Spring Intermonsoon Research Articles

Seasonal Dynamics of Trichodesmium and Phytoplankton Communities in the Eastern Arabian Sea: Winter Vs Spring-inter monsoon

The filamentous marine cyanobacterium Trichodesmium forms extensive blooms in tropical seas, where it plays an important role in nitrogen fixation. This study investigated Trichodesmium-associated phytoplankton in the Eastern Arabian Sea (EAS) during bloom and nonbloom conditions. Trichodesmium blooms over the continental shelf primarily comprised of T. erythraeum and T. thiebautii during the Spring Inter Monsoon (SIM) and T. erythraeum during the Winter Monsoon (WM). T. erythreaum blooms have rarely been reported from the north-east Arabian Sea during WM. Trichomes comprised of 143-1552 cells during the WM, with filaments ranging from 1339 to 13163 µm in length. Non-diazotrophic groups like diatoms and dinoflagellates were associated with Trichodesmium during both seasons within the bloom waters. The study found a novel symbiotic relationship between the heterocystous diazotroph Richelia intracellularis and Nitzschia sicula during WM blooms. This research expands knowledge of lesser-known phytoplankton species and their associations in the EAS.

Distribution and sea-to-air fluxes of nitrous oxide and methane from a seasonally hypoxic coastal zone in the southeastern Arabian Sea

The seasonal variability, pathways, and sea-to-air fluxes of nitrous oxide (N2O) and methane (CH4) in the coastal environment, where coastal upwelling and mudbanks co-exist are presented based on the monthly time-series measurements from November 2021 to December 2022. Upwelling-driven hypoxic water's shoreward propagation and persistence were the major factors controlling the N2O concentrations, while the freshwater influx and sedimentary fluxes modulate CH4 concentrations. The N2O concentrations were high during the southwest monsoon (up to 35 nM; 19 ± 8 nM)), followed by spring inter-monsoon (up to 19 nM; 10 ± 5 nM), and lowest during the northeast monsoon (up to 13 nM; 8 ± 2 nM), whereas the CH4 levels were high during the spring inter-monsoon (8.4 to 65 nM), followed by southwest monsoon (6.8 to 53.1 nM) and relatively lower concentrations during the northeast monsoon (3.3 to 32.6 nM). The positive correlations of excess N2O with Apparent Oxygen Utilisation (AOU) and the sum of nitrate and nitrite (NOx) indicate that nitrification is the primary source of N2O in the mudbank regime. The negative correlation of CH4 concentrations with salinity indicates considerable input of CH4 through freshwater influx. CH4 exhibited a highly significant positive correlation with Chlorophyll-a throughout the study period. Furthermore, it displayed a statistically significant positive correlation with phosphate (PO43−) during the northeast monsoon while a strong negative correlation with PO43− during the spring inter-monsoon, pointing towards the role of aerobic CH4 production pathways in the mudbank regime. N2O and CH4 exhibited a contrasting seasonal pattern of sea-to-air fluxes, characterised by the highest N2O fluxes during the southwest monsoon (hypoxia) (13 ± 10 μM m−2 d−1), followed by spring inter-monsoon (12 ± 16 μM m−2 d−1), and the lowest during the northeast monsoon (0.6 ± 3 μM m−2 d−1). Conversely, the highest sea-to-air fluxes of CH4 were noticed during the spring inter-monsoon (74 ± 56 μM m−2 d−1), followed by the southwest monsoon (45 ± 35 μM m−2 d−1), and the lowest values during the northeast monsoon (19 ± 16 μM m−2d−1). Long-term time-series measurements will be invaluable in understanding the longer-term impacts of climate-driven variability on marine biogeochemical cycles in dynamic nearshore systems.

Contrasting patterns in pH variability in the Arabian Sea and Bay of Bengal.

