Seasonal variations in carbon biomass of bacteria, thraustochytrids and microzooplankton in the Northern Arabian Sea

As a part of the JGOFS-India Programme, abundance of heterotrophic bacteria (Hbac; 0–2000 m), thraustochytrids (0–2000 m), microzooplankton (0–200 m occasionally up to 300 m) as well as bacterial growth rates (0–500 m) were studied in the Northern Arabian Sea (NAS) during Spring (SpIM) and Fall intermonsoon (FIM), Northeast Winter (Win) and Summer (SuM) monsoon seasons. Hbac abundance was mostly in the range of 108–109 cells l−1 in the surface (0–100 m) and intermediate (150–400 m) depths. Thraustochytrids (heterotrophic protists rich in polyunsaturated fatty acids) were roughly in the range of 103 l−1 (nil to ∼2.7×105). Below 200 m, they decreased drastically to values between nil and 10 l−1. Seasonal differences in carbon biomass of Hbac, thraustochytrids and microzooplankton were more pronounced than those seen in the mesozooplankton. The highest Hbac C biomass was observed during FIM. Bacterial carbon demand (BCD, mg C m−3 d−1; at an assumed 33% conversion efficiency) varied from 0.45 to 40.6 in FIM, 23.43 to 100 in SpIM, 2.31 to 17.5 in Win and from 6.6 to 25.4 in SuM in the top 100 m. In general, with increasing depth the BCD decreased quite rapidly ranging from 0.007 to 1.65 mg C m−3 d−1 below 100 m. In the NAS, heterotrophic bacteria appear to play a significant role in sustaining microzooplankton and the so-called ‘Arabian Sea mesozooplankton stable-biomass paradox’ through microbial loop. Sizable quantities of Hbac, thraustochytrids and microzooplankton C might be suggestive that Hbac support these microheterotrophs, in mesopelagic zone in particular. Their role in sustaining a variety of deep-water micro- and mesozooplanktonic communities as well as in some ways, the myctophids ingesting marine snow (enriched with bacteria, thraustocytrids and dead organic matter besides zooplankters) is brought to attention.