digestion resistance hypothesis. Digestion resistance is further supported by a positive correlation between assimilation efficiency and growth. Our results show that stoichiometric theory needs to be integrated with theory on plant and algal defenses against herbivores.

Stoichiometric Constraints vs. Algal Defenses: Testing Mechanisms of Zooplankton Food Limitation

The nature of resource limitation for zooplankton grazers is strongly debated. We provide a test of stoichiometric constraints and three alternative mechanisms of resource limitation in six lakes that represent a natural gradient in resource quality. Growth assays measured the responses of juvenile Daphnia to concentrated natural resources and to supplements of phosphate (P), essential fatty acids (EFA), and P-deficient and P-rich algae. Assimilation experiments quantified the digestibility of 32P-labeled resources. Daphnia growth was much lower on resources from deep compared with shallow lakes (0.13 ± 0.01 d–1 and 0.30 ± 0.03 d–1, respectively) but was independent of resource concentration. Growth was negatively correlated with the resource C:P ratio, but the relationship was not causal as responses to the P supplement were very weak. Adding EFA also produced weak responses. In contrast, supplements of readily digested algae stimulated moderate (P-deficient) and strong (P-rich) increases in growth that accounted for growth variation among the lakes. Changes in Daphnia P balance were also contrary to P limitation. In all, our growth assays implicate energy limitation unrelated to food concentration and, therefore, support the digestion resistance hypothesis. Digestion resistance is further supported by a positive correlation between assimilation efficiency and growth. Our results show that stoichiometric theory needs to be integrated with theory on plant and algal defenses against herbivores.

The effects of Daphnia on nutrient stoichiometry and filamentous cyanobacteria: a mesocosm experiment in a eutrophic lake

1. Stoichiometric theory predicts that the nitrogen : phosphorus (N : P) ratio of recycled nutrients should increase when P‐rich zooplankton such as Daphnia become dominant. We used an enclosure study to test the hypothesis that an increased biomass of Daphnia will increase the relative availability of N versus P sufficiently to decrease the abundance of filamentous cyanobacteria. The experiment was conducted in artificially enriched Lake 227 (L227) in the Experimental Lakes Area (ELA), north‐western Ontario, Canada. Previous studies in L227 have shown that the dominance of filamentous, N‐fixing cyanobacteria is strongly affected by changes in the relative loading rates of N and P.2. We used a 2 × 2 factorial design with the addition or absence of D. pulicaria and high or low relative loading rates of N and P (+NH4, –NH4) in small enclosures as treatment variables. If Daphnia can strongly affect filamentous cyanobacteria by altering N and P availability, these impacts should be greatest with low external N : P loading rates. The phytoplankton community of L227 was predominantly composed of filamentous Aphanizomenon spp. at the start of the experiment.3. Daphnia strongly reduced filamentous cyanobacterial density in all enclosures to which they were added. The addition of NH4 had only a small impact on algal community composition. Hence, we conclude that Daphnia did not cause reductions in cyanobacteria by altering the N : P ratio of available nutrients.4. Despite the lack of evidence that Daphnia affected filamentous cyanobacteria by altering the relative availability of N and P, we found changes in nutrient cycling consistent with other aspects of stoichiometric theory. In the presence of Daphnia, total P in the water column decreased because of an increase in P sedimentation. In contrast to P, a decrease in suspended particulate N was offset by an increase in dissolved N (especially NH4). Hence, dissolved and total N : P ratios in the water column increased with Daphnia as a result of differences in the fate of suspended particulate N versus P. There was minimal accumulation and storage of P in Daphnia biomass in the enclosures.5. Our experiment demonstrated that Daphnia can strongly limit filamentous cyanobacterial abundance and affect the biogeochemical cycling of nutrients. In our study, changes in nutrient cycling were apparently insufficient to cause the changes in phytoplankton community composition that we observed. Daphnia therefore limited filamentous cyanobacteria by other mechanisms.

Interplay between Energy Limitation and Nutritional Deficiency: Empirical Data and Food Web Models

