Organic carbon dynamics in the continental shelf waters of the eastern Arabian Sea.

The seasonal and spatial distribution of total organic carbon (TOC) is presented for the coastal waters of the eastern Arabian Sea, which experiences seasonal suboxia during the late southwest monsoon (SWM). This study reveals that high TOC was observed off Kochi as compared to Goa and Mangalore transects, and may be attributed to stronger upwelling along the Kerala coast. This is also supported by the excess carbon due to upwelling during the late SWM that varied from 37 µM (Goa), 39 µM (Mangalore), to 51 µM (Kochi). Our seasonal data from 2014 to 2020 at the Goa transect indicates that high TOC is seen during late SWM to fall inter monsoon (FIM) and between the late northeast monsoon (NEM) to the early spring inter monsoon (SIM). The high TOC concentrations and C/N ratios observed during the FIM are a combination of high primary production, the buildup of remnant organic matter from the previous season (due to prevailing low oxygen conditions), accumulation of refractory organic carbon, and release from diatoms (especially Chaetoceros sp.). Inter-annual variations indicate that phytoplankton blooms resulted in higher TOC concentrations, especially during the year 2020. Based on a comparison with an Elnino-Southern Oscillation (ENSO) year (2015), we can infer that the partitioning of carbon may increase from particulate to dissolved phase in future warming scenarios.

Sea foam-associated pathogenic bacteria along the west coast of India.

Anthropogenic activities release effluents containing nutrients and pathogenic bacteria that change the characteristics of coastal ecosystems. An important type of marine pollution which has occurred in 3 different states in India during 2019 is sea foam. Sea foam was found on Hole beach, Goa (Lat: 15.404° N, Long: 73.787° E), where nutrients (NO3- = 137 µM and organic nitrogen = 121 µM) from a garbage dumpyard are released directly via streams/gutters to coastal waters. This resulted in a bloom of the diatom Thalassiosira pseudonana, associated with high concentration of total organic carbon and fucoxanthin. Decay of this bloom along with strong agitation due to rocks and wave action resulted in sea foam. We isolated foam-associated bacteria and identified pathogenic bacteria including Enterobacter cancerogenus through 16S rRNA gene sequencing. Such foam-associated pathogenic bacteria, could be antibiotic resistant, and may have adverse effects on human health. This can also hamper the tourism industry of a small state like Goa that relies heavily on tourism.

Total organic carbon and its role in oxygen utilization in the eastern Arabian Sea.

We report seasonal and temporal variation of total organic carbon (TOC) in the eastern Arabian Sea (AS). In comparison to the deep, TOC in the top 100 m showed spatial variation with higher concentrations towards northern AS during North east monsoon (NEM) and South west monsoon (SWM). A comparison with the US-JGOFS data (1995) shows warmer temperatures, enhanced TOC and low chlorophyll in the recent years. High TOC is associated with Arabian Sea high saline waters (ASHSW), advected from the Arabian Gulf, might have resulted in an enhancement of TOC in the eastern AS. This excess TOC supports a high abundance of bacteria despite the low primary productivity. TOC oxidation accounted for 14.3% and 22.5% of oxygen consumption for waters with potential density between 24.5 and 27.3 kg/m3. This study attains great significance considering the missing links with respect to the role of transport processes in ocean deoxygenation under ongoing warming scenarios.

Anthropogenic spherules in Zuari estuary, south west coast of India.

In this study we report silica rich anthropogenic spherules from the marine environment. We found spherical, dumbbell, teardrop and fused spherules in Zuari estuary (near the Dona paula jetty), south west coast of India. The spherules were composed of SiO2 (69.8%), Na2O (13.2%), CaO (8.8%), MgO (3.8%), and traces of Al2O3, and FeO. Their high Na and Ca contents rules out the possibility of being an impact spherule or microtektite, or anthropogenic spherules coming from fly ash. Their elemental composition suggests that these are glass micro beads that have many applications including production of road and pavement marking materials, such as traffic paints. Considering that the glass micro beads are known to have high concentrations of Pb, As, and Sb that can leach into the marine environment, this study also raises questions regarding the impact of such spherules on marine biota, and highlights the need for further detailed study.

Variability of organic nitrogen and its role in regulating phytoplankton in the eastern Arabian Sea.

The relative importance of organic nitrogen (ON) and inorganic nitrogen (IN) as nitrogen sources for the phytoplankton communities were studied through a seasonal time series of nitrogen species in the eastern Arabian Sea. Seasonal data on nitrogen species showed that ON dominates the system throughout the year and account for >90% of the total nitrogen (TN) during north east monsoon (NEM). The average ON concentration in the study region was 11.5 µM during NEM as compared to 7.2 µM during south west monsoon (SWM). The contribution of picoplankton to the total phytoplankton pool (fpico) increased from 19% during SWM to 36% in NEM. Along with cyanobacteria, the diatoms Thalassionema nitzchoides and Thalassiosira sp. and among the dinoflagellates Pronoctiluca sp. were found to proliferate in ON rich waters. We suggest atmospheric deposition, riverine-input, resuspension of bottom sediment and ground water intrusion as the possible sources of ON in the study region.