Spatial variability of microzooplankton in the central Arabian Sea during spring intermonsoon

Abstract

Spatial variability of microzooplankton (MZP; organisms in size 20-200 -m) along with total heterotrophic bacteria (THB) and picophytoplankton in the upper 120-m water column of the central Arabian Sea (AS) was studied during the spring intermonsoon period (March 2015). Six stations divided into three regions (northern, southern, and central) were sampled along 68˚E transect, between 9˚N and 21˚N. A total of 25 species of MZP were recorded corresponding to loricate ciliates (11 species), aloricate ciliates (two species), and heterotrophic dinoflagellates (12 species). Along the transect, MZP total abundance that varied between 5 and 9 x 103 org./m3 was largely contributed by ciliates, especially loricate ciliates (30%-62%) followed by heterotrophic dinoflagellates (23%-26%). MZP counts were lower in the central region (SD ± 0.4, 7-8 x 103 org./m3) than in the northern and the southern regions (SD ± 0.7, 8-9 x 103 org./m3) of the Arabian Sea. Heterotrophic bacterial counts were higher in the northern (28-202 x 103 cells/ml) and southern (16-92 x 103 cells/ml) and lowest in the central (13-89 x 103 cells/ml) region. In contrast, picophytoplankton showed higher counts in central (3-54 x 103 cells/ml) and southern (4-113 x 103 cells/ml) regions and lowest in the north (1-45 x 103 cells/ml). Strong positive coupling of MZP-THB (0.05 > p > 0.01) than that of MZP-picophytoplankton density and MZP-chlorophyll a concentration suggests the spatial scale variability and functioning of microbial loop in the central Arabian Sea during the spring intermonsoon period.