Acoustically measured seasonal patterns of Zooplankton in the Arabian Sea

Zooplankton biovolumes and sizes were determined from high-frequency acoustical measurements taken from an undulating SeaSoar vehicle on three cruises in the northern Arabian Sea. Cruises in 1994 and 1995 encompassed the periods of the winter Northeast Monsoon (November 28–December 17, 1994), the summer Southwest Monsoon (June 21 to July 13, 1995), and the Fall Intermonsoon (September 18 to October 11, 1995). Data are discussed from three sampling grids and their interconnecting transects in offshore waters. Zooplankton biovolumes varied seasonally, with lowest biovolumes during the summer Southwest Monsoon, intermediate during the Fall Intermonsoon and highest during the winter Northeast Monsoon. Biovolumes were highest near the surface and decreased with increasing depth. The smallest size-class (0.05–0.16 mm equivalent spherical radius) dominated the biovolume throughout most of the water column. Larger size-classes sometimes dominated the near-surface layers. Mesoscale variability was consistently observed in all areas on all cruises.