Continuous cultures of the green algae Selenastrum capricornutum were grown at different concentrations of total phosphorus (1-50 mol P/L) and different light levels (10-200 mol quanta·m Ϫ2 ·s Ϫ1 ). Growth yields in terms of C, N, and P were positively correlated to total P level. Total biomasses in terms of C and N, but not P, were also positively correlated to light level. The cell quotas of C were positively correlated with light, whereas cell quotas of P were negatively correlated with light. The resulting elemental ratios gave a quite consistent pattern, where high light caused significant reductions in both N:C and P:C ratios, as well as strong reductions in chlorophyll to carbon ratios. The increase in P:C ratio with increasing total P and decreasing light can be interpreted as an adaptive response to self-shading. In that case, our results indicate that light adaptation not only involves a cost in terms of increased chlorophyll synthesis, but also in terms of increased P demands. This provides new insight not only into the physiological regulation of C and P uptake in algae, but it could also explain deviations from the Redfield ratio.The growth of juvenile Daphnia magna fed S. capricornutum from the different light and phosphorus treatments was studied in a series of short-term assays (7 d) covering a gradient of food concentrations. The response of Daphnia growth rate along this quantity (0.5-5.0 mg C/L) and quality (0.5-12 g atomic P·[mg atomic C] Ϫ1 ) gradient gave a close fit to a double hyperbola model. Changes in elemental ratios of the algae were reflected in the growth rate of Daphnia, such that up to 40% reduction in its growth rate could be attributed to increased C:P ratios. This study demonstrates that the physiological responses of phototrophs in terms of chlorophyll content and elemental composition depend strongly on ambient light and nutrient regimes. It also confirms that these patterns can yield contrasting responses on herbivore growth responses along the food quantity and quality axes.

Animals that maintain near homeostatic elemental ratios may get rid of excess ingested elements from their food in different ways. C regulation was studied in juveniles of Daphnia magna feeding on two Selenastrum capricornutum cultures contrasting in P content (400 and 80 C:P atomic ratios). Both cultures were labelled with 14 C in order to measure Daphnia ingestion and assimilation rates. No significant difference in ingestion rates was observed between P-low and P-rich food, whereas the net assimilation of 14 C was higher in the treatment with P-rich algae. Some Daphnia were also homogeneously labelled over 5 days on radioactive algae to estimate respiration rates and excretion rates of dissolved organic C (DOC). The respiration rate for Daphnia fed with high C:P algae (38.7% of body C day -1 ) was significantly higher than for those feeding on low C:P algae (25.3% of body C day -1 ). The DOC excretion rate was also higher when animals were fed on P-low algae (13.4% of body C day -1 ) than on P-rich algae (5.7% of body C day -1 ) . When corrected for respiratory losses, total assimilation of C did not differ significantly between treatments (around 60% of body C day -1 ). Judging from these experiments, D. magna can maintain its stoichiometric balance when feeding on unbalanced diets (high C:P) primarily by disposing of excess dietary C via respiration and excretion of DOC.

Summary 1. Elemental composition (carbon : nitrogen : phosphorus, C : N : P) was analysed in eggs and juveniles of two crustaceans, Daphnia magna (Cladocera) and the crayfish Astacus astacus (Decapoda). Stoichiometry was also analysed for the carapace, muscle tissue, hepatopanchreas and gills of Astacus. 2. For both species the C : P ratio was significantly higher in eggs than juveniles, but there was a constant, homeostatic elemental ratio in eggs during embryogenesis (Astacus) and with different C : N : P in maternal food (Daphnia). 3. Differences in the stoichiometry of major tissue categories in Astacus suggest that there are distinct allocation strategies of elements to various somatic tissues as well as to reproduction versus somatic tissues overall. 4. There are strong ontogenetic shifts in the allocation of energy and elements in both species, as for crustaceans in general. During maturity there may be a trade‐off with regard to the allocation of C, N or P to somatic or reproductive tissue, and poor food quality (high C : P in food) could pose other constraints on reproductive capacity than does food shortage (low C). 5. Egg production may be at least as sensitive to low P as is somatic growth and could result in a marked decrease in overall population growth rate more severe than would be expected from individual growth rate alone.

A dramatic increase in the breeding population of geese has occurred over the past few decades at Svalbard. This may strongly impact the fragile ecosystems of the Arctic tundra because many of the ultra-oligotrophic freshwater systems experience enrichment from goose feces. We surveyed 21 shallow tundra ponds along a gradient of nutrient enrichment based on exposure to geese. Concentrations of total phosphorus (P) and dissolved inorganic nitrogen (DIN) in the tundra ponds ranged from 2-76 to 2-23 g l ¡1 respectively, yet there was no signiWcant increase in phytoplankton biomass (measured as chlorophyll a; range: 0.6-7.3 g l ¡1 ) along the nutrient gradient. This lack of response may be the result of the trophic structure of these ecosystems, which consists of only a two-trophic level food chain with high biomasses of the eYcient zooplankton grazer Daphnia in the absence of Wsh and scarcity of invertebrate predators. Our results indicate that this may cause a highly eYcient grazing control of phytoplankton in all ponds, supported by the fact that large fractions of the nutrient pools were bound in zooplankton biomass. The median percentage of Daphnia-N and Daphnia-P content to particulate (sestonic) N and P was 338 and 3009%, respectively, which is extremely high compared to temperate lakes. Our data suggest that Daphnia in shallow arctic ponds is heavily subsidized by major inputs of energy from other food sources (bacteria, benthic bioWlm), which may be crucial to the persistence of strong top-down control of pelagic algae by Daphnia.

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1. The effect of ultraviolet (UV) radiation (280-400 nm) on fatty acid composition and elemental ratios of carbon (C), nitrogen (N) and phosphorus (P) of a unialgal culture of the chlorophyte Selenastrum capricornutum was investigated. Algae were cultured in the presence or absence of UV radiation and were subsequently fed to Daphnia magna to assess potential effects of UV on zooplankton fatty acid composition, growth and reproduction. 2. Algal growth rate was substantially reduced by UV radiation, probably because of a severe inhibition of photosynthetic efficiency (measured as optimal quantum yield). 3. Algae exposed to UV radiation had a significantly reduced content of 18 : 1 n-9, while C18 polyunsaturated fatty acids (PUFAs) were higher under UV radiation. These observations point at an increased demand for and synthesis of PUFAs under UV stress. 4. The C : P and N : P ratios showed a remarkable decrease in UV-exposed cells primarily owing to an increased uptake of P. The nutritional quality in terms of both fatty acid composition and stoichiometry was therefore higher in the UV treatment relative to the control. 5. Despite the UV-induced changes in nutritional quality of S. capricornutum, no significant effects on D. magna growth or reproduction were detected. The fatty acid composition of Daphnia fed on UV irradiated algae showed a significantly lower content of 18 : 1 n-9, but no changes in the essential PUFAs.

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