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Threshold food N : C or P : C ratios, below which net production is N or P limited, were calculated for two cladocerans, Daphnia galeata and Bosmina longirostris, based on the N and P contents of body tissue and the C balance between ingestion and net production rates under experimental conditions with various food concentrations from 0.05 to 2.50 mg C liter-'. With changing food concentration, the threshold N: C and P: C ratios for D. galeata changed in weight ratio from 0.039 to 0.077 and 0.0067 to 0.0120, respectively, with the lowest value at the high food concentration. Changes in threshold ratios for B. longirostris, however, were small and mean values were 0.085 and 0.0080 for the N: C and P: C ratios. As a result, the threshold N: C ratio for D. galeata was lower than for B. longirostris at high food concentrations, while the P : C ratio for B. longirostris was lower than for D. galeata at lower food concentrations. Although the threshold elemental ratios calculated here are theoretically minimum estimates, they were higher than the lower end of the sestonic elemental ratios in some lakes, suggesting that it is not unusual for these cladocerans to be N or P limited at certain periods of the year. Sestonic elemental ratios may have potential importance in determining the community structure of zooplankton.

The classical Redfield ratio of carbon 106 : nitrogen 16 : phosphorus 1 is a cornerstone of biogeochemistry. With the use of .2,000 observations of the chemistry of particulate matter from small and large lakes, as well as nearand off-shore marine environments, we found that the best model to describe seston stoichiometry depended on the scale of analysis. We also found that there were better estimates for seston chemistry than the classical ratio for all habitats, whether freshwater or marine. Across the entire data set, a constant proportionality of C 166 : N 20 : P 1 (6error) described the data, which implies higher C sequestration per unit of N and P in surface waters than given in the classical ratio. At a regional scale, however, C : P and C : N often declined with increasing seston abundance, rejecting a constant ratio model. Within both freshwater and marine habitats, higher seston abundance is often associated with lower C : P and C : N ratios (higher nutrient content). The difference in appropriateness of the constant ratio model with respect to the entire data compared with subsets of the data indicates a scale dependence in stoichiometric relationships in seston C : N : P ratios. Given these consistent shifts in seston chemistry with particle abundance, the narrower variation in seston chemistry associated with marine seston chemistry could occur because of a reduced range of particulate nutrient concentration. For all but the largest scales, the classical Redfield model of biogeochemical cycling should be replaced with a more general power function model.

Based on the observation that organism‐specific elemental content creates ecologically relevant mismatches such as between plant and animal tissue, it was postulated—and experimentally verified—that this would profoundly affect trophic efficiency and nutrient fluxes in ecosystems. From its beginnings as a Daphnia‐centered perspective, the field of ecological stoichiometry (ES) has widened to include many organism groups, and ecosystem types, and the questions it addresses have broadened. We address some of the development of ES in aquatic sciences especially over the past 10 yr, focusing on homeostasis and mass balance in the consumer, and its effect on trophic efficiency and nutrient recycling in aquatic communities. We also discuss how ES has provided novel insights into genomic, proteomic, and cellular responses at one end of the biological scale as well as into large‐scale effects related to biogeochemical couplings at the ecosystem level. The coupling of global C, N, and P cycles via their biotic interactions and their responses to climate change accentuate ES as an important toolkit for ecosystem analysis. We also point to some of the major topics and principles where ES has provided new insights. For each of these topics we also point to some novel directions where the ES concepts likely will be useful in understanding and predicting biological responses.

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.