Continuous understanding of the ongoing ocean acidification (OA) is essential for predicting the future impact of OA on marine ecosystems. Here we report the results of open ocean time-series measurements (19 cruises) of seawater pH in total hydrogen ion scale (pHT) and associated parameters in the Arabian Sea (AS) and the Bay of Bengal (BoB). During southwest monsoon (SWM), the pHT within the 30 to 100 m water column shows the maximum difference between the two basins with BoB pHT being lower (up to ~0.39 units) than AS which could be due to freshwater influx from rivers, mixed layer dynamics, and cold-core eddies. However, during Spring inter-monsoon (SIM), the pHT of BoB follows the trend of AS. A contrasting finding is that the lowest pHT occurs at 350 to 500 m in the BoB while it is ~1000 m in the AS. The pHT within the 150 to 1500 m layer of these two basins shows lower values by 0.03 (±0.02) in the BoB as compared to the AS. The possible reasons for the low pHT within the BoB oxygen minimum zone (OMZ) could be due to intrusion of western Pacific water in the BoB, freshwater influx from rivers, variations in OMZ of the two basins, higher temperature (~2°C) within the OMZ of the AS, anddenitrification in the AS. The pHT in both the basins (500 to 1000 m) is lower than in the North Atlantic and higher than in the North Pacific waters; however, the pHT in the 200 to 500 m is lower in the BoB than in all these basins. This study highlights the under-saturation of calcium carbonate at very shallow depths (~ 100 m) in the BoB, indicating that the plankton in the BoB are facing a major risk from OA compared to the AS and need further investigation.

Characterization of phytoplankton functional types in the western Bay of Bengal: HPLC- and CHEMTAX-based approach.

This study investigates phytoplankton functional group variations in the western Bay of Bengal (WBoB) during the Spring Intermonsoon. Samples were collected from four cross-shore transects: Mahanadi (MN), Vamsadhara (VD), Godavari (GD), and Krishna (KS). East India Coastal Current and warm gyre influenced the southern transects (KS, GD), VD was experiencing moderate upwelling and MN was characterized by low salinity and oligotrophic conditions due to freshwater input. In response to hydrography, phytoplankton biomass and functional types differed within and between the transects. Chlorophyll-a (Chl-a) was spatially high in VD and low in MN. The subsurface Chlorophyll-a maxima (SSCM) was prominent and shallow in the MN and VD, compared to the southern transects. Total diagnostic pigments concentration was high in VD, followed by GD, KS and MN. Phytoplankton functional groups and each groups contribution to Chl-a was calculated through CHEmical Taxonomy (CHEMTAX). Diatoms and cyanophytes were the dominant functional types in the surface layers. Progressive shift from diatoms in the nearshore region to cyanophytes in the offshore was observed. The low saline and low-nutrient conditions were conducible for the growth of cyanophytes, while nutrient-rich optimum light layer of SSCM and upper layer of VD were favorable for diatoms. Cryptophytes contribution to Chl-a was higher in southern transects compared to the north. Prymnesiophytes and prasinophytes were high in the subsurface and deep layers could be due to their adaptions to light and nutrients. The present study highlights the significance of physical processes associated hydrography in structuring the phytoplankton functional types.

Vertical distribution of nitrogen sources for new and regenerated production in the western Bay of Bengal

The study investigated the new and regenerated production using 15N tracers in the western Bay of Bengal, with observations were made in a vertical water column and sunlight gradients under stratified oligotrophic conditions, which are the major changes that distinguish the hydrobiological parameters associated with the deep chlorophyll maximum (DCM). In this study, new and regenerated production were measured in at different zones such as the surface, the DCM region and the euphotic depth (below 1% of light intensity) during the high stratification period. The results showed that new and regenerated productivity at the surface was low, while higher light intensity decreased productivity at the surface and productivity in the DCM layer increased significantly. Productivity at different location within regions is primarily controlled by the supply of nitrogen nutrients from deep within the region. In contrast, productivity at a given location with the available nutrient pool, at different depths is highly influenced by light availability. The study region exhibited potential for moderate export production during spring inter-monsoon, as is the case in other parts of the Indian Ocean. The f-ratio was estimated to be 0.2, which is a very low value in deep water and plays an important role in the Earth’s carbon cycle, as carbon transported to the deep ocean is absorbed from the atmosphere, and the ocean thus acts as a carbon reservoir. The results also found that the availability of nitrogen nutrients is capable of exporting a significant part of the total production to the deep ocean when conditions are favorable. Our results suggest that the Bay of Bengal may be equally important in its efficiency to export productivity to the deep sea and the DCM layer. Statistical analysis revealed a positive correlation between nitrogen productivity and chlorophyll-a, dissolved inorganic nitrogen and total nitrogen: phosphorous ratio.

