The late summer crustacean zooplankton in western U.S.A reservoirs reflects ecoregion, temperature and latitude

Abstract

Summary We tested whether the crustacean zooplankton species composition and size structure (which are important determinants of energy flow in plankton food webs) during late summer in reservoirs would be related to ecoregional designations, catchment land use, temperature and latitude. We also examined whether the predictions of decreasing cladoceran body size with decreasing latitude (as observed for natural lakes) would be valid for reservoirs. Zooplankton samples were collected in late summer 2010 from 102 western U.S. reservoirs distributed over three major ecoregions from 32.5 to 48.6°N latitude. Large‐bodied cladocerans (Daphnia pulex complex) and cyclopoid copepods (Diacyclops thomasi) were found in deeper, cooler reservoirs with forested catchments (Northwestern Forested Mountains). Small‐bodied cladocerans (Daphnia retrocurva, Diaphanosoma spp. and Chydorus sphaericus) and cyclopoid copepods (Tropocyclops prasinus) were more common in productive reservoirs in catchments influenced by agriculture (Great Plains). Small‐bodied cladocerans (Bosmina longirostris) dominated the otherwise modest crustacean zooplankton in reservoirs located in catchments dominated by evergreen shrub vegetation with limited forestation (Western Xeric). Calanoid copepods also displayed clear distribution by ecoregion. Increasing water temperature and decreasing latitude were associated with smaller mean Daphnia and cladoceran body lengths and lower absolute biomass of daphniids. Daphniids were rare or absent in warm waters (i.e. below 36o N latitude and/or from sites located <500 m altitude). Species composition and mean body size of the cladoceran zooplankton were consistent with the feeding habits of the common planktivorous fish – salmon (Oncorhynchus spp.) and lake trout (Salvelinus spp.) in the Northwestern Forested Mountains, gizzard shad (Dorosoma cepedianum) in the Great Plains and threadfin shad (Dorosoma petenense) in the Western Xeric ecoregion. Our study indicates that the taxonomic composition and size structure of the cladoceran zooplankton in reservoirs are highly connected to ecoregional attributes such as catchment land use, temperature and latitude as has been observed for natural lakes.