Primary productivity and its regulation in the Arabian Sea during 1995

The annual cycle of monsoon-driven variability in primary productivity was studied in 1995 during the Arabian Sea Expedition as part of the United Sta tes Joint Global Ocean Flux Studies (US JGOFS). This paper describes the se asonal progression of productivity and its regulation on a section which ra n from the coast of Oman to about 1000km offshore in the central Arabian Se a at 65 degreesE. During the SW Monsoon (June-mid-September), the coolest w ater 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 - 1000km offshore. As expected, the SW Monsoon was the most productive seas on (123 +/- 9 mmol Cm-2 d(-1)) along the southern US JGOFS section from the coast to 1000km offshore, but productivity in the NE Monsoon was surprisin gly high (112 +/- 7mmol Cm-2 d(-1)). There was no onshore/offshore gradient in primary productivity from 150 to 1000km off the Omani coast in 1995, an d there was no evidence of light limitation of either primary productivity or photosynthetic performance (P-opt(B)) from deep convective mixing during the NE Monsoon, deep wind mixing during the SW Monsoon or offshore Ekman d ownwelling during the SW Monsoon. Productivity during the Spring Intermonso on (86 +/- 6 mmol Cm-2 d(-1)) was much higher than in oligotrophic regions such as the tropical Pacific Ocean (29 +/- 2 mmol Cm-2 d(-1)) or the North Pacific gyre region (32 +/- 8 mmol Cm-2 d(-1)).The 1995 annual mean product ivity (111 +/- 11mmol Cm-2 d(-1)) along this section from the Omani coast t o the central Arabian Sea was about equal to the spring bloom maximum (107 +/- 23 mmol Cm-2 d(-1)) during the 1989 North Atlantic Bloom Experiment (NA BE) and the equatorial, 1 degreesN-1 degreesS wave guide maximum (95 +/- 6m mol Cm-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 t he same region in 1994 by the Arabesque Expedition (127 +/- 14 mmol Cm-2 d( -1)) and in 1964 by the ANTON BRUUN Expedition (115 +/- 27 mmol Cm-2 d(-1)) . During the 1995 SW Monsoon, strong, narrow and meandering current filamen ts extended from the region of coastal upwelling to about 700km offshore; t hese filaments had levels of biomass, primary productivity, chlorophyll-spe cific productivity and diatom abundance that were elevated relative to othe r locations during the SW Monsoon. The SW Monsoon was the most productive p eriod, but SW Monsoon primary productivity values were lower than predicted because efficient grazing by mesozooplankton kept diatoms from accumulatin g the biomass necessary for achieving the high levels of primary productivi ty characteristic of other coastal upwelling regions. The high rates of chl orophyll-specific productivity (P-opt(B) > 10 mmol C mg Chl(-1) d(-1)) obse rved in the 1995 SW Monsoon, together with the observed dust flux and iron concentrations, indicate that the Arabian Sea was more iron replete than th e equatorial Pacific Ocean or the Southern Ocean. (C) 2001 Elsevier Science Ltd. All rights reserved.