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Winter cooling and persistent mixing for more than a quarter of year (November to early March) along the North Eastern Arabian Sea (NEAS) results in nutrient enrichment of the euphotic column thereby triggering biological production. Hydrographic characteristics of NEAS during Late Winter Monsoon (LWM) and Early Spring Inter Monsoon (ESIM) and the influence on biological production are overviewed here. Winter convective mixing signatures were evident during LWM with low SST (24°C), high SSS (36.4), deep mixed layers (>100 m) and increased surface nitrate (~1 µM). Open ocean waters observed high chlorophyll a (1–2 mg m−3) and microphytoplankton abundance (1.2–1.5 × 104 cells l−1). Diatoms and green Noctiluca scintillans were the major microphytoplankton identified. ESIM observed gradual stabilization of water column with curtailment of winter signatures and strengthening of Noctiluca scintillans blooms. Mesozooplankton biomass was higher during LWM and decreased towards ESIM with intensification of Noctiluca blooms. However during ESIM, abundance of gelatinous zooplankton occurred in the bloom region. Inter-annual variations were observed in the biological responses along with the hydrographic changes. Thus the convective process during winter monsoon and stabilization of the water column during ESIM plays a significant role in the production pattern of NEAS. [ABSTRACT FROM PUBLISHER]

Carbon biogeochemistry of a tropical ecosystem (The Cochin Estuary, India) undergoing increased human intervention was studied during February (premonsoon), April (early monsoon) and September (monsoon) 2005. The Cochin estuary sustains high levels of pCO2 (up to 6000 μatm) and CO2 effluxes (up to 274 mmolC m−2 d−1) especially during monsoon. A first-order estimate of the carbon mass balance shows that net production of dissolved inorganic carbon is an order of magnitude higher than the net loss of dissolved and particulate organic carbon from the estuary. This imbalance is attributed to the organic inputs to the estuary through anthropogenic supplies. The bacteria-mediated mineralization of organic matter is mainly responsible for the build-up of pCO2 and increased CO2 emission to the atmosphere indicating heterotrophy. The linear correlation between excess CO2 and apparent oxygen utilization indicates respiration as the chief mechanism for CO2 supersaturation. An increase in the net negative ecosystem production (–ve NEP) between premonsoon (−136 mmolC m−2 d−1 or −376 MgC d−1) and monsoon (−541 mmolC m−2 d−1 or −1500 MgC d−1) is supported by a corresponding increase in O2 influxes from 17 mmol O2 m−2 d−1 (126 MgC d−1) to −128 mmol O2 m−2 d−1 (−946 MgC d−1) and CO2 emissions from 65 mmolC m−2 d−1 (180 MgC d−1) to 267 mmolC m−2 d−1 (740 MgC d−1). There is a significant north-south gradient in metabolic rates and CO2 fluxes attributable to the varying flow patterns and anthropogenic inputs into the estuary. The study reveals that the Cochin estuary, a previously autotrophic (CO2 sink) system, has been transformed to a heterotrophic (CO2 source) system following rapid urbanization and industrialization. Moreover, the export fluxes from the Cochin estuary appear to be quite important in sustaining net heterotrophy in the southeastern Arabian Sea. [ABSTRACT FROM AUTHOR]

Studies on biogeochemical cycling of carbon in the Chilka Lake, Asia’s largest brackish lagoon on the east coast of India, revealed, for the first time, strong seasonal and spatial variability associated with salinity distribution. The lake was studied twice during May 2005 (premonsoon) and August 2005 (monsoon). It exchanges waters with the sea (Bay of Bengal) and several rivers open into the lake. The lake showed contrasting levels of dissolved inorganic carbon (DIC) and organic carbon (DOC) in different seasons; DIC was higher by ∼22% and DOC was lower by ∼36% in premonsoon than in monsoon due to seasonal variations in their supply from rivers and in situ production/mineralisation. The DIC/DOC ratios in the lake during monsoon were influenced by physical mixing of end member water masses and by intense respiration of organic carbon. A strong relationship between excess DIC and apparent oxygen utilisation showed significant control of biological processes over CO2 production in the lake. Surface partial pressure of CO2 ( pCO2), calculated using pH–DIC couple according to Cai and Wang (Limnol and Oceanogr 43:657–668, ), exhibited discernable gradients during monsoon through northern (1,033–6,522 μatm), central (391–2,573 μatm) and southern (102–718 μatm) lake. The distribution pattern of pCO2 in the lake seems to be governed by pCO2 levels in rivers and their discharge rates, which were several folds higher during monsoon than premonsoon. The net CO2 efflux, based on gas transfer velocity parameterisation of Borges et al. (Limnol and Oceanogr 49(5):1630–1641, ), from entire lake during monsoon (141 mmolC m−2 d−1 equivalent to 2.64 GgC d−1 at basin scale) was higher by 44 times than during premonsoon (9.8 mmolC m−2 d−1 ≈ 0.06 GgC d−1). 15% of CO2 efflux from lake in monsoon was contributed by its supply from rivers and the rest was contributed by in situ heterotrophic activity. Based on oxygen and total carbon mass balance, net ecosystem production (NEP) of lake (−308 mmolC m−2 d−1 ≈ −3.77 GgC d−1) was found to be almost in consistent with the total riverine organic carbon trapped in the lake (229 mmolC m−2 d−1 ≈ 2.80 GgC d−1) suggesting that the strong heterotrophy in the lake is mainly responsible for elevated fluxes of CO2 during monsoon. Further, the pelagic net community production represented 92% of NEP and benthic compartment plays only a minor role. This suggests that Chilka lake is an important region in biological transformation of organic carbon to inorganic carbon and its export to the atmosphere. [ABSTRACT FROM AUTHOR]

