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Carbon isotopic composition of dissolved inorganic carbon (δ13CDIC) in the Cochin estuary, a tropical eutrophic estuary along the southwest coast of India, and the adjacent coastal Arabian Sea was measured to understand spatio-temporal variability in sources and processes controlling inorganic carbon (C) dynamics in this estuarine-coastal system. δ13CDIC in the Cochin estuary showed wide variation during three different seasons (premonsoon: − 12.2 to − 3.26‰; monsoon: − 13.6 to − 5.69‰; and postmonsoon: − 6.34 to + 0.79‰). Detailed mixing curve approximation modeling along with relationships of δ13CDIC with dissolved oxygen and nutrients suggest dominant role of freshwater mixing and degassing of CO2 on DIC dynamics during wet seasons (premonsoon and monsoon). Excess CO2 brought in by rivers and in situ production due to respiration in the Cochin estuary result into one of the highest pCO2 observed in estuarine systems, leading to its degassing. During postmonsoon, a relatively dry period with high salinity, calcite precipitation was a major process with calcite saturation index > 1 at few locations. Relatively lower average surface values of δ13CDIC in the coastal Arabian Sea (premonsoon: + 0.95‰; monsoon: + 0.88‰; and postmonsoon: + 0.66‰) compared to the predicted open ocean value along with mixing curve modeling suggest dominance of respiration/organic matter (OM) degradation over primary productivity. Estuarine influence on coastal DIC dynamics was observed in nearshore region (~ 10 km), whereas evidence of upwelling was found at farther locations.

The seasonal upwelling along the southeastern Arabian Sea (SEAS) brings cold, nutrient-rich low oxygen subsurface water to the continental shelf. The subsurface oxygen deficiency due to upwelling is severe in some years, the intensity of which could profoundly influence the nutrient cycling along the SEAS. Herein, we studied the effect of seasonal anoxia on fractionation of phosphorus during the peak upwelling period of August 2013. Abundance of five fractions of phosphorus (P), namely exchangeable or loosely sorbed P (Pads), iron-bound P (PFe), authigenic P (Paut), detrital apatite plus other inorganic P (Pdet) and organic P (Porg), in surface sediments of SEAS shelf has been studied using a sequential extraction procedure (SEDEX) to examine their distributions and sources. Total P (TP) concentrations ranged from 209 to 1081 µg g−1 with an average of 508 ± 256 µg g−1. Among the five P fractions, the authigenic P was the dominant species, representing about 60% of TP. The relative abundance of P fractions was in the order: Paut>Pdet>Porg>PFe> Pads. Multivariate analyses revealed that the P fractions were primarily associated with the organic constituents and fine sediments. The hypoxic bottom condition associated with summer monsoon upwelling significantly reduced the concentrations of PFe, Pads and Porg in the surface sediments. The enhanced release of organic bound and iron oxides bound P under low oxygen condition was evident from the enhanced C/P and N/P ratios and lower C/N ratio in the sediments. The intense biomineralization of organic matter and reduction of Fe-Mn oxides due to the periodic anoxia resulted in the transformations of their associated P fractions and enhanced accumulation as authigenic calcium phosphate mineral.

The paper describes the community composition of phytoplankton in the eastern Arabian Sea (EAS) during winter monsoon (WM, January–February 2018), encompassing the entire eastern part of the Arabian Sea basin (~6–22°N and 77–67°E) with high-resolution sampling (7 transects, 56 sites), from coastal to offshore regions. The phytoplankton pigment concentrations estimated by high-performance liquid chromatography were interpreted using ‘CHEMTAX’ (Mackey et al., 1996) to determine relative abundance and contribution of phytoplankton functional groups at the class level. A distinct spatial heterogeneity was observed in the distribution of phytoplankton groups in the coastal and offshore regions of the northern as well as southern part of the EAS. The basin-scale column integrated chlorophyll a (Chl a) concentration in the upper 100 m was higher (16–118 mg m−2) in the northeastern Arabian Sea (NEAS) as against 9–79 mg m−2 in the southeastern Arabian Sea (SEAS). The chromatographic analysis revealed the dominance of diatoms in the coastal waters of the EAS basin. A closer evaluation of the hydrodynamics and nutrient chemistry of the offshore regions revealed that entrainment of sub-halocline nutrients into the sunlit water column through convective mixing supported the dominant cyanobacterial population in the NEAS. In the warm oligotrophic waters of the SEAS, the substantially high NH4+ concentration indicates that the dominant prochlorophytes were sustained by the regenerated production. The dominance of large phytoplankton like diatoms indicates a herbivorous control in the food chain in coastal waters of the EAS. Offshore waters are characterised by large proportions of pico-nano plankton along with considerable micro phytoplankton, which suggests a mixotrophic ecosystem. The statistical analysis showed that the silicate (Si/N ratio) concentration played a major role in controlling the diatom abundance in the coastal areas of the north (south) EAS while temperature and salinity together with nutrients structured the phytoplankton assemblage in the offshore waters during WM.

The Cochin estuarine system (CES) has experienced rapid changes over the last few decades from extensive urbanisation, industrialisation, agricultural development activities and aquaculture practices. Intense pollution and other anthropogenic disturbances have led to an accelerated increase in organic matter (OM) and dissolved inorganic nitrogen (DIN) in the estuarine complex. This study explores nitrous oxide (N 2 O) fluxes and its drivers in the CES. Relatively higher N 2 O concentrations were observed in the inner estuary when compared to the outer estuary. Higher DIN with varying salinity gradients likely controlled the production and flux of N 2 O into the inner portions of the system. Hypoxia was never established in the water column indicating that conditions were favourable for nitrification, which in turn possibly led to increased N 2 O concentrations. It was observed particularly during the monsoon season that the inorganic nutrients and dissolved greenhouse gases from the river Periyar at the head of the estuary, contributed to the increased N 2 O concentrations. The N 2 O water to air flux ranged from 0.49 to 29. 73 μ m ol m−2 day−1, which was higher than what was documented in earlier reports. Although the CES exports a substantial amount of nutrients to the adjacent Arabian Sea, the estuary effectively processes nitrogen inputs as N 2 O making CES a net source of N 2 O emissions.