Spatial and temporal distributions of acoustically estimated Zooplankton biomass near the Marine Light‐Mixed Layers station (59°30′N, 21°00′W) in the North Atlantic in May 1991

Abstract

Temporal and spatial acoustic backscatter estimates of Zooplankton biomass were made using an unmodified hull‐mounted 153‐kHz acoustic Doppler current profiler (ADCP) during the May 1991 Marine Light‐Mixed Layers (MLML) cruises to the North Atlantic. Relative backscatter from the ADCP was converted to Zooplankton biomass estimates using individual plankton taxa abundances and weights from Zooplankton samples collected during the cruises. There was a small but consistent diel pattern in the 20 to 250‐m depth‐integrated backscatter, with highest values during darkness. Removal of the diel signal with harmonic analysis revealed slightly higher Zooplankton biomass to the southwest and west of the mooring than to the northeast, in common with gradients in surface temperature and chlorophyll during the mapping cruise. Overall however, depth‐integrated Zooplankton biomass during the mapping cruise varied by only a factor of 2, comparable to what one observes in replicate plankton tows. The nightly 0 to 250‐m obliquely collected Zooplankton samples (May 16–24) indicated increasing densities (and biomasses) of probable Zooplankton scatterers (especially the copepod Calanus fintmarchicus ) during middle to late May, soon after the peak in the spring phytoplankton bloom. This increase in May was mirrored by a comparable increase in depth‐ integrated acoustic backscatler. The distribution of Zooplankton changed following two 50+ kn (1 kn = 1.85 km h−1 ) wind storms on May 19 and May 21; Zooplankton biomass was higher and extended much deeper in the water column at night following these strong mixing episodes. Before the storm events, the patterns of Zooplankton diel vertical redistribution were consistent from day to day. Diel patterns of Zooplankton variability measured using shipboard acoustics are qualitatively similar to patterns observed from an ADCP on the MLML mooring, presenting the possibility of calculating a nearly continuous seasonal measure of Zooplankton biomass from the mooring ADCP data.