India's vast exclusive economic zone (EEZ) with its rich bounty of living and non-living resources, offers a great opportunity to expand its economy and food security, and could enable sustainable development and create resilient jobs. Marine fisheries, mariculture and marine bioprospecting are significant sectors which could act as the next big drivers of the GDP and public welfare. Marine biodiversity documentation sporadically requires innovative techniques using genomics in combination with underwater imagery to cover the full spectrum of marine living resources. Deep waters beyond 500 m depth are not optimally exploited and offer immense scope for commercial exploitation. The mariculture sector, despite the wide array of candidate species, is still in its infancy and requires the use of genetic engineering to develop breeds suitable for the mariculture systems. Employing new analytical technologies and a wide range of 'multiomics' tools can enhance the bioprospecting of numerous marine living resources, which have the potential to unravel hundreds of new compounds for human well-being. Ascertaining the continuity of the blue economy demands judicious management of oceanic resources through innovative and sustainable practices. [ABSTRACT FROM AUTHOR]

The largest continental shelf Oxygen Minimum Zone (OMZ) in the world is formed along the Indian western shelf in the eastern Arabian Sea during the Southwest Monsoon [(SWM); June-September], which is a natural pollution event associated with the coastal upwelling. This study examines the composition, abundance, and distribution of copepods during the Northeast Monsoon [(NEM); November to February] and SWM in 50 m depth zones along the Indian western shelf in the eastern Arabian Sea. The NEM was characterised by warm, stratified, and low-salinity waters in the southeast Arabian Sea and cold, high-salinity, and well-mixed waters in the northeastern Arabian Sea. During the SWM, cold and Dissolved Oxygen (DO) deficient waters (<22 μM/0.5 ml L -1 ), which are the signs of coastal upwelling, were evident all along the study zone, but with more intensity off Kochi, Mangalore, and Goa in the south than off Mumbai and Okha in the north. The zooplankton total biomass and abundance showed seasonality with a general decrease during the SWM (av. 3.68 ± 1.29 ml m -3 and av. 5711 ± 3096 Ind. m -3 , respectively) compared to the NEM (av. 7.37 ± 2.17 ml m -3 and av. 14,473 ± 4966 Ind. m -3 , respectively). At the same time, the abundance of Polychaeta and Siphonophora showed an increase during the SWM (av. 1187 ± 1055 Ind. m -3 and av. 169 ± 119 Ind. m -3 , respectively), probably a result of the DO deficient waters associated with upwelling. Two striking seasonal features in Copepoda community were evident in this study: (a) a compositional shift from Cyclopoida dominant during the NEM to Calanoida dominant during the SWM, and (b) the coastal OMZ along the Indian western shelf during the SWM was dominated by Calanoida, which include oceanic OMZ species such as Pleuromamma indica, Lucicutia flavicornis, L.paraclausii, Eucalanus elongatus, Subeucalanus pileatus, S.subcrassus, and Clausocalanus furcatus. This forms a clear imprint for the extension of the oceanic OMZ into nearshore waters during the SWM due to coastal upwelling., Competing Interests: Declaration of competing interest The authors declare that there is no conflict of interest with this manuscript., (Copyright © 2024 Elsevier Ltd. All rights reserved.)

