Vertical Distributions of Zooplankton at the Mouth of Chesapeake Bay and Calibration of Backscatter From an Acoustic Doppler Current Profiler

This study investigated the physical factors influencing the vertical distribution of zooplankton at the mouth of the Chesapeake Bay and attempted to derive biological data from acoustic Doppler current profiler (ADCP) backscatter. A single site was occupied in the North Channel at the mouth of Chesapeake Bay (37° 04\u27 N, 75° 58\u27 W) from 17.0–19.7 November 1999. Temperature, salinity, fluorescence, light transmission, current velocity and backscatter were measured throughout the sampling period. Plankton samples were collected every hour from 3.5, 7.5, and 11.5 m. We found little association between ADCP backscatter and plankton concentrations, but stronger correlations between backscatter and light transmission suggesting the backscatter was not biological in origin. Time series analysis of tidal currents and backscatter near the bottom revealed strong correlations at 6-hr intervals, suggesting resuspension of particles due to semidiurnal tidal flows. Backscatter from a single frequency echosounder such as the ADCP may only produce a coarse estimate of suspended particles in estuaries such as Chesapeake Bay