Integrative Foraging in Complex Landscapes: Nutrient acquisition and mixing across food patches at the primary producer-Daphnia interface

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Consumers such as the keystone aquatic herbivore Daphnia, continuously experience spatial and temporal variability in their food availability and quality. Depending on the observational scale, this variability is driven by various factors such as successions in algal species, variation of abiotic factors (light, nutrients) and/or to movements between food patches of different quality. As a result, the uptake of limiting nutrients by Daphnia is temporally fluctuating and in the case of co-limiting nutrients the uptake might even be decoupled. In such complex nutritional environments, it is difficult to predict Daphnia responses only based on lab nutrition studies were food quality is kept constant and on punctual (and usually very distant) estimates of seston nutritional quality. Predicting Daphnia responses, requires understanding (a) how the resources fluctuate at temporal scales which are relevant to Daphnia, and (b) how Daphnia integrate such spatio-temporally heterogeneous resources into long-term fitness i.e. how nutrients acquired in different patches are be taken up, stored, mixed and used for growth and reproduction. This project aims to characterise the natural variability of the nutritional quality (in terms of Phosphorus, essential fatty acids and sterol contents) of seston at the poorly studied, but relevant to Daphnia, hourly and daily scale. Additionally, through a series of life table experiments, we aim to understand how fluctuation and decoupling in the uptake of co-limiting nutrients affect Daphnia growth rates, fecundity and nutrient assimilation efficiency. Finally, this project also aims to increase the very limited knowledge on Daphnia digestive enzyme physiology, which might potentially be their most important tool for optimizing the acquisition of various nutrients from a continuously fluctuating diet. We expect that outcomes of the project will improve our understanding of the scale at which temporal and/or spatial heterogeneity in resource quality matters for zooplankton. Moreover knowledge will be gained on how such heterogeneity should be averaged to obtain estimates of the overall habitat quality for zooplankton which is essential for accurate model building.