A twenty-four month long observational study conducted in an Asia's largest brackish water ecosystem, Chilika Lagoon, India, aimed to unravel dissolved organic matter (DOM) dynamics in this tropical brackish water ecosystem. The study assessed the interplay between allochthonous and autochthonous DOM sources during lean and active flow periods based on regional rainfall. Dissolved organic carbon (DOC), nitrogen (DON), and phosphorus (DOP) fluxes were analyzed, considering catchment runoff, phytoplankton production, benthic-pelagic interactions, and sea-lagoon exchanges as contributors. Contrary to conventional thinking, the study found autochthonous processes to be more significant than conservative mixing in shaping DOM dynamics. It introduced a novel conceptual model illustrating the multifaceted origins of DOM, encompassing catchment runoff, phytoplankton, benthic-pelagic interactions, bacterial activity, and sea-lagoon exchanges. These findings underscore the importance of holistic management strategies for Chilika Lagoon to preserve its ecological health, given its vital role in global carbon cycling, fisheries, and aquaculture., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 Elsevier Ltd. All rights reserved.)

This review has been undertaken to understand the effectiveness of ocean acidification on oceanic micronutrient metal cycles (iron, copper and zinc) and its potential impacts on marine biota. Ocean acidification will slow down the oxidation of Fe(II) thereby retarding Fe(III) formation and subsequent hydrolysis/precipitation leading to an increase in iron bioavailability. Further, the increased primary production sustains enzymatic bacteria assisted Fe(III) reduction and subsequently the binding of weaker ligands favours the dissociation of free Fe(II) ions, thus increasing the bioavailability. The increasing pCO 2 condition increases the bioavailability of copper ions by decreasing the availability of free CO 3 2- ligand concentration. The strong complexation by dissolved organic matter may decrease the bioavailable iron and zinc ion concentration. Since ocean acidification affects the bioavailability of essential metals, studies on the uptake rates of these elements by phytoplankton should be carried out to reveal the future scenario and its effect on natural environment., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the research reported in this paper., (Copyright © 2023 Elsevier Ltd. All rights reserved.)

Despite the omnipresence of microplastics (MPs), the studies around the western continental shelf of Indian Ocean (Eastern Arabian Sea-EAS) are uncovered and understudied. Thus, the present study was focused to understand the spatial distribution, characterization and risk assessment of MPs in sediment across seven coastal transects (10 to 50 m) all along the EAS shelf. The highest MPs concentration (MPs/kg d.w.) was detected in the northern EAS (NEAS; 2260 ± 1050) followed by central (CEAS; 1550 ± 1012) and southern (SEAS; 1300 ± 513) shelves. Among all distinct locations, the highest concentration of MPs (2500 ± 1042) was detected in the north coastal sediments off Mumbai, followed by off Mangalore (1480 ± 1169) in the center and off Kochi (1350 ± 212) in the south. MPs were found in the form of fibres, fragments and films with a predominance of fibres (~70-80 %). Approximately 74.6 % of the total MPs were in the size range of 300 μm to 5 mm. The surface of detected MPs was rough, irregular, and mechanical weathering features such as pits, grooves also observed and spotted with bacterial community structures. Polypropylene (PP; 34 %), polyisoprene (PIP; 19 %), butyl rubber (18 %), and low-density polyethylene (LDPE; 13 %) were dominant polymers. The pollution load index highlighted minor risk while the polymer hazard index exhibited a hazard level of V. Litter discharge, fishing activities, and active marine navigation are among the many high-risk sources of plastic contamination in this region. Due to the prevailing winds, currents, low sea surface height, and high precipitation, the conditions in the EAS are favorable for the accumulation of both sea-based and land-based particles. Hence, this study provides novel insights into the potential risks posed by MP to the IO rim and associated marine ecosystem which will enhance our knowledge of the ecological implications and consequences of MP pollution, ultimately aiding in developing effective management and mitigation strategies., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2023. Published by Elsevier B.V.)

The study assessed the Cochin estuary and adjacent coastal Arabian Sea for their seasonal variation in nitrate (NO 3 - ) and ammonium (NH 4 + ) uptake rates by total and nano + picoplankton using the 15 N tracer technique. The results suggested that the NO 3 - and NH 4 + uptake rates in the Cochin estuary are higher than those in the adjacent coastal Arabian Sea. NO 3 - and NH 4 + uptake rates in the nearshore stations in the off Cochin station were high, indicating the influence of the eutrophic estuary. NO 3 - and NH 4 + uptake rates conducted in off Mangalore transect were significantly lower than those of the off Cochin as it does not have an exchange with eutrophic systems. The nano + picoplankton's contribution to the total DIN uptake rates in the Cochin estuary was 77-98 %, indicating the relevance of nano + pico phytoplankton in the N cycling of the region., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2023 Elsevier Ltd. All rights reserved.)

