Contrasting pattern in chlorophyll a distribution within the Polar Front of the Indian sector of Southern Ocean during austral summer 2010

A contrasting pattern in chlorophyll a (CM a) distribution observed within the Polar Front (52 degrees S and 56 degrees S along 57 degrees 30'E) in the Indian sector of the Southern Ocean during the 2010 austral summer is reported here. At 52 degrees S, Chl a concentration ranged from 0.08 (30 m) to 0.69 mg m(-3) (70 m), whereas at 56 degrees S it was below detection level at all depths. Nutrients and light intensity were not found to be limiting the primary production at both stations. It appears that different types of food webs operating in the two locations within the front might have been responsible for causing the contrasting pattern in Chl a distribution. At 56 degrees S, large adult copepods and salps were abundant in the mesozooplankton community, which might have been exerting high grazing pressure on the phytoplankton community (conventional food web), thereby lowering the Chl a concentration. At 52 degrees S, on the other hand, Chl a concentration was relatively high because the small-sized copepods and copepodites which were more abundant here, might have preferred feeding on microzooplankton (20-200 mu M size) over phytoplankton (microbial food web). The high total bacterial count observed at this station is also in support of the prevalence of the microbial food web. It could be deduced from the study that distinct biological regimes may be existing within the same frontal regions in the Southern Ocean, depending on the type of food-web dynamics prevailing. More intensive sampling within the frontal regions is imperative in understanding the various biological processes so as to evaluate the productivity potential of the Southern Ocean.