Spatial and temporal variability of plankton stocks from acoustic backscatter intensity and direct measurements in the northeastern Gulf of Mexico

Abstract

To investigate the utility of Acoustic Doppler Current Profilers (ADCPs) for estimating the spatial and temporal variability of zooplankton stocks in the northeastern Gulf of Mexico, acoustic data from a moored array of ADCPs were used in conjunction with sea truth zooplankton samples. As part of the DeSoto Canyon Eddy Intrusion Study, twelve Broadband ADCPs were moored along four cross-margin lines in the DeSoto Canyon area of the northeastern Gulf of Mexico. These upward-looking ADCPs were moored at approximately 90-m depth and deployed March 1997 through April 1999. To obtain sea truth for the backscatter data, net tows were made during spring, summer, and fall of 1998 as well as spring 1999. Results of this combined physical and biological research show that acoustic backscatter intensity data recorded by ADCPs do represent a biological signal that serves as an approximation for zooplankton standing stocks in the northeastern Gulf of Mexico. Spectral analysis of acoustic data reveals daily variability in acoustic backscatter intensity as well as low frequency, weekly to monthly, variability. This low frequency variability in acoustic backscatter intensity is linked to current flow in the DeSoto Canyon area, and correlations between acoustic backscatter intensity and current velocity vary with the movement of warm slope eddies and other mesoscale circulation features in the Gulf. Wind mixing associated with hurricane-strength storm events appears to affect the acoustic backscatter recorded by ADCPs, and increases in acoustic backscatter intensity during these periods are attributed to the downward mixing of bubbles from surface turbulence. From this research the interactions between physics and biology are evident, and we recommend that data recorded by moored ADCPs enable further investigation of such interactions.