Dynamics of nitrous oxide and methane in the southeastern Arabian Sea

This study examined the factors controlling the intra- and inter-seasonal variations of dissolved methane (CH4) and nitrous oxide (N2O) in the southeastern Arabian Sea (SEAS). Time-series measurements of CH4, N2O and allied biogeochemical parameters were carried out during the monthly campaigns in the coastal waters and a seasonal campaign in the shelf waters of the SEAS. The southwest monsoon period (SWM) brought drastic changes in the regional hydrography through the incursion of hypoxic waters due to coastal upwelling, which increased N2O concentrations substantially but reduced CH4 levels. The ranges of N2O and CH4 during the upwelling period were 8–89 nM and 9–165 nM, respectively, and the non-upwelling period was 2–27 nM and 5–271 nM, respectively. The significant positive correlations of N2O with apparent oxygen utilisation (AOU), the sum of dissolved nitrate and nitrite (NO2− + NO3−) and excess N2O (ΔN2O), as well as a negative correlation with dissolved oxygen indicates that nitrification is the major process in this region during the non-upwelling period. In contrast, during SWM, N2O did not correlate with NO2− + NO3−; however, it exhibited a significant negative correlation with dissolved nitrite (under hypoxia), suggesting the possibility of nitrifier-denitrification as an active process during hypoxia. The high (low) levels of CH4 recorded during the oxic spring inter-monsoon (hypoxic during the SWM) period showed a direct dependency on the changes in the benthic community. The high abundance of the adult macrofauna and active bioturbation resulted in high sedimentary CH4 release, which led to enhanced water column CH4 concentrations (17–271 nM) during the spring inter-monsoon period. In addition, the breakdown of methylated organic compounds under nutrient-limited conditions may also support the elevated CH4 levels in surface waters. A low macrofaunal abundance and reduced bioturbation led to a considerable reduction of subsurface CH4 concentrations during hypoxia. Overall, the SEAS is found to be a net source of CH4 and N2O to the atmosphere, with its sea-to-air fluxes ranging from 1.7 to 85.8 μM m−2 d−1 (19.88 ± 22.20 μM m−2 d−1) for N2O and 4–756 μM m−2 d−1(133 ± 158 μM m−2 d−1) for CH4.

Diversity and community structure of microzooplankton in the eastern Indian Ocean during the inter-monsoon period

Microzooplankton (MZP) are an important part of the microbial food web and play a pivotal role in connecting the classic food chain with the microbial loop in the marine ecosystem. They may play a more important role than mesozooplankton in the lower latitudes and oligotrophic oceans. In this article, we studied the species composition, dominant species, abundance, and carbon biomass of MZP, including the relationship between biological variables and environmental factors in the eastern equatorial Indian Ocean during the spring intermonsoon. We found that the MZP community in this ocean showed a high species diversity, with a total of 340 species. Among these, the heterotrophic dinoflagellates (HDS) (205 species) and ciliates (CTS) (126 species) were found to occupy the most significant advantageous position. In addition, CTS (45.3%) and HDS (39.7%) accounted for a larger proportion of the population abundance, while HDS (47.1%) and copepod nauplii (CNP) (46.4%) made a larger contribution to the carbon biomass. There are significant differences in the ability of different groups of MZP to assimilate organic carbon. In this sea area, MZP are affected by periodic currents, and temperature is the main factor affecting the distribution of the community. The MZP community is dominated by eurytopic species and CNP. CTS are more sensitive to environmental changes than HDS, among which Ascampbelliella armilla may be a better habitat indicator species. In low-latitude and oligotrophic ocean areas, phytoplankton with smaller cell diameters were found to occupy a higher proportion, while there was no significant correlation between the total concentration of integrated chlorophyll a and the biological variables of MZP. Therefore, we propose that the relationship between size-fractionated phytoplankton and MZP deserves further study. In addition, the estimation of the carbon biomass of MZP requires the establishment of more detailed experimental methods to reflect the real situation of organisms. This study provides more comprehensive data for understanding the diversity and community structure of MZP in the eastern equatorial Indian Ocean, which is also of good value for studying the adaptation mechanism and ecological functions of MZP in low-latitude and oligotrophic ocean ecosystems.

