Mechanism of the biological response to winter cooling in the northeastern Arabian Sea

THE Arabian Sea is one of the most biologically productive ocean regions1, mainly due to the upwelling of nutrients during the summer (southwest) monsoon. But the northern Arabian Sea continues to sustain fairly high biological production after the upwelling season and during much of the winter (northeast) monsoon2–4. The processes that enable this high winter productivity have hitherto been poorly understood, being variously attributed to surface cooling effects2,3 or wind-driven changes in ocean circulation4. Here we present physical, chemical and biological data that indicate that sea surface cooling drives convection processes that lead to the injection of nutrients up into the surface waters of the northeastern Arabian Sea during winter, and that this mechanism of nutrient supply is a dominant control on winter productivity. Observed seasonal changes in bacterial and microzooplankton populations may provide an explanation for the Arabian Sea 'paradox'5–8 that mesozooplank-ton biomass remains more or less invariable throughout the year.

[1]  J. G. Field,et al.  The Ecological Role of Water-Column Microbes in the Sea* , 1983 .

[2]  Andrew F. Bunker,et al.  Computations of Surface Energy Flux and Annual Air–Sea Interaction Cycles of the North Atlantic Ocean , 1976 .

[3]  S. Naqvi,et al.  Chemical oceanography of the Indian Ocean, North of the equator , 1984 .

[4]  Baars,et al.  Monsoons and Pelagic Systems , 1994 .

[5]  S. Painting,et al.  Simulation and field measurements of phytoplankton-bacteria-zooplankton interactions in the southern Benguela upwelling region , 1993 .

[6]  K. Banse,et al.  Winter blooms of phytoplankton in the Arabian Sea as observed by the Coastal Zone Color Scanner , 1986 .

[7]  Taro Takahashi,et al.  Primary production at 47°N and 20°W in the North Atlantic Ocean: a comparison between the 14C incubation method and the mixed layer carbon budget , 1993 .

[8]  H. Ducklow,et al.  Stocks and dynamics of bacterioplankton carbon during the spring bloom in the eastern North Atlantic Ocean , 1993 .

[9]  B. Irwin,et al.  The western North Atlantic bloom experiment , 1993 .

[10]  Ralph J. Slutz,et al.  A Comprehensive Ocean-Atmosphere Data Set , 1987 .

[11]  Jim Aiken,et al.  Size-fractionated primary production and nitrogen assimilation in the northwestern Indian Ocean , 1993 .

[12]  S. Hastenrath.  Climatic atlas of the Indian Ocean , 1979 .

[13]  H. Ducklow,et al.  Plankton succession and carbon cycling at 47°N 20°W during the JGOFS North Atlantic Bloom Experiment , 1993 .

[14]  B. N. Desai.  Oceanography of the Indian Ocean , 1992 .