Zooplankton abundance: A neglected key element in the evaluation of reservoir water quality

Based on our results, we propose the use of zooplankton abundance (density or biomass) as an indicator to complement the information currently being used concerning the quality of water in reservoirs. Until now, the Water Framework Directive (EU) for lakes and reservoirs has not included zooplankton because the classification of the water trophic state is based on a bottom-up model: an increase in nutrients implies an increase in primary producers and, therefore, poorer water quality. The use of zooplankton has recently been claimed due to their sensitivity to environmental changes and their control over primary producers. From our work, carried out from 2006 to 2009 (summer and winter seasons) in 20 reservoirs found in various Mediterranean river basins, we prove the relationship of the abundance of zooplankton with the trophic state. Zooplankton abundance, with or without interaction with other agents, explained much of the distribution of total phosphorus in the reservoirs, thus relating the trophic status with the aquatic food chain. In addition, we have found, illustrated by the zooplankton: phytoplankton ratio, how the top-down control masked high production situations in the system. Zooplankton's ability to cover up these cases of poor water quality highlight that the indicators presently being used are frequently insufficient.