Abstract
The trophic transfer efficiency at the pelagic phytoplankton‐zooplankton interface is—among other factors—governed by the dietary availability of polyunsaturated fatty acids (PUFAs) to herbivorous zooplankton. As most zooplankton taxa cannot synthesize PUFAs de novo, the phytoplankton PUFA composition is crucial for the herbivores' fitness. As PUFA synthesis is taxon‐specific in phytoplankton, alterations in phytoplankton biodiversity lead to changes in the phytoplankton trait “nutritional quality” and can thus be expected to affect the relative fitness and intraspecific competition of herbivore populations. To study this potentially important link between producer diversity and consumer competitive interactions, we performed a freshwater mesocosm experiment with diversity‐manipulated natural phytoplankton communities and naturally coexisting Daphnia longispina genotypes. We found that phytoplankton diversity is correlated with its PUFA composition, suggesting that a biodiversity loss on the primary producer level leads to decreased PUFA diversity and ultimately reduced dietary quality for consumers. This change in dietary PUFA availability further affected competitive interactions between co‐occurring D. longispina genotypes, which was most evident for the ω3‐PUFA α‐linolenic acid (18:3 n‐3) and the ω6‐PUFA arachidonic acid (20:4 n‐6). Interestingly, these two PUFAs had opposite effects on competition by favoring different genotypes. Our study thus demonstrates that phytoplankton PUFA content is a trait that affects consumers' functional traits and therefore represents a potential link between biodiversity and competition in aquatic herbivores.