Ecology and trophic preference of picoplankton and nanoplankton in the Gulf of Mannar and the Palk Bay, southeast coast of India

The Gulf of Mannar (GoM) and the Palk Bay (PB) are two least studied marine environments located between India and Sri Lanka. The environmental and smaller plankton (0.2–20μm) data from 30 locations in the GoM and PB during the northeast monsoon (November–February) are presented in this paper. Coastal currents during the study period was from the east to west and as a result, the PB had Bay of Bengal (BoB) waters, which was low saline (av. 28.98±1.34) as compared to the GoM (av. 31.96±0.58). The BoB waters caused significantly higher turbidity in the PB (av. 7.84±13.59NTU) as compared to the GoM (av. 1.76±1.38NTU). Multivariate analyses of hydrographical parameters demarcated two separate clusters in the study area clearly segregating the GoM and PB. This was mainly due to the physical barriers (Rameswaram Island, Ramsethu and Mannar Island) that inhibit the mixing of waters between the GoM and PB. The fluorescence microscopy and flow cytometry data showed a high abundance of picoeukaryotes, heterotrophic bacteria and autotrophic nanoplankton in the GoM whereas, Synechococcus and heterotrophic nanoplankton were higher in the PB. The picoplankton and nanoplankton carbon biomass was higher in the GoM (av. 62.2mgCm−3) as compared to the PB (av. 47.6mgCm−3). The carbon biomass in the GoM and PB was mainly contributed by nanoplankton (>70%) signifying their trophic preference in the study area. The carbon contribution of different plankton components in the GoM was autotrophic nanoplankton>heterotrophic bacteria>heterotrophic nanoplankton>Synechococcus>picoeukaryotes. On the other hand, heterotrophic nanoplankton was the second most dominant component in the PB followed by heterotrophic bacteria, Synechococcus and picoeukaryotes. The redundancy analysis (RDA) showed that picoeukaryotes, heterotrophic bacteria and autotrophic nanoplankton are positively correlated with salinity and nitrate, whereas Synechococcus and heterotrophic nanoplankton are positively correlated with turbidity, phosphate and dissolved oxygen. The data presented in this paper forms the first information on the relative trophic preference of various fractions of smaller plankton in Indian coastal waters.