Food quality may play an important role in consumer population dynamics. The frequently large differences in elemental and biochemical composition observed between autotrophs and their grazers suggest that food quality may be of particular importance for herbivores. Under nutrient-depleted conditions the carbon-to-nutrient ratios of autotrophs can increase to such an extent that consumers become nutrient rather than energy limited. Estimating the importance of this effect in situ in pelagic food webs is complicated by the omnivory of many consumers and by rapid nutrient recycling. Isolated predator–prey studies inadequately represent this interaction; instead, an ecosystem perspective is required. We used seven years of data from large, deep Lake Constance to develop seasonally resolved flux models of the pelagic food web and analyzed the balance between energy and nutrient constraints. The carbon (C) and phosphorus (P) flows were simultaneously quantified and balanced. C represented food quantity/energy. P was taken as a surrogate of food quality, because algal C:P ratios exceeded the threshold above which P limitation of herbivores is predicted by stoichiometric theory throughout summer and autumn. Primary production exceeded bacterial C production by a factor of 3, but autotrophs and bacteria took up approximately equal amounts of P during summer and autumn. As a consequence, the C and P supplies of suspension-feeding zooplankton were decoupled: Consumer C demands were largely met by phytoplankton whereas P was mostly obtained from bacteria and their protist predators. The degree of consumer P deficiency varied according to supplementation of their algal diet with P-enriched bacteria or bacterivores. This favored the occurrence of omnivores, i.e., organisms that minimized P deficiencies at the cost of enhanced energy limitation. In contrast with previous perceptions, P remineralization during P-depleted summer conditions was dominated by bacterivorous flagellates, carnivorous crustaceans, and fish, which fed on prey with an elemental composition similar to their own, whereas herbivores contributed only 30% of P cycling despite their large biomass and C production. Our results suggested a co-limitation of predominantly herbivorous consumers by C and P and a mutual dependence of the two types of deficiency at the individual and system level. This pattern is not specific to pelagic systems but appears to be applicable across ecosystem types.

INTERPLAY BETWEEN ENERGY LIMITATION AND NUTRITIONAL DEFICIENCY: EMPIRICAL DATA AND FOOD WEB MODELS

Food quality may play an important role in consumer population dynamics. The frequently large differences in elemental and biochemical composition observed between autotrophs and their grazers suggest that food quality may be of particular importance for herbivores. Under nutrient-depleted conditions the carbon-to-nutrient ratios of autotrophs can increase to such an extent that consumers become nutrient rather than energy limited. Estimating the importance of this effect in situ in pelagic food webs is complicated by the omnivory of many consumers and by rapid nutrient recycling. Isolated predator–prey studies inadequately represent this interaction; instead, an ecosystem perspective is required. We used seven years of data from large, deep Lake Constance to develop seasonally resolved flux models of the pelagic food web and analyzed the balance between energy and nutrient constraints. The carbon (C) and phosphorus (P) flows were simultaneously quantified and balanced. C represented food quantity/energy. P was taken as a surrogate of food quality, because algal C:P ratios exceeded the threshold above which P limitation of herbivores is predicted by stoichiometric theory throughout summer and autumn. Primary production exceeded bacterial C production by a factor of 3, but autotrophs and bacteria took up approximately equal amounts of P during summer and autumn. As a consequence, the C and P supplies of suspension-feeding zooplankton were decoupled: Consumer C demands were largely met by phytoplankton whereas P was mostly obtained from bacteria and their protist predators. The degree of consumer P deficiency varied according to supplementation of their algal diet with P-enriched bacteria or bacterivores. This favored the occurrence of omnivores, i.e., organisms that minimized P deficiencies at the cost of enhanced energy limitation. In contrast with previous perceptions, P remineralization during P-depleted summer conditions was dominated by bacterivorous flagellates, carnivorous crustaceans, and fish, which fed on prey with an elemental composition similar to their own, whereas herbivores contributed only 30% of P cycling despite their large biomass and C production. Our results suggested a co-limitation of predominantly herbivorous consumers by C and P and a mutual dependence of the two types of deficiency at the individual and system level. This pattern is not specific to pelagic systems but appears to be applicable across ecosystem types.

The evolution of ecosystem processes: growth rate and elemental stoichiometry of a key herbivore in temperate and arctic habitats

Abstract Understanding the reciprocal interactions between the evolved characteristics of species and the environment in which each species is embedded is a major priority for evolutionary ecology. Here we use the perspective of ecological stoichiometry to test the hypothesis that natural selection on body growth rate affects consumer body stoichiometry. As body elemental composition (nitrogen, phosphorus) of consumers influences nutrient cycling and trophic dynamics in food webs, such differences should also affect biogeochemical processes and trophic dynamics. Consistent with the growth rate hypothesis, body growth rate and phosphorus content of individuals of the Daphnia pulex species complex were lower in Wisconsin compared to Alaska, where the brevity of the growing season places a premium on growth rate. Consistent with stoichiometric theory, we also show that, relative to animals sampled in Wisconsin, animals sampled in Alaska were poor recyclers of P and suffered greater declines in growth when fed low-quality, P-deficient food. These results highlight the importance of evolutionary context in establishing the reciprocal relationships between single species and ecosystem processes such as trophic dynamics and consumer-driven nutrient recycling.

