Zooplankton abundance and composition is sensitive to eutrophication, and its top-down effect in the planktonic food web can alter the classification of the trophic state of waterbodies based on the microscopic primary producers. However, this relevant group in aquatic ecosystems is overlooked when lakes and reservoirs are monitored to describe and improve water quality as required by the European Water Framework Directive (WFD). Fortunately, we have data on monitored zooplankton and the physical, chemical, and biological variables prescribed by the WFD in 20 Mediterranean reservoirs over a 4-year period (summer and winter sampled). We related zooplankton composition (90 species) with these limnological descriptors and found that zooplankton species assemblages are mainly driven by trophic gradients. Different compositions were found within the eutrophic range, suggesting alternative states when systems are more productive, but no composition was typical of oligotrophy. Based on these results, 10 species were selected based on their significant indicator value for trophic state; they are good indicators of mesotrophic, eutrophic, and hypertrophic states. Our results support the use of zooplankton as a metric to establish water quality and provide concrete data of the indicator value of some species in reservoirs in the Mediterranean basin.