The possibility of P limitation for zooplankton growth has many implications for understanding changes in production efficiency and feedback dynamics between consumers and resoutxes. However, there have been no direct tests to determine whether the putative P limitation is real. To answer this question, we directly supplied inorganic P to Daphnia magna apart from food algae, Scenedesmus acutus, and then evamined changes in body mass. During the period from birth to age 6 d, D. magna were fed on live algae For 19 h and placed in water of high inorganic P (4 mM: P treatment) for 5 h each day. We used P-free water as a control treatment. Growth rate estimated from initial and final body mass during the 6-d incubation was significantly larger in the P treatment than in the control treatment when Daphnia fed on P-deficient algae, whereas a significant dij'ference was not detected between the treatments for Daphnia fed on P-sufficient algae (C : P atomic < 300). The results clearly demonstrate that Daphnia growth is in fact limited by P itself when they feed on P-deficient algae.Recently, controversy has sparked over the possibility of direct P limitation for zooplankton production. Several studies have demonstrated lower individual and population growth rates of Daphnia when they feed on algae with low P content relative to carbon, i.e. high C : P ratios (Sommer 1992;Sterner 1993;Sterner et al. 1993). Using mass-balance models, Olsen et al. (1986) and Urabe and Watanabe (1992) estimated the threshold food C : P ratio, above which net production of cladocerans is limited by P content rather than C in the food. In both these studies, the threshold was calculated to be -300 (atomic ratio) for Daphnia, although the ratio changes according to food concentration and digestibility of carbon in the food (Hessen 1992; Urabe and Watanabe 1992) and because of changes in the proportion of metabolic cost (respiration) within assimilated carbon (Sterner and Robinson 1994;Sterner 1997). Seston has C : P ratios >300 in many north temperate lakes (Hecky et al. 1993; Elser and Hassett 1994;Sterner et al. 1997), implying that P limitation of Daphnia growth may be common.Direct P limitation of zooplankton growth, however, has been called into question (Brett 1993; Mtiller-Navarra 1995a, b) in part because of the lack of direct evidence showing whether P is in fact the actual substance limiting the growth rate of Daphnia. Although direct demonstrations of the precise limiting factors for zooplankton feeding on low-quality foods have been lacking, several studies suggest that the growth rate of Daphnia is affected by the availability of individual long-chain unsaturated fatty acids such as EPA (eicosapentaenoic acid, 20 : 503) and DHA (docosahexaenoic acid, 22 : 603) (Ahlgren et al. 1989; Miiller-Navarra 1995b). Fatty acids such as these are essential substances for

Experiments using planktonic organisms revealed that the balance of radiant energy and available nutrients regulated herbivore growth rates through their effects on abundance and chemical composition of primary producers. Both algae and herbivores were energy limited at low light/nutrient ratios, but both were nutrient limited at high light/nutrient ratios. Herbivore growth increased with increasing light intensity at low values of the light/nutrient ratio due to increases in algal biomass, but growth decreased with increasing light at a high light/nutrient ratio due to decreases in algal quality. Herbivore production therefore was maximal at intermediate levels of the light/nutrient ratio. The results contribute to an understanding of mass transfer mechanisms in ecosystems and illustrate the importance of integration of energy-based and material-based currencies in ecology.Both light and nutrients are essential in sustaining ecosystems, but very little is known about how relative changes in these abiotic factors extend into food chains (1, 2). Plants use solar radiation to fix carbon while they acquire nutrients at appropriate rates to maintain their biological integrity. However, photosynthesis and nutrient uptake are not perfectly coupled, and thus the contents of bioelements relative to carbon (C) in plant biomass vary within species (3-6). Because foraging and growth of many herbivore species respond to the chemical composition of their diet (7-10), the balance between photosynthesis and nutrient uptake may in turn regulate herbivores through the interplay of food quantity and quality. In this report, we test the hypothesis that herbivore growth is dependent on the light/nutrient balance supplied to laboratory ecosystems.We focused on phosphorus (P) as a limiting nutrient because algal growth is frequently limited by P in freshwater systems (11,12), and because the algal P/C ratio has been most strongly implicated in regulating planktonic herbivores (13)(14)(15)(16)(17). We first considered the likely responses of algae to light intensity for a given P supply with a moderate but constant loss rate (Fig. 1A). Here, we expressed the response of algae by a rectilinear form to show the essence of trends and qualitative differences along the light gradient. Precise response to light and nutrients would depend on the identity of the algal species and other environmental factors. At low light, algal growth should be limited by irradiance such that algal biomass increases with light intensity. At high light, algal growth should be limited by finite P and algal biomass should reach a plateau. At extremely high light, algal growth may decrease due to photoinhibition, but Fig. 1A assumes light is below the photoinhibition point. The response of algal P/C ratio is also shown in Fig. 1A. At low light, the algal P/C ratio is expected to be high, close to the Redfield ratio (0.0094 by atoms), because algal growth is limited by irradiance alone and thus P supply is sufficient relative to algal biomass. Because the ...

Laboratory studies are increasingly indicating that the quality of nutrientlimited algae is suboptimal for zooplankton production. However, little is known about how quality is affected by nutrient limitation of phytoplankton in more natural situations. To test for phosphorus (P) limitation of zooplankton growth under realistic food conditions, we performed a set of 5-d experiments using Daphnia dentifera and suspended particulate matter (seston) from three lakes at the Experimental Lakes Area (Ontario, Canada). Neonate Daphnia fed for 6 h per day on freshly collected seston enriched or unenriched with PO 4 and spent the rest of the day feeding on unaltered natural seston. PO 4 enrichment did not affect food abundance or concentrations and composition of essential fatty acids but dramatically lowered seston C:P ratio and significantly stimulated Daphnia growth. These results demonstrate that, even with field-collected seston, the effects of algal phosphorus limitation can extend to herbivores through reduced food quality.