Physical forcing controls spatial variability in primary production in the Indian Ocean

Abstract

In order to examine the spatial variability and controlling factors of primary production in the Indian Ocean, deck-incubation primary production (13C method) and hydrographic properties were measured at 25 locations during inter-monsoon period (April-May 2014). Low concentration of nutrients were observed in the upper 30 m due to stratification of water column. Chlorophyll-a (Chl-a) in the mixed layer was, however, relatively higher in the Bay of Bengal (BoB) than in the Arabian Sea (AS) due to influence of river discharge and eddies. A prominent deep Chl-a maximum (DCM) was observed in the entire study region centred around 75 m of depth. The mixed layer integrated primary production displayed linear relation with nitrate and phosphate indicating that nutrients enriched through mixing enhanced primary production in the upper ocean. The photic zone integrated production was higher in the AS (1483 � 202 mg C m?2 d?1) followed by Equatorial Indian Ocean (EIO; 1182 � 360 mg C m?2 d?1), BoB (878 � 390 mg C m?2 d?1), and South Indian Ocean (SIO; 740 � 281 mg C m?2 d?1) in the Indian Ocean. Eddies were found to modify primary production in the Indian Ocean as production observed was lower in the anticyclonic eddies (ACE) by ~43% while higher by ~52% in the cyclonic eddy (CE) than no-eddy (NE) region. The primary production in the AS was only 70% higher than in the BoB compared to the hitherto estimated as 150%. We believe our estimate as more accurate because it takes into account the high spatial variability caused by eddies and sampling representation during different phases of the Indian Ocean Dipole (IOD). Vertically Generalized Primary Production model (VGPM) underestimated photic zone primary production by 40-70% whereas carbon-based primary production model (CbPM) estimated close to that of measured primary production in the BoB, EIO and SIO whereas underestimated by ~30% in the AS.. There is need for generating more data in order to test the performance of remote sensing models in the Indian Ocean