Environmental Filtering Links Zooplankton Community Assembly and Resource‐Use Efficiency in the River Danube

Terrestrial studies and pelagic studies in lakes generally report positive biodiversity–ecosystem functioning relationships, where more diverse communities perform better. In large rivers, strong environmental constraints constantly filter plankton communities, promoting species sorting and the dominance of well‐adapted taxa that may alter the mechanisms linking biodiversity to ecosystem functioning. Here, we evaluated the link between environmental filtering and zooplankton community composition, and then tested whether ecosystem functioning is primarily associated with complementarity or selection effects associated with dominant species in the middle section of the River Danube.

We quantified resource use efficiency as an ecosystem functioning measure using the ratio of zooplankton to phytoplankton carbon biomass, alongside taxonomic (species richness and evenness) and functional (functional richness and evenness) diversity measures as predictors. Our analysis relied on a multi‐year high‐frequency dataset, covering a long environmental gradient across contrasting hydrological years.

Our results demonstrated that local environmental variables, linked primarily to discharge, were associated with constrained zooplankton community composition based on both taxonomic and functional data. Assessing diversity measures individually, resource use efficiency scaled positively with zooplankton taxonomic and functional richness; meanwhile, evenness only mattered in Rotifera. Concurrently, a negative relationship with taxonomic evenness revealed that pronounced community dominance, associated with biomass peaks of a few highly competitive opportunistic taxa, maximised community‐level resource use via mass ratio effect. Considering all diversity measures jointly, the best‐performing models combined trait richness and taxonomic evenness, providing a better explanation of ecosystem functioning than either predictor alone. Taxonomic and functional trait richness were almost perfectly correlated, explaining why functional richness retained explanatory power whereas traditional species richness did not in combined models.

Since dominance‐driven effects can only act on the available species pool, we argue that high functional richness helps maintain complementary ecological strategies that maximise resource use under varying environmental conditions, while environmental filtering promotes the dominance of the highly competitive opportunistic taxa, that is, Rotifera in large rivers.

Our findings indicate that community dominance does not replace biodiversity effects; rather, environmental filtering operates upon a functionally diverse species pool, making functional richness and community dominance complementary qualities influencing ecosystem functioning in large river zooplankton.