Temporal and spatial patterns of zooplankton in the Chesapeake Bay turbidity maximum

We measured the distribution of hydrographic parameters, currents, phytopla nkton fluorescence, suspended sediments and zooplankton in axial transects through the Chesapeake Bay estuarine turbidity maximum (ETM) seasonally (Ma y, July and October 1996) and over tidal cycles within seasons. Zooplankton abundance was estimated with a g-frequency, Tracer Acoustical Profiling Sy stem (TAPS-6) at the same vertical (0.25 to 0.50 m) and horizontal (0.5 to 1.5 km) resolution as hydrographic parameters and suspended sediments. The general pattern exhibited in axial transects through the Chesapeake Bay ETM is that sediments, fluorescence and zooplankton are in higher concentratio ns up-Bay of the salt wedge (defined as the intersection of the 1 isohaline with the bottom). The salinity front appears to trap these particles in th e upper portion of Chesapeake Bay. The highest acoustically determined zoop lankton biomass generally occurred near the bottom, at the toe of the salt wedge. The convergence zone associated with this feature concentrates sedim ents and zooplankton (primarily the copepod Eurytemora affinis). Advection appeared to dominate changes in zooplankton abundance during time series st udies at a fixed station in the ETM. Zooplankton biomass at the fixed ETM s tation increased/decreased with the tidal excursion of the salt wedge. Wate r column zooplankton concentrations and the Vertical distribution of zoopla nkton biomass appeared to be influenced by currents. We often found that du ring maximum ebb and flood tidal currents, zooplankton biomass and sediment s in the mid and upper water column increased. Thus the hydrodynamic proces ses that resuspend, advect and trap suspended sediments in the ETM likely h ave the same effects on zooplankton. The ETM of the Chesapeake Bay appears to act as an entrapment zone for zooplankton. The lack of diel vertical mig ration, carrying eggs until they are ready to hatch, possible reduced preda tion by visual predators in the turbid waters, and the ability to consume p hytoplankton, protozoa and detritus all may allow Eurytemora to persist at high concentrations in the Chesapeake Bay ETM.