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This study describes an assessment of an ocean-ecosystem model in simulating marine ecosystem dynamics in the Indian coastal waters. Long-term sustained in-situ observations of temperature, salinity, chlorophyll-a and dissolved oxygen (DO) collected in the coastal waters of India, and ship-based observations are used for this assessment. The model captures observed trend of temperature, salinity and chlorophyll-a with high correlation in both eastern and western shelf waters except for salinity and DO along west and DO along east coast. The model performs very well in simulating the Indian coastal shelf ecosystem dynamics. The seasonal occurrence of prominent phytoplankton bloom in the central-east coast during pre-monsoon, south-west coast during monsoon and north-west coast during post-monsoon is realistically reproduced by the model. The model also reproduces the seasonal presence of significantly low DO content in the upwelled water at a shallow depth leading to coastal hypoxia in the south-west and central-east coast of India during summer monsoon. A fine-tuned model is useful in understanding better Indian coastal shelf ecosystem and predicting future changes in time and space, like the occurrence of coastal hypoxia or anticipating phytoplankton blooms. Such information is useful for predicting potential fishing grounds in coastal waters and fisheries management. [ABSTRACT FROM AUTHOR]

• Year-long variation of OMZ and influencing factors assessed in eastern Arabian Sea. • OMZ occupied between 60 and 1350 m, and thickness increased from south to north. • Core-OMZ (denitrifying zone, DO < 5 µM) oscillated seasonally between 12° and 15°N. The oxygen minimum zone (OMZ) in the eastern Arabian Sea (EAS, ∼6° to 21°N), within Indian Exclusive Economic Zone (EEZ), is mapped, for the first time, for one year through ten repeated ship-based observations between December 2017 and January 2019 at seven to ten stations along the 2000 m depth contour. On an annual basis, the OMZ (<20 µM oxygen) in the EAS varied between 60 and 1350 m; its thickness decreased from north to south. During the winter monsoon, the upper boundary of the OMZ in the north and south was deeper (150–160 m) than the central EAS (∼110 m). Consequent to the summer monsoon upwelling and resultant Ekman transport, this upper boundary shoaled to 50–70 m in the south and central EAS. In contrast, relatively weaker upwelling in the north restricted the shoaling to ∼ 110 m. The core-OMZ (<5 µM oxygen), representing the core-denitrifying zone, has been found to have a perennial southern boundary in the central EAS at ∼ 15°N, which was thinner (150–750 m) than in the north (190–1000 m). The restriction of perennial core-OMZ to the central EAS is augmented by the relatively stronger upwelling south of it during the summer monsoon; the resultant upliftment of the water column has facilitated the progression of poleward undercurrent that ventilates this region. The aeration due to this poleward undercurrent enhanced the oxygen levels of OMZ (150 to 500 m) by ∼ 5 µM between 6.5°N and 13°N, and further north it increased by ∼ 2.5 µM up to ∼ 15°N. But this core-OMZ was extended to the south up to 12°N when a weak equatorward undercurrent acted in the region during the non-monsoon seasons. The study affirms the importance of sustained ship-based studies on OMZ in the EEZs, where no autonomous observatories like Bio-Argo floats are available, as such studies are critical to assess not only the future changes in the extent and behaviour of OMZ (core-OMZ) and its consequent advection onto the western continental shelf of India developing severe hypoxic to anoxic conditions but also its implications in modifying their biogeochemistry and food chain. [ABSTRACT FROM AUTHOR]

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. • Influence of coastal upwelling on the macrofaunal taxonomic and functional patterns were studied in the SE Arabian Sea. • Upwelling peaked during June–July and withdrew rapidly by September. • Upwelling fuelled organic matter along with sediment texture and dissolved oxygen influenced macrofaunal patterns. • Abundance driven differences in functional trait expression revealed importance of habitat filtering. [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. • Considered Copepoda as an indicator of coastal hypoxia along the Indian western shelf. • A compositional shift of Copepoda was evident during different seasons. • Many Copepoda of oceanic OMZ were found in the coastal hypoxic zones. • The study shows the biological continuity of the coastal and oceanic OMZ. [ABSTRACT FROM AUTHOR]

Changes in macrozoobenthic standing stock and composition along the south-eastern Arabian Sea continental shelf (30–200 m) are investigated, based on data collected from the same sites during the same season 14 years apart (winter monsoon, 1998 and 2012), using the same platform, sampling gear, methods etc. During both surveys, polychaetes dominated among the macrozoobenthos, followed by crustaceans and molluscs. A decline in macrozoobenthic density and biomass with increasing depth was noted in both surveys. Macrozoobenthic density increased significantly between 1998 and 2012, especially in the inner and mid shelf (30–50 m), due to the increase in density of polychaetes and molluscs, while the density of crustaceans decreased. The mean individual body weight of polychaetes decreased between 1998 and 2012, most notably in the inner and mid shelf (30–50 m). This reflected an overall increased dominance of small-sized opportunists such as spionids, magelonids, paraonids, cirratulids and pilargids in 2012 across the entire shelf. Such an increase in dominance of opportunists and decline in abundance of sensitive taxa is indicative of long-term environmental stress and anthropogenic pressure. However, no significant changes were noted in the measured environmental variables which are known to influence benthic faunal distribution in the region, such as sediment nature and oxygen availability. The observed changes may rather be indicative of the long-term effect of intense bottom trawling, which is practiced in the study area year-round. This is the first study of its kind for soft-sediment shelf benthos from the tropical belt. • Revisit of same sites after 14-year gap revealed increase in macrozoobenthic density. • Temporal change more prominent in the inner and mid-shelf. • Caused by increase in density of smaller-sized polychaetes and molluscs. • No change in measured environmental factors e.g. sediment texture, dissolved oxygen. • Observed changes in community composition may be an impact of chronic bottom trawling. [ABSTRACT FROM AUTHOR]

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. • Seasonal subsurface hypoxia established over shelf by peak of summer monsoon. • Increase in macrozoobenthic density from onset to peak of summer monsoon. • Direct result of increase in abundance and dominance of hypoxia-tolerant taxa. • Settlement and survival of these taxa regulated by substrate nature. • Shelf edge fauna play major role in regulating these community changes. [ABSTRACT FROM AUTHOR]