Stoichiometric theory extended to micronutrients: Comparison of the roles of essential fatty acids, carbon, and nitrogen in the nutrition of marine copepods

Conventional stoichiometric theory, which is used to study the limitation of zooplankton production by C, N, and other elements, is extended to include the essential polyunsaturated fatty acids (PUFAs) 20:5(n−3) and 22:6(n−3). Using typical biochemical compositions of consumer (Calanus helgolandicus) and algal food (hypothetical diatom‐dinoflagellate mixtures), the analysis shows that PUFAs or macronutrients can be important in limiting zooplankton production, depending on the biochemical compositions of consumer and food and the efficiencies with which dietary components are used. Predicted limitation by fatty acids is strongest when zooplankton use a monospecific diet, indicating that such limitation may be of particular significance in laboratory zooplankton, which are often fed phytoplankton monocultures. The analysis illustrates several factors that operate to minimize the limiting potential of fatty acids to the extent that limitation by C or N could occur: selective grazing to obtain a nutritionally balanced diet, plasticity in consumer biochemical composition, and high C requirements for respiration. The possibility of macronutrient limitation is increased further if zooplankton are able to actively synthesize essential PUFAs, although this is not thought to be the case in most aquatic systems. The work highlights the need for complete data sets incorporating fatty acids and bulk elemental properties of consumers and prey, as well as an improved understanding of the roles and cycling of essential fatty acids, if we are to be able to provide a unified view of zooplankton nutrition.

Elemental stoichiometry of lower food web components in arctic and temperate lakes

Lakes were surveyed to assess the potential patterns of latitudinal variation in carbon:nitrogen:phosphorus (C:N:P) stoichiometry of lower food web components. Thirty-four lakes were surveyed at an arctic latitude (68°38′N, 149°38′W) and 10 lakes at a temperate latitude (46°13′N, 89°32′W) during 1997. The temperate data set was augmented with earlier survey results employing similar methods. It was hypothesized that differences in environmental variables across latitude would cause differences in community C:N:P ratios, leading to differences in trophic interactions. Physical measurements (light, temperature), seston and zooplankton were collected from each lake. Seston and zooplankton were analyzed for C, N and P content, and zooplankton were counted and measured for biomass estimates. The degree of trophic interaction between seston and zooplankton was assessed by (i) measuring elemental imbalances between seston and zooplankton and (ii) calculating the potential recycling ratio by the zooplankton community available for seston. Seston C:nutrient, but not N:P, ratios were higher in temperate than arctic lakes. Conversely, arctic zooplankton had higher C:nutrient, but not N:P, ratios than zooplankton in temperate lakes. Elemental imbalances were greater in temperate than in arctic lakes, but N:P stoichiometry of potential zooplankton recycling was nearly identical between the two latitudes. Zooplankton community C:N:P ratios were not related to either latitude or seston C:N:P. In accordance with stoichiometric theory, relative abundances of calanoid copepods were positively correlated with seston C:N in temperate lakes. Additionally, relative abundances of Daphnia were negatively correlated with seston C:N ratios in temperate and arctic lakes, and positively correlated with N:P ratios in the arctic. In general, these results suggest that seston and zooplankton community stoichiometry differ across latitude, and these differences have the potential to affect trophic interactions.

THE STOICHIOMETRY OF CONSUMER-DRIVEN NUTRIENT RECYCLING: THEORY, OBSERVATIONS, AND CONSEQUENCES

Ecologists are increasingly recognizing the importance of consumers in regulating ecosystem processes such as nutrient cycling. Ecologists have recently made considerable progress in understanding nutrient cycling and trophic interactions in pelagic systems by application of a new concept, ecological stoichiometry, to consumer-driven processes. In this paper we synthesize these conceptual advances within pelagic ecology and attempt to illustrate how they may be usefully applied in other ecosystems. Stoichiometric theory shows that both grazer and algal elemental composition are critical parameters influencing rates and ratios of nutrient release. Thus, the stoichiometry of nutrient recycling is a feedback mechanism linking grazer dynamics and algal nutritional status. Incorporation of such effects into a fully dynamic stoichiometric model generates profound changes in the predicted dynamics of algae and grazers, suggesting that adoption of a stoichiometric view may substantively alter our view of the interaction between trophic dynamics and nutrient cycling. The basic predictions of stoichiometric models of nutrient release are generally supported by experimental data showing that N:P release ratios are primarily a function of algal N:P ratio and secondarily a function of grazer N:P ratio, and that rates of P release by grazers are also related to food P:C. Furthermore, evidence for effects of nutrient release stoichiometry on phytoplankton communities and pelagic ecosystem function is accumulating, including data showing consistent alterations in algal physiological status and ecosystem-scale changes in N fixation in response to altered grazer community structure and elemental composition. As the general features of the stoichiometry of algae–zooplankton interactions reflect fundamental biological processes linked to plant and animal mineral nutrition, the stoichiometric view of consumer-driven nutrient recycling can easily be applied to other ecosystems, including terrestrial and benthic food webs. A suite of potential applications of stoichiometric thinking to benthic and terrestrial habitats is suggested.

