Acoustically measured seasonal patterns of Zooplankton in the Arabian Sea

Zooplankton biovolumes and sizes were determined from high-frequency acoust ical measurements taken from an undulating SeaSoar vehicle on three cruises in the northern Arabian Sea. Cruises in 1994 and 1995 encompassed the peri ods of the winter Northeast Monsoon (November 28-December 17, 1994), the su mmer Southwest Monsoon (June 21 to July 13, 1995), and the Fall Intermonsoo n (September 18 to October II, 1995), Data are discussed from three samplin g grids and their interconnecting transects in offshore waters. Zooplankton biovolumes varied seasonally, with lowest biovolumes during the summer Sou thwest Monsoon, intermediate during the Fall Intermonsoon and highest durin g the winter Northeast Monsoon. Biovolumes were highest near the surface an d decreased with increasing depth. The smallest size-class (0.05-0.16mm equ ivalent spherical radius) dominated the biovolume throughout most of the wa ter column. Larger size-classes sometimes dominated the near-surface layers . Mesoscale variability was consistently observed in all areas on all cruis es. (C) 2001 Elsevier Science Ltd. All lights reserved.