The role of crustacean zooplankton in water quality monitoring: Findings from two Portuguese reservoirs

Zooplankton is a key biological element in the aquatic food web. Although currently the European Water Framework Directive (WFD) does not include zooplankton in the biological assessments, the investigation of its potential contribution to water quality is crucial for an integrated evaluation of aquatic ecosystems. For one year, zooplankton communities from Portuguese reservoirs Rabagão and Aguieira, were sampled seasonally and characterized with several metrics and indexes (proportion of Calanoid/Cyclopoid ratios, feeding strategies and Zooplankton Reservoir Trophic Index - ZRTI). This strategy aimed to evaluate the ability of these endpoints to detect differences among trophic status and ecological potential in heavily modified water bodies. The two target reservoirs presented different classifications according to the WFD. Rabagão has good ecological potential, with higher Calanoid/Cyclopoid ratios and a greater proportion of large Cladocera to total Cladocera. On the other hand, Aguieira showed a poor ecological potential, mainly due to high phosphate concentrations, as well as low quality of phytoplankton (e.g., cyanobacteria concentration). In this reservoir, a greater abundance and diversity of zooplankton, was observed, which include highly efficient filter feeders organisms. Additionally, the ZRTI proved to be a sensitive discriminating element regarding the zooplankton community, which vary according to the quality of the two studied reservoirs. Our data, although limited to two reservoirs, indicate that ZRTI can complement the information given by the phytoplankton community in the water reservoir evaluation and, can be included in monitoring programs providing an effective indicator of bottom-up and top-down water quality.