Human induced eutrophication has strongly altered aquatic ecosystems. With increasing eutrophication, plant nutrient concentrations increase, making them more attractive as food for herbivores. However, most aquatic consumers are omnivorous. Ecological stoichiometry theory predicts that animals prefer to consume food which has a similar nutrient (N and P) composition or C:nutrient ratio compared to their own bodies, hence omnivorous animals may prefer to eat animal prey instead of plants. We asked whether aquatic omnivores would shift their diet towards more plant consumption when plants are more nutritious and their stoichiometry becomes more similar to the stoichiometry of the omnivore. We hypothesized that: (1) the omnivore increases plant consumption as the plant C:nutrient ratio decreases when there is only plant material available; (2) the omnivore generally prefers animal food over plant material; (3) the omnivore will increase its relative plant consumption as the plant C:nutrient ratio decreases, in the presence of animal food. As a model system, we used the pond snail Lymnaea stagnalis (omnivorous consumer), the aquatic plant Potamogeton lucens (plant food to the consumer, cultured at different nutrient regimes to obtain different plant C:nutrient ratios), and the crustacean Gammarus pulex (animal food to the consumer, using freshly dead individuals). When there was only plant material available, the consumers increased their relative consumption rate with decreasing plant C:nutrient ratio from no measurable amount to about 102 mg g-1 day-1. When plant material was offered simultaneously with animal food, even though the omnivores always preferred animal food over plant material, the omnivores still increased their relative intake of plant material as plant C:nutrient ratio decreased, from virtually nothing at the highest to on average 16% of their diet at the lowest plant C:nutrient ratio, with a maximum of 28%. Therefore, we conclude that as nutrient loading increases in aquatic ecosystems, plant-eating omnivorous animals may shift their trophic position towards increased plant consumption and alter the food web structure. As a result, we may observe increased top-down control on aquatic plants.
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