Madhupratap, M.

National Institute of Oceanography, Dona Paula, Goa 403 004, India, E-mail: madhu@darya.nio.org

Carbon transfer and food web dynamics of the Arabian Sea

Different aspects of food web dynamics from central and eastern Arabian Sea such as phytoplankton (primary production, chlorophyll a, phytoplankton counts), bacteria (abundance, production rates), microzooplankton (abundance, species composition) and mesozooplankton (biomass, abundance) were studied. These were studied during three seasons viz., spring intermonsoon, northeast, and southwest monsoon. Microzooplankton abundances (700 x 10³ m^-3) and carbon content (25 mg C m^-3) during intermonsoon were higher when phytoplankton biomass was the lowest (average 6 mg C m^-3). Overall, higher biomass of microzooplankton occurred in the coastal region (21 mg C m^-3) than in the open ocean (14 mg C m^-3) with marked seasonal shifts. Analyses of microzooplankton along with other biological parameters suggest that they are nutritionally facultative and may feed on phytoplankton, bacteria or detritus depending on the available food. Their predominant food seems to be phytoplankton during southwest and northeast monsoon and bacteria during spring intermonsoon. Their seasonal average carbon biomass (8 - 30 mg C m^-3) accounted for 23 – 50% of the total living carbon in the pelagics, and they appear to act as a major source of food to higher trophic levels. Chlorophyll a and bacteria counts based on carbon fall short of meeting the microzooplankton demand. We believe that microzooplankton may alternatively depend on particulate organic carbon and detritus as well to meet their energy requirement.

Similar to microzooplankton, bacterial growth was also higher during spring intermonsoon following the build up of an organic carbon pool from northeast monsoon primary production when nutrients were available in the surface waters. Coastal and open ocean upwelling during southwest and cooling and convection during northeast monsoon increased the primary productivity of the region. But the region was net heterotrophic during spring intermonsoon. Mesozooplankton biomass was generally invariable in the upper layers irrespective of seasons. This leads us to suggest that a microbial loop, with mesozooplankton feeding alternately on microzooplankton or phytoplankton and microzooplankton on bacteria or phytoplankton, seems to be operating on a seasonal basis in the Arabian Sea. A prominent seasonal shift from heterotrophy during spring intermonsoon to autotrophy in southwest and northeast monsoon could be discerned.