Is Primary Productivity in the Indian Ocean Sector of Southern Ocean Affected by Pigment Packaging Effect

Abstract

The probable cause for photoinhibition of primary productivity (PP) in the surface layers of the Indian Ocean sector of the Southern Ocean (SO) was studied during the austral summer (February) 2010. Chlorophyll a (Chl a ) and PP values were higher for polar stations compared to offshore stations and showed surface maxima; however, subsurface Chl a maxima was observed in two of the offshore stations. Biomass explained 36% of variance in PP and was not the sole controlling factor for PP variability. Euphotic zone integrated PP showed increasing trend from offshore to polar stations and varied from 159.56 to 1083.57 mg C m -2 d -1 . The relationship between Chl a -specific PP (P B ) and the corresponding photosynthetically active radiation in the water column was linear for offshore and curvilinear for polar stations, indicating the occurrence of 'photoinhibition' in the surface waters of polar stations. This could be ascribed to the onset of pigment packaging (the 'package effect') as larger phytoplankton (diatoms) dominated the polar stations, where macronutrients ratio was ideal (N : P ∼ 16 and N : Si ∼ 1) for growth of diatoms. Despite high Chl a in the polar waters, the corresponding PP was proportionally not high compared to the offshore stations. We suggest that larger phytoplankton are susceptible to pigment packaging, which in turn decreases their light-absorption/photosynthetic efficiency, resulting in lower PP, which is otherwise expected to be higher in the presence of elevated biomass.