Rethinking aquatic bioindicators: testate amoebae versus metazoans in plankton samples for assessing anthropogenic impacts on freshwater ecosystems

Freshwater ecosystems are globally threatened, necessitating effective biomonitoring. Planktonic metazoans are key bioindicators; however, the potential of protists within the zooplankton community remains largely overlooked. We assessed the relative contribution of testate amoebae to net-zooplankton richness and density across a hydrodynamic gradient, comparing 34 lentic environments (reservoirs) with 44 lotic environments (from first- to sixth-order rivers) from a large geographic scale in Brazil. We tested the hypotheses that (i) Metazoans (rotifers, cladocerans, and copepods) would dominate in lentic systems and (ii) the contribution of testate amoebae would be greater in lotic systems, showing an inverse relationship with river order due to the exchange of fauna between compartments. The first hypothesis was confirmed, with conventional zooplankton increasing in reservoirs. The second was partially supported, while testate amoebae richness and density peaked in intermediate-order rivers, their relative contribution to the entire community was highest in lower order streams (exceeding 90%) and decreased to a maximum of 20% in larger rivers and reservoirs. Our study demonstrated that lotic ecosystems show a remarkable relative contribution of testate amoebae in plankton samples, compared with other zooplanktonic groups. Our findings emphasize the importance of incorporating protists, at least testate amoebae, into ecological assessment and river monitoring, conservation, and restoration programs. Further research is needed to elucidate how testate amoebae (and other protists) respond to different anthropogenic stressors, ideally through functional approaches and multicommunity analyses.