Carbon cycling in the northern Arabian Sea during the northeast monsoon: significance of salps

Winter cooling and mixing brings nutrients (nitrate and phosphate) to the surface waters over large parts of the northern Arabian Sea, but the rates of primary production as well as carbon export from the euphotic zone are not especially high during the northeast monsoon. A multi-disciplinary time-series study conducted over a period of 13 d around 21° N, 64° E during the northeast monsoon of 1997 revealed substantial decreases in chlorophyll stocks in the mixed layer following the occurrence of a massive swarm of salps. A large increase in DOC was also observed during the same period. We propose that a deficiency of silicate relative to nitrate during this season may often limit the growth of diatoms, creating an ecological niche for filter feeders that can efficiently utilize abundant smaller plankton. This may lead to periodic removal of chlorophyll, thereby moderating primary production as well as episodic build-up of DOC in the upper layers. This DOC pool may be used as a nutrient source for the microbial loop in the surface layer during the following spring intermonsoon season and by the denitrifying bacteria in the oxygen-minimum zone.