Zooplankton Community in Delaware Bay During Summer: Spatial Distribution and Environmental Correlates

Zooplankton are an extremely diverse assemblage of organisms and are important trophic intermediates in estuaries. Yet, zooplankton community structure in Delaware Bay has been poorly documented over the past 65 years, despite marked changes in land use, nutrient inputs, and water quality. We report on the summer distribution of zooplankton and environmental parameters sampled at 16 stations throughout Delaware Bay in June 2015. Zooplankton were sampled by double oblique bongo net tows. Both nets were size-fractionated, and one net processed for species composition and abundance semi-autonomously using a ZooScan system, and the other net processed for dry weight. Environmental parameters were captured as water column profiles from casts of a YSI 6600V2 sonde and HydroRad hyperspectral radiometer at each station. Trends in the environmental variables (temperature, salinity, dissolved oxygen, pH, turbidity, chlorophyll a, and K_dPAR) were indicative of a well-mixed estuary post spring bloom. The bay-wide pattern of zooplankton diversity showed an increase moving from oligohaline water near Wilmington, DE to polyhaline water outside of the bay mouth. Zooplankton community composition was dominated by 5 taxa (copepods - Acartia tonsa, Centropages spp., Eurytemora spp., copepodites; and marine cladocerans), each of which contributed at least 30% of the total relative abundance in at least one sampling station. Using redundancy analysis (RDA), Acartia was positively correlated with temperature, dissolved oxygen, and chlorophyll a; Centropages spp. and cladocerans were positively correlated with salinity; while Eurytemora spp., and copepodites were positively correlated with turbidity and K_dPAR. Summer environmental conditions throughout Delaware Bay likely influence zooplankton community composition and provide a means for predicting zooplankton dynamics in this estuarine system.