Spatial variability in plankton metabolic balance in the tropical Indian Ocean during spring intermonsoon

The tropical Indian Ocean consists of three basins, namely the Arabian Sea (AS), Bay of Bengal (BoB) and Southern Indian Ocean (SIO), with relatively nutrient-rich waters in the former two basins. It is hypothesized that the excess carbon produced in the northern Indian Ocean may support heterotrophic carbon demand in the SIO. In order to test this hypothesis, deck incubation experiments were conducted during the spring intermonsoon under the aegis of the Indian-GEOTRACES program. Nutrients in the mixed layer were low in the SIO compared to AS and BoB due to strong thermal stratification in the former region. Dominant net autotrophy was noticed in the AS whereas net heterotrophy in the BoB and SIO. High community respiration (CR) was observed in the BoB which may be supported by riverine organic carbon, whereas in situ produced and advected excess carbon from the northern Indian Ocean may support in AS and SIO respectively. Net community production (NCP) displayed an inverse (linear) relationship with temperature (salinity) in the euphotic zone in the BoB and SIO suggesting that stratification driven by river discharge and equatorial currents, respectively, reduced nutrients inputs through vertical mixing in the upper ocean resulting in the formation of the strong net heterotrophy and contrast to this was found in the AS due to increase in primary production due to nitrogen fixation. The euphotic zone integrated nutrients displayed a linear relationship with NCP and Gross Primary Production (GPP) indicating that the availability of nutrients controlled the plankton metabolic rates in the tropical Indian Ocean. The threshold of GPP for plankton metabolic balance in the tropical Indian Ocean (1.9 mmol O2 m−3 d−1) was close to that of the global mean (2.2 mmol O2 m−3 d−1). The slope of the log-log relationship between GPP and CR was 0.5 and it is close to that of the global mean value of 0.60.

Spatial and temporal variation of dissolved iodine in the eastern Arabian Sea

Here we report the seasonal and spatial variation of iodide (I−) and total dissolved iodine (TI) along a transect covering the oxygen minimum zone (OMZ) in the Arabian Sea (AS). This data is generated after a gap of more than two decades since the US-Joint Global Ocean Flux Study (JGOFS) reported observations of iodine speciation in the region. Also, this is the first study to report on iodine speciation from the coastal region of the west coast of India. Here the water samples were collected along the meridional transect (68°E) during spring inter-monsoon (SIM), southwest monsoon (SWM), and fall inter-monsoon (FIM), and from the coastal Goa transect during late SWM and FIM. In addition, sediment porewater was also collected from the coastal Goa transect during late SWM. Iodide was found to be dominant in the OMZ, whereas TI was majorly represented by iodate below the OMZ. The I− and TI concentrations were higher around the 15°N stations during all three seasons sampled. Seasonal variability was observed in the study, with high I− and TI during the SWM and FIM as compared to SIM. The coastal region was observed to have high I− values during seasonal anoxia in FIM as compared to the late SWM. This may be due to the release of I− from the sediments which are extremely rich in I−. The porewater showed extremely high I− (up to 4545 nM) at the shelf stations, which is 10 times higher than what is measured in the overlying waters. The US-JGOFS study reported iodide values as high as 953 nM in the AS OMZ with excess iodine (difference between the measured TI and the predicted TI) of 400 nM and attributed this to the input from the shelf sediments. In the present study TI values up to 811 nM were observed in the AS. To ascertain the source of excess iodine, porewater extracted from the sediments covering shelf and open ocean stations along the Goa transect was studied for iodine speciation. The diffusive flux of I− calculated from the surface sediments ranged between 14 and 43 μmol I− m−2 d−1 from the shelf stations off Goa, which is high enough to explain the excess iodine concentrations seen in the OMZ. Apart from the seasonality observed in iodine concentrations, our study infers that the shelf sediments along the west coast of India act as a source of iodine to the AS OMZ.

Extensive Accumulation of Nitrous Oxide in the Oxygen Minimum Zone in the Bay of Bengal

AbstractThe production by microorganisms of nitrous oxide (N2O), a trace gas contributing to global warming and stratospheric ozone depletion, is enhanced around the oceanic oxygen minimum zones (OMZs). The production constitutes an important source of atmospheric N2O. Although an OMZ is found in the northern part of the eastern Indian Ocean, the Bay of Bengal (BoB), two earlier studies conducted during the later phase of winter monsoon (February) and spring intermonsoon (March–April) found quite different magnitudes of N2O accumulation. This study found two‐ to ten‐fold greater accumulation of N2O during the autumn intermonsoon (November) than for other seasons described in earlier reports. The maximum N2O concentration (136 nmol kg−1 at 16°N, 88°E) is comparable to those observed around the OMZ in the Arabian Sea or eastern tropical Pacific. Isotopic signatures suggest that bacterial denitrification and archeal nitrification play important roles in N2O production, but earlier studies using nitrate or nitrite analysis did not confirm denitrification in the BoB. Large seasonal variation of N2O implicates the BoB as an important N2O source, similar to the Arabian Sea and eastern tropical Pacific, if the accumulated N2O is emitted to the atmosphere during the subsequent monsoon season.

