ABSTRACT: An approach to a partial solution to the general problem of defining biogeochemical provinces for the accurate estimation of global-ocean primary production and realistic structuring of epipelagic plankton ecosystem models is presented for the northwestern Indian Ocean. This is accomplished through use of a new technique, biohydro-optical classification, that applies a rudimentary submarine light budget incorporating climatologies of incident light, mixed-layer thickness, and chlorophyll 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 through the spring intermonsoon (March through May), summer southwest monsoon (June through August), and fall intermonsoon (September through November) under climatic forcing and in response to the resulting biological variability. Virtually all of the open Arabian Sea is within the Typical Tropical Class at the close of the spring intermonsoon. This class type is intended to identify the epipelagic plankton ecosystem mode associated with tropical regions containing a deep chlorophyll maximum (DCM) maintained by active algal growth, and a light-rich, oligotrophic shallow zone containing a phytoplankton association which depends largely upon regenerated nutrients. At the close of the southwest monsoon in August, a Mixed-Layer Bloom Province covers much of the northern Arabian Sea. This province class corresponds to the ecosystem mode represented by tropical regions undergoing marginal or mid-ocean upwelling and greatly simplifies regional extrapolation of the local primary 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 At the onset of the fall intermonsoon, the Mixed-Layer Bloom Province in the northern and western Arabian Sea is superseded by a Transitional Province, which persists through the fall intermonsoon. We interpret the upper layer of the fall intermonsoon Transitional Province in the Arabian Sea as a special case of the shallow regenerative plankton ecosystem of oligotrophic ocean areas, where rates of zooplankton-driven nutrient regeneration and recycled production, key processes in the upper layer of the classic 2-layer euphotic zone of oligotrophic low-latitude oceans, reach extreme values.

KEY WORDS: Indian Ocean · Primary production · Biogeochemistry · Satellite remote-sensing · Modeling · Phytoplankton ecosystems

Published in MEPS Vol. 165 (1998) on May 7
ISSN: 0171-8630. Copyright © Inter-Research, Oldendorf/Luhe, 1998