Interplay between energy, mineral and biochemical constraints of herbivores in the pelagic food web of Lake Constance

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Understanding trophic interactions and food web regulation and their response to anthropogenetically altered nutrient and climate conditions demands information on the food quantity and quality. Stoichiometric theory suggests that the ratio between carbon (C, surrogate for energy) and essential nutrients, e.g. phosphorus (P), determines the food quality of primary producers for consumers. As recent studies showed that a shortage of essential fatty acids can also reduce food quality, we intend to extend the existing stoichiometric theory by additionally considering potential growth limitations by polyunsaturated fatty acids (PUFAs). For this purpose already available comprehensive long-term data on plankton abundance, production, and C and P fluxes of the pelagic food web of Lake Constance shall be combined with new laboratory measurements of species-specific, nutrient- and temperature-dependent fatty acid contents of major algal species, and with field measurements of plankton abundance and size-fractionated C, P and fatty acid contents in Lake Constance. This enables to estimate the contribution of different functional groups to the fluxes of energy, minerals and fatty acids throughout the food web, and to develop hypotheses on the interplay of consumer growth limitations by energy, nutrients and/or distinct PUFAs, which shall then be explicitly tested experimentally. This improves our ability to understand the response of individual plankton groups and the entire food web to the past decades of re-oligotrophication, and to predict future reactions to altered nutrient and climate conditions.