Variations in the inorganic carbon components in the thermal fronts during winter in the northeastern Arabian Sea

Abstract

In order to examine the variations in inorganic carbon components in the thermal fronts, seven fronts have been sampled in the northeastern Arabian Sea during winter. The Sea Surface Temperature (SST) was cooler by 0.2 to 1.03 °C within the fronts sampled and three out of seven fronts were relatively saltier (by 0.04 to 0.10) and others were fresher (by 0.03 to 0.13) compared to outside the front. Increase in chlorophyll-a (Chl-a) was observed by 0.05 to 0.6 mg m⁻³ in the four fronts and marginal decrease was noticed in other fronts (0.02–0.23 mg m⁻³). The increase in Chl-a within the front from that of outside showed linear relation with magnitude of difference in temperature and salinity suggesting that enhanced vertical mixing brought nutrients to surface to support phytoplankton biomass in the fronts. Dissolved oxygen was relatively under-saturated by ∼1.6% in the frontal zone compared to outside suggesting that dominant heterotrophy in the former region due to availability of high phytoplankton biomass. Dissolved Inorganic Carbon (DIC) was higher in the frontal zone by 3 to 41.5 μM than outside. The salinity normalized DIC displayed linear relation with Chl-a and inverse correlation with dissolved oxygen saturation suggesting that both auto and heterotrophic activities may be responsible for changes in DIC in the frontal zone. The higher levels (by 3 to 50 μatm) were noticed in the frontal zone compared to outside, however, the magnitude of difference was linearly correlated with difference in SST and Chl-a between within and outside fronts. Based on the simple model, it was noticed that biological and mixing effects were dominant controlling factors for pCO₂ followed by thermal and flux effects. The biological effect was dominant at fronts 1 and 3 while mixing effect controls at fronts 2, 4 and 6. This study suggests that fronts act as a source or sink depends on the strength of the mixing process associating with biological response and also age of the front.