Particulate Organic Carbon Composition in Temperature Fronts of the Northeastern Arabian Sea during Winter

In order to understand the major sources of particulate organic carbon (POC) in the frontal zones and to examine their variability with space and time, a total of five temperature fronts of different ages was sampled in the northeastern Arabian Sea during winter. Compared to the nonfrontal regions, POC and chlorophyll-a were higher within the coastal fronts, whereas chlorophyll-a was lower within the open ocean front (T1). The variation of POC between coastal and open ocean fronts is attributed to the combined influence of variable vertical mixing, heterotrophic transformation and age of the front. Relatively depleted C-13(POC) and N-15(PN) were observed within the fronts, suggesting that POC pool is contributed by in situ production supported by upwelling of nutrient-rich water and zooplankton biomass. Elemental C:N ratios, POC:Chl-a, C-13(POC), and N-15(PN) suggest that POC is mainly contributed from primary producers and heterotrophs in the study region. However, relative contributions from these two sources vary spatially from coastal to open ocean and with the age of the front. Stable Isotope Analysis in R (SIAR) model revealed that zooplankton biomass largely contributed to POC in the open ocean (60-80%) than phytoplankton (20-40%) and nearly equal contribution was observed in the coastal fronts (50-60% and 40-50%, respectively). This study, thus, demonstrates that dominant heterotrophy and autotrophy in the open ocean and coastal fronts and it is consistent with their biomasses. Predominant heterotrophy in the open ocean is attributed to deeper mixed layer resulting in upwelling of bacteria-rich and phytoplankton-poor water to surface leading to existence of microbial loop.