BIOHYDRO-OPTICAL CLASSIFICATION OF THE NORTHWESTERN INDIAN-OCEAN

An approach to a partial solution to the general problem of defining b iogeochemical provinces for the accurate estimation of global-ocean pr imary production and realistic structuring of epipelagic plankton ecos ystem models is presented for the northwestern Indian Ocean. This is a ccomplished through use of a new technique, biohydro-optical classific ation, that applies a rudimentary submarine light budget incorporating climatologies of incident light, mixed-layer thickness, and chlorophy ll to recognize fundamental modes of tropical plankton ecosystems. The 3 types of biohydro-optical classes found in the Arabian Sea, Typical Tropical, Mixed-Layer Bloom, and Transitional, are shown to evolve th rough the spring intermonsoon (March through May), summer southwest mo nsoon (June through August), and fall. intermonsoon (September through November) under climatic forcing and in response to the resulting bio logical variability. Virtually all of the open Arabian Sea is within t he Typical Tropical Class at the close of the spring intermonsoon. Thi s class type is intended to identify the epipelagic plankton ecosystem mode associated with tropical regions containing a deep chlorophyll m aximum (DCM) maintained by active algal growth, and a light-rich, olig otrophic shallow zone containing a phytoplankton association which dep ends largely upon regenerated nutrients. At the close of the southwest monsoon in August, a Mixed-Layer Bloom Province covers much of the no rthern Arabian Sea. This province class corresponds to the ecosystem m ode represented by tropical regions undergoing marginal or mid-ocean u pwelling and greatly simplifies regional extrapolation of the local pr imary production algorithm. At the onset of the fall intermonsoon, the Mixed-Layer Bloom Province in the northern and western Arabian Sea is superseded by a Transitional Province, which persists through the fal l intermonsoon. We interpret the upper layer of the fall intermonsoon Transitional Province in the Arabian Sea as a special case of the shal low regenerative plankton ecosystem of oligotrophic ocean areas, where rates of zooplankton-driven nutrient regeneration and recycled produc tion, key processes in the upper layer of the classic 2-layer euphotic zone of oligotrophic low-latitude oceans, reach extreme values.