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Changing Arctic climate may alter freshwater ecosystems as a result of warmer surface waters, longer open-water periods, reduced wintertime lake ice growth, and altered hydrologic connectivity. This study aims to characterize zooplankton community composition and size structure in the context of hydrologic connectivity and ice regimes in Arctic lakes. Between 2011 and 2016, we sampled the phytoplankton, zooplankton, and fish communities from a set of representative lakes on the Arctic Coastal Plain (ACP) of northern Alaska to determine potential food web responses to changing Arctic ecosystems. Multivariate analyses showed that time from ice-out had a strong influence on zooplankton community structure and that seasonal succession of zooplankton differed between lakes with varying hydrologic connectivity. Trends were observed suggesting that large-bodied zooplankton (Daphnia, calanoid copepods) may be more prevalent in poorly connected lakes with low fish diversity. Large-bodied zooplankton displayed higher biomass in lakes with high occurrences of bedfast ice, while small-bodied zooplankton (Bosmina, rotifers) displayed highest biomass in deeper lakes with low occurrences of bedfast ice. Our results contribute to limited knowledge of zooplankton in remote lakes of the ACP and suggest that the anticipated changes to aquatic ecosystems in the Arctic may include energetically less efficient plankton food webs.

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.

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