Ursula Gaedke, Silke Hochstädter, Dietmar Straile

• Due to differences in the biochemical composition of autotrophs and their grazers, food quality can strongly influence herbivore population dynamics. 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 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 analyze the balance between energetic 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 byDue to differences in the biochemical composition of autotrophs and their grazers, food quality can strongly influence herbivore population dynamics. 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 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 analyze the balance between energetic 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 supply of suspension-feeding zooplankton was 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.…