AN INTERMEDIATE NEPHELOID LAYER ASSOCIATED WITH HIGH MICROBIAL METABOLIC RATES AND DENITRIFICATION IN THE NORTHWEST INDIAN-OCEAN

Extensive optical, physical, chemical, and biochemical measurements ma de simultaneously in the northwest Indian Ocean reveal the occurrence of an intermediate nepheloid layer (INL) invariably associated with th e secondary nitrite maximum. Maxima in particulate protein and the act ivity of the respiratory electron transport system (ETS) are also foun d within the INL. Since the INL persists long distances from the conti nental margin with an offshore intensification, it may not be related to the transport of material resuspended along the continental margin. An apparent correlation of the INL with the previously reported subsu rface maximum in bacterial abundance suggests that a local increase in the abundance of bacteria may be responsible for the increased turbid ity. Positive correlations of the beam attenuation anomaly with nitrit e and nitrate deficit suggest that most of these bacteria may be denit rifiers. The organic carbon demand within the denitrifying layer, comp uted from the observed ETS activity, appears to be severalfold higher than the sinking carbon fluxes to the denitrifying layer, requiring ad ditional modes of supply of the biodegradable organic matter. It is pr oposed that a bacterial maximum could be maintained with efficient uti lization of the dissolved organic matter within the denitrifying water s.