Huge anthropogenic microbial load during southwest monsoon season in coastal waters of Kakinada, Bay of Bengal

To examine the influence of anthropogenic activities on the marine ecosystem near the coastal waters of the port city, Kakinada, a study was conducted to investigate the abundance of heterotrophic, indicator and pathogenic bacteria during the spring inter monsoon (SIM) and southwest monsoon (SWM) seasons. A drastic change in the marine bacteria due to the input of allochthonous bacteria during SWM was noticed. An order of magnitude higher abundance of indicators (Escherichia coli and Enterococcus faecalis) and bacterial pathogens (Proteus mirabilis and Pseudomonas aeruginosa) was observed during SWM. In contrast, Chlorophyll-a, heterotrophic bacterial abundance, Aeromonas hydrophila and Klebsiella pneumoniae were higher during SIM. A significant increase in some of the indicator and pathogenic bacterial abundance due to moderate rainfall suggests that the improper drainage system in the city could spread these bacteria, posing a considerable threat to both environment and human health.

Planktoniella sol, a New Host for the Ectoparasitic Dinophyta Paulsenella Chatton from the Northeastern Arabian Sea

Parasitic dinoflagellates, an essential component of the marine ecosystem, are mainly represented by five genera, Myxodinium, Paulsenella, Dubosquella, Coccidinium and Amoebophrya, infecting marine diatoms. All members of dinophyte Paulsenella are ectoparasites of diatoms and presently include three species. The marine diatom Planktoniella sol infection by a parasitic dinoflagellate Paulsenella chaetoceratis from the northeastern Arabian Sea was observed during the spring intermonsoon 2012. ~17 parasitic cells represented by their dinospores infect the host diatom. The diatom P. sol forms a new host record for the parasite, and the report is the first of its kind from the Indian waters.

Ecological Insights from Phytoplankton Diversity Off Veraval, Gujarat Coast, India

Phytoplankton species are sensitive to environmental and seasonal variability. This property of phytoplankton can be used to predict and study fluctuations in the ecological health of the system. In this study, phytoplankton diversity and its direct interactions with abiotic components of the ecosystem were studied to evaluate the health and stability of the coastal waters of Veraval, Gujarat coast, India. Results showed that diatoms dominated the ecosystem with a strong seasonal pattern of community succession. Waters were well churned and turbidity hindered penetration of light into deeper layers during the fall and winter monsoon season whereas stratification during spring inter-monsoon governed the distribution of different phytoplankton groups in the water column. These seasonal shifts resulted in a sincere pattern of annually re-occurring seasonal succession and formation of bloom events in the region. Apart from general phytoplankton diversity and its seasonal succession, the study intended to understand the forces governing the diversity and ecological system as a whole. To meet this objective, percent light intensities of chlorophyll-a, at various depths and temperatures were studied in relation to phytoplankton density. The correlation did not stand well for stations having active bloom formation showing that chlorophyll-a cannot be used as a proxy of phytoplankton biomass during bloom conditions. Thus excluding the points corresponding to active bloom conditions, second order polynomial equations were developed explaining the changes in chlorophyll-a concentration with variations in phytoplankton cell counts for study seasons. It was observed that most phytoplankton cells preferred upper well-lit layers of the water column with 100% light intensity followed by depths with 50% and 1% light intensities respectively. Study on temporal dynamics of the phytoplankton community in relation to temperature showed that the winter monsoon was characterized by a temperature range of 21.9 °C to 29 °C where Navicula distans was the most abundant phytoplankton species. Skeletonema costatum was the most abundant species at 27.2 °C during fall intermonsoon. Spring inter monsoon had an abundance of Navicula distans at both 28.35 °C and 28.9 °C.

Diversity and Quantitative Detection of Clade I Type nosZ Denitrifiers in the Arabian Sea Oxygen Minimum Zone.

