LAUSR

Abstract Phytoplankton community assessment using functional groups is a powerful tool for lake and reservoir biomonitoring, but its efficiency has only been tested in intersite, low-frequency datasets. Here, we took advantage of a gradient of spatially close, but environmentally contrasting, meso- to hypereutrophic reservoirs in Southeast Brazil, to compare the efficiency of morphology-based functional classification (MBFC), phylum-level, and species-level classification for high-frequency, i.e. monthly, biomonitoring. Monitoring of water physical and chemical characteristics revealed the presence of two independent environmental gradients, with the first being related to total phosphorus and suspended solids concentrations, as well as Secchi depth, and the second related to dissolved inorganic nitrogen, water temperature and pH. The species approach had the highest power to explain environmental variation in assemblage structure (21% vs 16% and 17%, in the phylum and MBFC approaches). Although both simplified classifications showed similar explanatory power, MBFC allowed a better differentiation of reservoir assemblages. As the MBFC approach is also the least resource intensive, it may be preferable for routine biomonitoring, or to process large sample numbers. Nonetheless, species-level classification provided the most detailed information on assemblage dynamics and is therefore an important option for scientific studies.