Modelling the seasonality of subsurface light and primary production in the Arabian Sea

Abstract

Seasonal changes in mixed-layer depth and phytoplankton biomass in the Arabian Sea are assessed with climatologies of ship-based hydrographic measurements and ocean-color observations from satellite. At the close of the intermonsoons in November and especially May, the open Arabian Sea resembles the stereotypic, unperturbed tropical ocean, with a thin oligotrophic mixed layer and a pronounced subsurface chlorophyll maximum. Both the northeast and southwest monsoons disrupt this typical tropical hydrography through mixed-layer deepening and eutrophication in the central and northern Arabian Sea. Computations using a spectral model of light penetration suggest that seasonal changes in mixed-layer thickness and phytoplankton concentration result in pronounced fluctuations through the annual cycle in the radiant flux reaching the base of the mixed layer. At the close of the fall and spnng intermonsoons the base of the model euphotic zone is in the thermocline across all of the open Arabian Sea. The euphotic zone appears to rise Into the mixed layer of the northern Arabian Sea during both the winter and summer monsoons. Strong seasonality in total primary production and its partitioning between the mixed layer and thermocline is predicted by a photosynthesis-irradiance model for a site in the western Arabian Sea (14.36 N, 57.38' E). Modeled mixedlayer primary production depicts an intense peak for the southwest monsoon and a secondary northeast monsoon peak separated by intermonsoon periods of low production. During the fall and spring intermonsoons, in the presence of a subsurface chlorophyll maximum, the model estimate of primary production in the thermocline exceeds that in the mixed layer. Our model calculations suggest that the subsurface chlorophyll maximum present in the Arabian Sea during the spring intermonsoon is a precursor of the regional, summer, phytoplankton bloom.