ADCP backscatter measurements in the northeastern Gulf of Mexico: comparison of near-surface with near-bottom biological backscatter

Abstract

In the Gulf of Mexico, acoustic backscatter intensity (ABI) from an acoustic Doppler current profiler (ADCP) can be a proxy for zooplankton and micronekton biomass, and sperm whale abundance is correlated with regions of locally high ABI. This analysis of ABI in sperm whale habitats during summer 2001 characterizes ABI temporal and spatial variability. During NOAA's Sperm Whale and Acoustic Monitoring Program (SWAMP) and follow on ichthyoplankton survey (SEAMAP), a ship-mounted 300kHz ADCP collected current velocity and ABI data from July to September 2001 from 16-56 m below sea surface. ABI averages were larger at night than during the day because of diel vertical migration of zooplankton and micronekton, and ABI was higher along the continental margin in features with cyclonic circulation than with anticyclonic circulation. During SEAMAP, wet displacement volumes were measured using Bongo net tows to get a first order estimate of the amount of plankton biomass corresponding to ABI data. Also investigated is ABI from near-bottom scatterers during the Deep Gulf of Mexico Benthic Ecology study (DGoMB). Four near-bottom deployments of a 300kHz ADCP in depths ranging from 755 to 2740 m were made in June 2001 each for 1-2 days. A fifth deployment was made in April 2002 at 300 m, and a sixth deployment for one month was done in June-July 2002 at 880 m depth. These deployments showed near-bottom ABI generally increased as the seabed was approached. Spectral and Empirical Orthogonal Function (EOF) analyses were performed on near-surface and near-bottom ABI data. Marked variability centered at 1 cycle per day (cpd) in the former was at frequencies associated with diel vertical migration of zooplankton and micronekton, and additional variability near-surface with 2-3 day periodicity corresponds to spatial scales of 10¹-10² km. Variability centered at 0.9-1.2 cpd was also evident in the month-long near-bottom record, even though at z = 880 m there is no day/night cue to migrating plankton and micronekton. EOF analysis showed the majority of variance ( >89%) was partitioned in the first three modes of the near-surface ABI data, versus 33.3% in the near-bottom data.