Acoustic Observations of Zooplankton Migration in Lake Superior

Abstract

The diel vertical migration (DVM) of aquatic organisms in marine environments is commonly studied using acoustic instruments because of their ability to sample with high spatial and temporal resolution for extended periods of time. The practice of using acoustic instruments in the Laurentian Great Lakes, however, is not as widespread. This research presents six years of Acoustic Doppler Current Profiler (ADCP) backscatter data from a site in Lake Superior to demonstrate that ADCPs can increase the resolution and robustness of data pertaining to the vertical migration of freshwater organisms, presumably zooplankton. We observed migratory behavior with sub-hour sampling over the course of entire seasons, including transitional seasons and the winter. The data distinctly reveals a diurnal signal consistent with DVM. In addition, with the high resolution of the data, we observe lunar and seasonal variation in the diurnal signal. We employ a simplified method using the change in measured intensity to isolate the diurnal signal, allowing us to compare data regardless of instrument modification, backscatter calibration, and range correction. We do not estimate abundance, biomass, population distribution, or species identification, instead we offer general observations on DVM patterns over days, months, and years to supplement standard biological sampling methods.