A significant amount of nitrous oxide (N2O) is effluxed into the atmosphere as a result of marine denitrification in the Arabian Sea (AS) oxygen minimum zone (OMZ). An assessment of temporal variations in the diversity and abundance of nosZ denitrifiers was performed to establish the relative importance of these bacteria in denitrification. Sampling was conducted at the Arabian Sea Time Series (ASTS) location and a quantitative PCR (qPCR) ana-lysis was performed. We detected a high abundance of the nosZ gene at core OMZ depths (250‍ ‍m and 500 m), indicating the occurrence of denitrification in the AS-OMZ. The maximum abundance of the nosZ gene was observed during the Spring Intermonsoon (SIM) at 250‍ ‍m (1.32×106 copies L-1) and 500‍ ‍m (1.50×106 copies L-1). Sequencing ana-lysis showed that nosZ denitrifiers belonged to the classes Alpha-, Beta-, and Gammaproteobacteria. Taxonomic ana-lysis revealed that most OTUs were affiliated with Pseudomonas, Rhodopseudomonas, and Bradyrhizobium. Diversity indices and richness estimators confirmed a higher diversity of nosZ denitrifiers at 250‍ ‍m than at 500‍ ‍m during all three seasons. The present results also indicated that dissolved oxygen (DO) and total organic carbon (TOC) are critical factors influencing the diversity and abundance of the nosZ-denitrifying bacterial community.

Nutrients and chlorophyll-a in the Gulf of Oman: High seasonal variability in nitrate distribution

As part of comprehensive oceanographic studies on the Persian Gulf and Gulf of Oman, this article documented the seasonal and vertical distribution of nutrients, chlorophyll-a, and nitrogen deficiency in the Strait of Hormuz and northern part of Gulf of Oman. During the three consecutive research cruises of RV Kavoshgar Khalij Fars (Persian Gulf Explorer) in September–October 2018, December 2018, and May 2019, 7–18 stations were surveyed. During the southwest monsoon season, the water column is characterized by a 10–20 m shallow mixed layer and high spatial variation in nutrients and their ratio in the upper 100 m. Shallow remineralization of nitrogen-rich organic matter and lateral advection from the Arabian Sea upwelling system lead to nutrient accumulation, and enrichment of nitrate-nitrogen compared to P and Si in the euphotic zone with some sign of Si limitation. On the other hand, deepening of the mixed layer due to the cool convective mixing during the northeast monsoon, brings nutrients to the surface and stimulate the phytoplankton blooms (i.e. with surface chlorophyll-a maximum of 1.46 ± 0.57 μg l−1). So, the primary production is likely to be controlled by grazing rather than nutrient availability during the northeast monsoon. In the spring inter-monsoon, the multilayer structure of the water column with a less well-characterized mixed layer has a surface layer severely depleted in nitrite and nitrate with low yet detectable concentrations of silicate (0.77 ± 0.44 μM) and phosphate (0.29 ± 0.09 μM). The distribution of surface chlorophyll-a concentration was patchy with generally low concentrations of 0.37 ± 0.10 μg l−1 with highly variable subsurface maxima occurring at 10–25 m depth level. Primary production seemed to be potentially limited by inorganic nitrogen availability in spring inter-monsoon, as severe nitrogen deficiencies extended from the surface to the bottom layer (17.54 ± 4.39 μM at 400–1000 m depth level).

Long-term variability of satellite derived total alkalinity in the southwest Bay of Bengal

The seasonal and inter-annual variability of Total Alkalinity (TA) concentration was studied in the Bay of Bengal from 2003 to 2019 by using MODIS-Aqua derived sea surface temperature (SST) and sea surface salinity (SSS) products. The satellite derived TA showed a positive relationship with in-situ TA with (R2 = 0.67, RMSE = ±27.53 μMol/kg, SEE = ±32.16 and uncertainty error = 2287μMol/kg). The seasonal SST, SSS and TA portray the clear seasonal pattern between the seasons without any rapid change increase or decrease in trend observed over the years. In contrast to other seasons, the spring inter-monsoon was observed to have a warm surface water temperature with high salinity and TA. Strong wind and excessive cloud cover during the summer monsoon result in the reduction of ocean surface heat, which favours sea surface cooling and shallow mixed layer depth, resulting in low SST, SSS, and TA compared to the spring inter-monsoon. During fall inter-monsoon, the reversal of East India coastal current directs warm water from north to south and the weak wind that prevails in this region enhances stratification. During winter, low-saline water compensates the static stability loss by thermal inversion from the sea surface resulting in surface cooling with coldest SST, low SSS and TA during this period.

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 51μM (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.

Variability of particulate organic carbon and assessment of satellite retrieval algorithms over the eastern Arabian Sea.