Limitation of zooplankton Production: Beyond Stoichiometry

Stoichiometry is a commonly adopted concept in studying food-related limitation of zooplankton production in both marine and freshwater systems (Sterner 1990, Hessen 1992, Elser and Hassett 1994, Sterner and Hessen 1994, Hassett et al. 1997). Stoichiometric studies typically place substrates into three major elemental categories: carbon, nitrogen and phosphorus. By employing stoichiometric arguments, some researchers have concluded that production of zooplankton in the ocean is limited by nitrogen whereas freshwater zooplankton are limited by phosphorus (Hessen 1992, Elser and Hassett 1994). Some commonly quoted evidence includes: 1) The C:N or P:N ratio of in situ marine zooplankton biomass is lower than the C:N or P:N ratio of suspended particulate matter (Redfield et al. 1966, Elser and Hassett 1994); thus nitrogen is always in relatively shorter supply than carbon and phosphorus and becomes limiting (sensu Liebig). On the other hand, higher P:N ratio in freshwater zooplankton relative to particulate matter is interpreted as evidence of phosphorus limitation among freshwater zooplankton (Elser and Hassett 1994). 2) In laboratory studies, the gross-growth efficiency (growth/ingestion) in terms of carbon of marine calanoid copepods decreases with an increase in the C:N ratio (range 5-30) of food of monospecific algal diets, but the grossgrowth efficiency in terms of nitrogen remains constant over the same C:N ratio of food (see Fig. 11 in Checkley 1980 and Fig. 3 in Ki0rboe 1989). This has been interpreted as the animals maximizing the uptake of the limiting element for growth: nitrogen (Ki0rboe 1989; similar argument by Sterner and Hessen 1994). Other researchers have applied stoichiometric arguments implicitly by quoting the lower C:N ratio of protozoa, when compared to other diets, as evidence of their better nutritional quality for metazoan zooplankton (Stoecker and Capuzzo 1990, Gifford 1991, Sanders and Wickham 1993). Here we will argue that the concept of stoichiometry, although powerful and useful, is not sufficient for understanding food-related limitation of zooplankton production. We will discuss the confusion created by some assumptions in stoichiometric arguments and how stoichiometric theory can benefit from biochemical and physiological studies. One possible drawback of stoichiometric arguments is that they ignore the biochemical characteristics of specific groups of substrates (see debate among Brett 1993, Hessen 1993 and Urabe and Watanabe 1993). In nature, food substrates do not exist as individual elements, but always as compounds. Therefore, how an element is processed by the consumer is dictated by the biochemistry of the compounds in which that element is present. If we take biochemistry and physiology into consideration, nitrogen limitation in marine zooplankton is not always warranted by simple stoichiometric

Effects of phosphorus‐deficient diets on the carbon and phosphorus balance of Daphnia magna

We used laboratory growth and feeding experiments to study the balance of carbon (C) and phosphorus (P) in Daphnia magna. Daphnia were fed high‐concentration mixtures of P‐sufficient and P‐deficient green algae (Scenedesmus acutus; molar C:P 80 and 900, respectively) or mixtures of P‐deficient Scenedesmus and a P‐rich cyanobacterium (Synechococcus elongatus; C:P = 60). P‐deficient diets resulted in, rapid declines in the growth rates of Daphnia and unexpected declines in Daphnia's P‐to‐dry‐weight ratio. P‐deficient Scenedesmus and P‐rich Synechococcus supported poor growth as sole foods but improved growth in mixtures. A 2‐d experiment with a dietary C:P gradient from 120 to 900 revealed a linear relation between Daphnia's final P‐to‐dry‐weight ratio and its growth rate (r2 = 0.92). Growth in mass (mean ± SE) ranged from 0.54 ± 0.01 to 0.17 ± 0.02 d−1 whereas final specific P content ranged from 1.47 ± 0.01% to 1.08 ± 0.02%.Supporting stoichiometric theory, experiments with dual‐labeled (14C/32P) Scenedesmus showed that C assimilation efficiency declines as the C:P ratio of the diet increases. Adding unlabeled P‐deficient algae reduced the C assimilation efficiency for labeled P‐sufficient algae, whereas adding unlabeled P‐rich algae improved the C assimilation efficiency for labeled P‐deficient algae. C gross growth efficiency (production/ingestion) steadily decreased with declining dietary P. In contrast, P gross growth efficiency exhibited a unimodal pattern with reduced values for both P‐rich and strongly P‐deficient resources. The unexpected decline with P‐deficient diets was apparently due to low but consistent P‐release rates by strongly P‐limited Daphnia. Mass‐balance calculations confirmed that Daphnia was a strong sink for P when resources exhibited intermediate levels of P deficiency. Our results support stoichiometric theory but show that P‐deficient diets lead to significant declines in Daphnia's P content.