Phytoplankton size classes (PSCs) are important in marine ecosystems because they organise the food chain and trophic pathways, which determine the overall biological environment. Based on three FORV Sagar Sampada cruises, the current study provides changes in PSCs in the Northeastern Arabian Sea (NEAS; north of 18 N) during different phases of the Northeast Monsoon [NEM (November-February)]. During all three phases of NEM such as early (November), peak (December), and late (February), in-situ chlorophyll-a fractionation data revealed that nanoplankton (2-20 μm) predominated, followed by microplankton (>20 μm) and picoplankton (0.2-2.0 μm). This was primarily because winter convective mixing in the NEAS maintains only a moderate level of nutrients in the surface mixed layer, which is more conducive to the dominance of nanoplankton. Brewin et al. (2012) and Sahay et al. (2017) have satellite-based PSC estimation algorithms; the former was developed for the entire Indian Ocean, while the latter is a modification of the former for the Noctiluca bloom-infested NEAS, with a claim that such blooms are typical of the NEM. When current in-situ PSCs data were compared to algorithm-based NEM data, Brewin et al. (2012) revealed a more realistic PSCs contribution pattern, especially in oceanic waters, with nanoplankton predominating except during early NEM. But the PSCs data from Sahay et al. (2017) showed a high degree of variation from the in-situ data, demonstrating the dominance of pico- and microplankton and a notably small contribution from the nano phytoplankton. The current study showed that Sahay et al. (2017) is inferior to Brewin et al. (2012) at quantifying PSCs in the NEAS without Noctiluca blooms, and provided evidence to show that Noctiluca blooms are not a typical feature of the region during the NEM., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2023 Elsevier Ltd. All rights reserved.)

The baseline study of Microplastics (MPs) in zooplankton (copepods, chaetognaths, decapods, and fish larvae) from six different zones along India's west coast (off Kanyakumari/Cape Comorin, Kochi, Mangalore, Goa, Mumbai, and Okha) in the Eastern Arabian Sea (EAS) is presented here with their vast ecosystem impacts. This investigation revealed that zooplankton in all six zones accumulated MPs pellets (52.14%), fibres (28.40%), films (10.51%), and fragments (8.95%). The highest average retention of MPs (MPs/individual) was found in fish larvae (av. 0.57 ± 0.18) while copepods had the lowest (av. 0.03 ± 0.01). The presence of low-density polyethylene, polypropylene, polystyrene, and polyethylene terephthalate was confirmed by Raman Spectra of MPs. The MPs in zooplankton found in this study (av. 22 ± 7 pieces/m 3 ) were nearly 2-fold greater than those found in some of the world's most densely populated areas. It is shown that the strong southerly coastal currents could advect the MPs contaminated water mass too far away, having the potential to affect the fish and corals., (Copyright © 2021 Elsevier Ltd. All rights reserved.)

Coastal upwelling that occurs in the eastern Arabian Sea (EAS) drive the complex dynamics of the food chain. Macrofauna plays a key role in the functioning of coastal ecosystems, but few studies explored the taxonomic and functional patterns of macrofaunal communities under the influence of upwelling. These patterns have been investigated in this study by sampling macrofauna and environmental variables during March-December 2012 across six depths (13-100 m) over the continental shelf off Kochi, south EAS. Upwelling, set over outer shelf prior to March, occupies the entire shelf by May, peaked during June-July and withdrew rapidly by September. A total of 203 macrofaunal taxa were collected in this study. Multivariate analysis revealed that the macrofaunal composition showed a spatiotemporal variation. Taxonomic diversity increases from nearshore to mid shelf whereas abundance and biomass decreased. Macrobenthic functioning, assessed through Biological Trait Analyses, displayed similar trait modalities between depths and seasons but abundance driven differences in trait expression revealed important habitat filtering. Increase in organic matter and decrease in dissolved oxygen influenced by upwelling and the spatial variation in sediment texture were the strongest drivers of the macrofaunal taxonomic pattern. We suggest that taxonomic and biological trait information needs to be considered in ecological studies as it provides a better understanding of how biodiversity responds to and interacts with environmental changes., (Copyright © 2021 Elsevier Ltd. All rights reserved.)