Particulate organic carbon (POC) and its variability were studied to assess the accuracy of ocean colour retrieval algorithms over the eastern Arabian Sea (EAS) as it controls the carbon sequestration, oxygen minimum zone and biogeochemical (C, N and P) cycles. The seasonality in the physical and biological processes strongly influenced the distribution of POC along the EAS. Higher POC and chlorophyll a (chl a) during the spring inter monsoon (SIM) in the north EAS were due to detrainment bloom. The lower POC:chl a ratios during the winter monsoon (WM) (299.8 ± 190.8) than the SIM (482.1 ± 438.3) were due to the influence of freshly derived organic matter with high nutrient levels. The moderate coefficient of regression values of POC with chl a (R2 = 0.49, N = 59) suggests the importance of dead organic materials in controlling the POC distribution in the EAS. Validation between satellite and in situ POC using the four ocean colour retrieval algorithms showed that the algorithm based on the ratio of remote sensing reflectance (Rrs) performed better (R2 = 0.6, N = 17). It also showed a linear trend of POC with absorption coefficients suggesting it as an optical proxy for the POC retrieval.

Vampyrophrya pelagica (Chatton and Lwoff 1930), an apostome ciliate infectious on calanoid copepod from the Northeastern Arabian Sea

BackgroundZooplankton provides a dynamic habitat to various kinds of epibionts in the marine ecosystem and also forms a source of food and nutrients to various parasites and parasitoids. The study was conducted to identify the association of ciliates with calanoid copepod of Northeastern Arabian Sea. Zooplankton samples collected were analysed under stereomicroscope for any kind of ciliate attachments. The infective stage and major characteristics of the ciliates were discussed. The present observation is the first report of apostome ciliate Vampyrophrya pelagica on calanoid copepod Bestiolina similis from the open ocean waters off Tarapur, Northeastern Arabian Sea, during the early spring intermonsoon season and also a new host record for the ciliate.ResultsAnalysis of ~ 2530 copepod revealed the presence of 12 copepod specimens infested with apostome ciliate Vampyrophrya pelagica. All the host copepod, identified as Bestiolina similis, were females. Approximately 50–60 ciliates occupied the host endocuticular region. V. pelagica was represented by fully grown trophont stage characterized by the presence of a large food vacuole surrounded by a thin layer of cytoplasm with numerous lipid bodies.ConclusionThe study establishes a new host record, calanoid copepod B. similis for V. pelagica. The report is the first of its kind from the Indian waters and extends the biogeographic boundary of the apostome ciliate.

Unraveling the sources of atmospheric organic aerosols over the Arabian Sea: Insights from the stable carbon and nitrogen isotopic composition

The isotopic composition of stable carbon (δ13C) and nitrogen (δ15N) in marine aerosols influenced by the continental outflows are useful proxies for understanding the aging and secondary formation processes. Every winter, the haze pollutants transported from South Asia significantly affect the chemical composition of marine atmospheric boundary layer of the Arabian Sea. Here, we assessed the δ13C of total carbon (TC) and δ15N of total nitrogen (TN) in marine aerosols collected over the Arabian Sea during a winter cruise (6–24 December 2018). TC (2.1–13.4 μg m−3) is strongly correlated with TN (0.9–5.0 μg m−3), likely because of their common source-emissions, biomass burning and fossil-fuel combustion in the Indo-Gangetic Plain and South Asia (corroborated by backward-air mass trajectories and satellite fire counts). Besides, the linear relationship between the mass ratios of water-soluble organic carbon (WSOC) to TC (0.04–0.65) and δ13CTC (−25.1‰ to −22.9‰) underscores the importance of aging process. This means oxidation of organic aerosols during transport not only influences the WSOC levels but also affects their δ13CTC. Likewise, the prevalent inverse linear relationship between the equivalent mass ratio of (NH4+/non-sea-salt- or nss-SO42−) and δ15NTN (+15.3‰ to +25.1‰) emphasizes the overall significance of neutralization reactions between major acidic ([nss-SO42−] ≫ [NO3−]) and alkaline species (NH4+) in aerosols. Higher δ15NTN values in winter than the spring inter-monsoon clearly emphasizes the significance of the anthropogenic combustion sources (i.e., biomass burning) in the South Asian outflow. A comparison of δ13CTC and δ15NTN with the source emissions revealed that crop-residue burning emissions followed by the coal fired power plants mostly dictate the atmospheric abundance of organic aerosols in the wider South Asian outflow.