Phytoplankton, Bacteria and Heterotrophic Nanoflagellate studies using ship and OCM-2 data along a coastal transect in the Arabian Sea

Abstract

The coastal region of the central West coast of India is biologically productive and well known for its Mackerel and Sardine fishery. December is an active fishing month and fall in the post-monsoon period when coastal waters are influenced by the riverine discharge of fresh-water. The impacts of fresh-water on the components of the food chain were studied. A total of 40 stations distributed over 3 transects were sampled off the coast of Goa, off Ratnagiri and off Dabhol. Physical conditions like salinity and temperature were monitored. Salinity of coastal waters ranged from 33.93-34.59 psu which lowered sigma sup(-t) values from 24.08-21.49 kg m sup(-3) to 22.57-21.92 kg m sup(-3) due to fresh-water run-off during the extended monsoon of 2010. Chlorophyll a (Chl a) values of the coastal waters were much higher than shelf stations. The total bacteria of coastal waters ranged from 0.93x109 - 4.359x109 cells L sup(-1). A strong positive correlation was noticed in Chl a and bacterial load, especially in shelf waters (R2 = 0.839, n=13, p less than 0.0001). Further, HNF as grazer showed a negative pattern with the bacteria and hence the phytoplankton biomass. The bacterial load showed an appreciably higher relationship with CDOM in the shelf waters off the west coast of Ratnagiri when compared to the coastal waters. CDOM Ac325 in coastal water ranged from 0.51-1.32 m sup(-1) which is reasonably high compared to the CDOM values in the shelf waters of 0.228-1.209 m sup(-1). The use of OCM-II data for understanding the impact of the monsoon on the food-chain was explored and has been presented in order to understand the pattern, in both the bacterial and the HNF biomass based on the chlorophyll values retrieved from OCM images.