Spatial and temporal variability in factors affecting mesozooplankton dynamics in Chesapeake Bay: Evidence from biomass size spectra

Zooplankton biomass in Chesapeake Bay was estimated with an optical plankton counter mounted on a towed body (Scanfish). Normalized zooplankton biomass size spectra were calculated for three Bay regions three times a year across 4 yr. Zooplankton biomass was maximum during April, and the zooplankton size at peak biomass was large compared with July and October. The variability of the normalized zooplankton biomass size spectrum in April was related to freshwater input, a proxy for nutrient loading, lower temperatures, and salinities. The normalized zooplankton biomass size spectrum showed little interannual variability in July, and the curvature of the biomass size spectrum was reduced. The lack of variability in July normalized zooplankton biomass size spectra was related to gelatinous zooplankton and fish predators. Normalized zooplankton biomass size spectra in October were similar to April and had the lowest total zooplankton biomass. October normalized zooplankton biomass size spectra were affected by gelatinous predators in the upper Chesapeake Bay and by fish predators in the middle to lower Chesapeake Bay. Food limitation did not appear to affect normalized zooplankton biomass size spectra because measured particulate carbon concentrations were always in excess of estimated maintenance food concentrations. The ratio of phytoplankton biomass to zooplankton biomass was higher than in other aquatic systems and was consistent across the year. The zooplankton biomass–to–fish biomass ratio varied seasonally, with April samples having the highest ratios and October the lowest. The normalized zooplankton biomass size spectra in Chesapeake Bay have more negative, and a wider range of, linear regression slopes than other aquatic systems. Normalized zooplankton biomass size spectra in Chesapeake Bay were influenced by climatologically driven variation in the densities of predators and prey. The variability of the normalized zooplankton biomass size spectrum was indicative of a highly eutrophic ecosystem. The biomass size spectrum has been used to assess the biological structure of pelagic ecosystems (Sheldon et al. 1972; Kerr and Dickie 2001). The biomass structure of the North Atlantic Ocean pelagic community was found to be flat or have a slope of zero; thus, biomass was evenly distributed from ‘‘bacteria to whales’’ (Sheldon et al. 1972). The slope estimate was later refined by normalizing the bio1 Corresponding author ([email protected]). 2 Present address: NOAA, Cooperative Oxford Laboratory, 904 South Morris Street, Oxford, Maryland 21654.