Riverscape heterogeneity explains spatial variation in zooplankton functional evenness and biomass in a large river ecosystem

Ecologists have long focused on local-scale phenomena (i.e. local environment variables) and assumed that spatial processes were unimportant factors influencing both the community structure and the functional diversity of aquatic communities. In this paper we used zooplankton assemblages in a typical large river (St. Lawrence River) as a biological model to examine the roles of (1) local environmental conditions (physicochemical characteristics of the water column), (2) broad-scale connectivity (a proxy for dispersion potential), and (3) habitat heterogeneity (a proxy for niche diversity) on the structure and the diversity of lotic communities. Together, these three sets of descriptors explained respectively 52, 49 and 59 % of the variation in zooplankton total biomass, functional diversity and community structure. After partialling out the roles of local environmental conditions and broad-scale connectivity, we demonstrated that habitat heterogeneity alone is a key driver of zooplankton total biomass and functional evenness at the riverscape level. In homogeneous and temporally stable habitats, zooplankton communities had higher biomass and functional evenness but lower species richness. Conversely, zooplankton had lower biomass and higher species richness in heterogeneous and unstable habitats, suggesting that zooplankton species can coexist because disturbances prevent competitive exclusion from occurring. This is the first study to reveal how local environmental conditions, spatial connectivity and habitat heterogeneity operate jointly to determine the functional diversity and structure of aquatic communities in a natural ecosystem.