Coastal upwelling in the south eastern Arabian Sea (SEAS) leads to oxygen depletion over the continental shelf during the summer monsoon season (June-September), with latitudinal gradients in intensity. Based on two surveys in the onset (June) and peak (August) phases of the summer monsoon, the present study evaluates the response of macrozoobenthic communities (size >500 μm) to upwelling and consequent hypoxia (dissolved oxygen <0.2 ml/l) in the central sector of the SEAS shelf (10-12°N, 30-200 m). From the onset to the peak monsoon, macrozoobenthic density increased five-fold in the mid-shelf (50 m water depth), and nearly doubled in the outer shelf (100 m water depth) and shelf edge (200 m water depth). This was found to be a direct consequence of recruitment and proliferation of opportunistic polychaetes, particularly the spionid Paraprionospio pinnata, which was the single dominant species (52-78%) at all depths during the peak monsoon. With the establishment of the monsoon, the shelf communities (particularly 50-100 m depth sites) are thus transformed from relatively diverse assemblages to dense, single-species dominated ones. The shelf-edge communities (150-200 m depths), which are impacted with the perennial Arabian Sea oxygen minimum zone, and therefore harbour opportunist-dominated communities year-round. It is postulated that larvae of hypoxia-tolerant taxa are transported from the shelf edge by the process of upwelling onto the shelf. The settlement and survival of these larvae are regulated by the nature of shelf sediments and by the prevailing hypoxia. Thus, substantial recruitment of opportunists occurred in the outer and mid-shelf (50-100 m), but not in the inner shelf (30 m), where sedimentation from river discharge hindered settlement and survival of juveniles., (Copyright © 2021 Elsevier Ltd. All rights reserved.)

As projected by climate change models, increase in sea surface temperature and precipitation in the future may alter nutrient cycling in the coastal regions due to potential changes in phytoplankton community structure and their ability to assimilate nitrogen (N) and carbon (C). An experiment simulating different temperature and salinity conditions (28 ° C-35 ambient conditions, 28 º C-31, 31 º C-35 and 31 º C-31) in mesocosms containing 1000 L of coastal water from the Arabian Sea was performed and N uptake rates were measured using 15 N tracer technique on 2 nd , 5 th , 7 th and 10 th day of the experiment. The results show that, under all conditions, the total N (NO 3 -  + NH 4 + ) uptake rates were lower in the beginning and on the final day of the tracer experiment, while it peaked during middle, consistent with chlorophyll a concentrations. Total N uptake rate was significantly lower (p = 0.003) under ambient temperature-lower salinity condition (28 º C-31) than the others. This indicates that lowering of salinity in coastal regions due to excessive rainfall in the future may affect the N uptake potential of the phytoplankton, which may change the regional C and N budget., (Copyright © 2018 Elsevier Ltd. All rights reserved.)

Bioavailable dissolved organic carbon (B DOC ), nitrogen (B DON ) and their degradation rate constants were measured for the Chilika Lagoon, India. Long-term laboratory incubation experiments (90 days) were conducted at a constant temperature (25 °C) to quantify the bioavailable dissolved organic matter (DOM) and the possible degradation rate coefficients. The results showed that 41 ± 12% of dissolved organic carbon (DOC) and 47 ± 17% of dissolved organic nitrogen (DON) were B DOC and B DON respectively, with their stoichiometry found to be higher than the Redfield ratio. A first order exponential non-linear fitting routine was used to estimate pool sizes. The degradation rate constant (k) for the B DOC varied from 0.127-0.329 d -1 and B DON from 0.043-0.306 d -1 during the study period. Half-lives of the B DOC and B DON ranged from 2.1-5.4 and 2.2-15.9 days, respectively. Overall, the results showed that a fraction of the labile DON was transported from the lagoon to the adjacent coastal sea., (Copyright © 2018 Elsevier Ltd. All rights reserved.)

The objectives of the present study were to evaluate the ecological status of tropical coastal waters using the temperate benthic indices and examine the effect of seasonal variability on the performance of benthic indices. Macrobenthic samples were collected from northwest to southeast coast of India during 2003-2012 and we tested different univariate indices, ecological strategies, indicator species and multimetric indices. AMBI and multimetric indices performed satisfactorily in evaluating the ecological status. Seasonal variability on the biotic indices was observed during the southwest monsoon and fall intermonsoon period due to recruitment. Therefore, we recommended the non-monsoon period (January-May) as a suitable time of the year to use the indices for effective assessment of the Indian coastal waters. Results show that, the temperate benthic indices are efficient in assessing the tropical environmental status. However, complementary use of different indices is suggested for accurate assessment of the environmental status., (Copyright © 2016 Elsevier Ltd. All rights reserved.)