Net plankton community production in the Arabian Sea based on O₂ mass balance model

Abstract

Monthly Net Community Production (NCP) rates were computed using oxygen mass balance model for the euphotic zone in the Eastern (EAS) and Western Arabian Sea (WAS). NCP showed large seasonal and spatial variability in the Arabian Sea and it is consistent with gross productivity. Net autotrophic conditions were found during high productive seasons, while net heterotrophic conditions were prevalent during non-monsoon seasons in both EAS and WAS. Though gross productivity is higher during SW monsoon (June to September), NCP decreased from June to September due to increase in grazing pressure. This suggests that production to respiration ratio controls NCP in the Arabian Sea. NCP is found to be higher in the WAS compared to the EAS and it is consistent with gross productivity. Incubation experiments suggested that phytoplankton growth was higher than zooplankton grazing in the WAS due to high concentrations of nutrients and it is opposite in the EAS, suggesting that a different ecosystem exists in the east and west probably due to existence of different physical forcing. In general, picoplanktons are more dominant in the EAS, whereas diatoms are major contributors to primary production in the WAS due to low nitrate concentration in the former region. As a result, most of the produced carbon is recycled in the euphotic zone in the EAS, whereas it is exported to the deeper layers in the WAS. These results are consistent with the sinking fluxes of carbon at 100 m depth in both the regions. On an annual scale, WAS is net autotrophic, whereas EAS is net heterotrophic. NCP amounts to approximately one fourth of the gross primary production, and it is consistent with the new production estimated using f-ratios in the Arabian Sea.