Zooplankton shifts from headwater to lowland streams: Insights into the role of water quality to assist the protection and restoration of agricultural waterways

Despite the critical importance of zooplankton for many ecological processes, knowledge of its structure and distribution patterns in small streams subject to intensive land uses remains limited. Here we analysed changes in zooplankton community and composition in 10 sites located in streams from first-order to sixteenth-order, according to Shreve's classification, in a small catchment influenced by intensive agriculture. Using univariate and multivariate methods, we investigated the extent to which local scale environmental factors explained the spatial patterns of zooplankton community. We found that environmental conditions of headwaters differed markedly from those of lowland streams, especially with respect to nutrient levels, which were higher in the former than the latter. Zooplankton community was mainly composed by microphagous rotifers, and the overall species composition differed with the stream order. While zooplankton diversity and evenness increased with stream order and correlated negatively with nutrient level, zooplankton abundance and biomass showed the opposite trend. The mechanistic processes creating these patterns remain unknown but are likely linked to the effects of greater water mixing and dilution in lowland streams, as well as ecological processes such as predation and resource diversification. No clear pattern was found for presence or concentrations of pesticides in relation to the stream order. However, water quality seemed to be an important factor influencing zooplankton assemblages, which were significantly associated with pesticides and trophic state variables. Our findings suggest that agricultural pollution is impacting the zooplankton community, which may have implications for the food webs and overall health of the stream systems.