Primary productivity and its regulation in the Arabian Sea during 1995

Abstract

Abstract The annual cycle of monsoon-driven variability in primary productivity was studied in 1995 during the Arabian Sea Expedition as part of the United States Joint Global Ocean Flux Studies (US JGOFS). This paper describes the seasonal progression of productivity and its regulation on a section which ran from the coast of Oman to about 1000 km offshore in the central Arabian Sea at 65°E. During the SW Monsoon (June–mid-September), the coolest water and highest nutrient concentrations were close to the coast, although they extended offshore to about 800 km; during the January NE Monsoon, deep convective mixing provided nutrients to the mixed layer in the region 400 – 1000 km offshore. As expected, the SW Monsoon was the most productive season (123±9 mmol C m−2 d−1) along the southern US JGOFS section from the coast to 1000 km offshore, but productivity in the NE Monsoon was surprisingly high (112±7 mmol C m−2 d−1). There was no onshore/offshore gradient in primary productivity from 150 to 1000 km off the Omani coast in 1995, and there was no evidence of light limitation of either primary productivity or photosynthetic performance (PoptB) from deep convective mixing during the NE Monsoon, deep wind mixing during the SW Monsoon or offshore Ekman downwelling during the SW Monsoon. Productivity during the Spring Intermonsoon (86±6 mmol C m−2 d−1) was much higher than in oligotrophic regions such as the tropical Pacific Ocean (29±2 mmol C m−2 d−1) or the North Pacific gyre region (32±8 mmol C m−2 d−1). The 1995 annual mean productivity (111±11 mmol C m−2 d−1) along this section from the Omani coast to the central Arabian Sea was about equal to the spring bloom maximum (107±23 mmol C m−2 d−1) during the 1989 North Atlantic Bloom Experiment (NABE) and the equatorial, 1°N–1°S wave guide maximum (95±6 mmol C m−2 d−1) in the Pacific Ocean during the 1992 EqPac study. The 1995 SW Monsoon primary productivity was similar to the mean value observed in the same region in 1994 by the Arabesque Expedition (127±14 mmol C m−2 d−1) and in 1964 by the ANTON BRUUN Expedition (115±27 mmol C m−2 d−1). During the 1995 SW Monsoon, strong, narrow and meandering current filaments extended from the region of coastal upwelling to about 700 km offshore; these filaments had levels of biomass, primary productivity, chlorophyll-specific productivity and diatom abundance that were elevated relative to other locations during the SW Monsoon. The SW Monsoon was the most productive period, but SW Monsoon primary productivity values were lower than predicted because efficient grazing by mesozooplankton kept diatoms from accumulating the biomass necessary for achieving the high levels of primary productivity characteristic of other coastal upwelling regions. The high rates of chlorophyll-specific productivity (PoptB>10 mmol C mg Chl−1 d−1) observed in the 1995 SW Monsoon, together with the observed dust flux and iron concentrations, indicate that the Arabian Sea was more iron replete than the equatorial Pacific Ocean or the Southern Ocean.