SPATIAL AND TEMPORAL DISTRIBUTIONS OF ACOUSTICALLY ESTIMATED ZOOPLANKTON BIOMASS NEAR THE MARINE LIGHT-MIXED LAYERS STATION (59-DEGREES-30'N, 21-DEGREES-00'W) IN THE NORTH-ATLANTIC IN MAY-1991

Temporal and spatial acoustic backscatter estimates of zooplankton bio mass were made using an unmodified hull-mounted 153-kHz acoustic Doppl er current profiler (ADCP) during the May 1998 Marine Light-Mixed Laye rs (MLML) cruises to the North Atlantic. Relative backscattcr from the ADCP was converted to zooplankton biomass estimates using individual plankton taxa abundances amid weights from zooplankton samples collect ed during the cruises. There was a small but consistent diet pattern i n the 20 to 250-m depth-integrated backscatter, with highest values du ring darkness. Removal of the diet signal with harmonic analysis revea led slightly higher zooplankton biomass to the southwest and west of t he 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 varie d by only a factor of 2, comparable to what one observes in replicate plankton rows. The nightly 0 to 2.50-m obliquely collected zooplankton samples (May 16-24) indicated increasing densities (amid biomasses) o f probable zooplankton scatterers (especially the copepod Calanus finm archicus) during middle to late May, soon after the peak in the spring phytoplankton bloom. This increase in May was mirrored by a comparabl e increase in depth-integrated acoustic backscatter. The distribution of zooplankton charged following two 50+ kn (1 kn = 1.85 km h(-1)) win d storms on May 19 amid May 21; zooplankton biomass was higher and ext ended 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 pat terns of zooplankton variability measured using shipboard acoustics ar e 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.