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The influence of mesoscale dynamics on variability of phytoplankton biomass in terms of chlorophyll-a (chl-a) concentration was studied in the coastal waters of the South Eastern Arabian Sea (SEAS) using long-term satellite data. Satellite-derived chl-a, sea level anomaly, sea surface temperature, and sea surface wind data for the period 1998–2016 were compiled from various sources and analysed to investigate the chl-a variability associated with coastal upwelling and mesoscale eddies. The Empirical Orthogonal Function and Morlet wavelet analyses were performed to estimate the quantitative variability and the result showed strong seasonal and interannual modulation in chl-a concentration and associated environmental variables. The Okubo–Weiss criterion was applied for the identification of mesoscale eddies. The results indicated the presence of cyclonic (cold core) eddies during the summer monsoon season (June–September). The wind-induced upwelling and the cyclonic eddies were most intense during the summer monsoon season, causing higher values of chl-a compared with other season. It is revealed that the variability of chl-a, which might be attributed to seasonal and interannual differences in the surface and sub-surface nutrients, is caused either by coastal upwelling or cyclonic eddies. In particular, the wind-induced upwelling strongly controls the spatial and temporal variability of chl-a compared with mesoscale eddies along the SEAS. The regression model we adopted points out the dominant role played by the wind and its forcing bring about variability in chl-a. The occurrence of extreme climatic events such as El Nino, La Nina and Indian Ocean Dipole (IOD) was noticed during the study period and particularly taken into account to understand the interannual fluctuations in chl-a and associated environmental variables. The relative variability in chl-a concentration was prominent during strong El Nino, La Nina, and IOD. We have attempted to determine the relationship between chl-a with coastal upwelling and mesoscale eddies, the overall importance of such physical forcings, and their influence on bio-production in the SEAS.

Near reef abundance and coast–offshore distribution of tuna larvae around Minicoy Island were studied based on three cruises carried out on board FORV Sagar Sampada during 2014–15. The samples were...

Convective mixing, mesoscale eddies and regenerated production sustain an above-average biological productivity in the North East Arabian Sea (NEAS) during the winter-spring transition period. Satellite-derived long-term data sets on Chlorophyll-a (Chl-a), Sea Surface Height Anomaly (SSHA), Sea Surface Temperature (SST) and Okubo-Weiss parameterization show existence of number of mesoscale eddies, propagating and non-propagating, that contribute to the regional production. The dominance of Eddy Kinetic Energy (EKE) over the Available Potential Energy (APE) in the core depth and the diameter (120 km) of the observed eddy being wider than the Rossby Radius of Deformation (RRD, 55 km), it is suggested that the baroclinic instability is a possible mechanism for the eddy formation. Spatial variation in APE and its influences on the regional dynamics, including chemical and biological response are explained. In the non-eddy areas, where convective mixing is active, diatoms (96.74%) dominated than dinoflagellates (3.14%), and the Chl-a in the Cold Core Eddy (CCE) were two to three folds higher to non-eddy regions. The abundance increased from core (58,152 cells L−1) to periphery (5.95 × 105 cells L−1) where the water column is less dynamic. Extensive blooms of the dinoflagellate green Noctiluca (N. scintillans) contribute to the very high cell density in the periphery of the CCE, where the currents were comparatively weak, and water column was more stable. Active mixing is associated with diatom dominance, followed by Noctiluca when the mixing slackens, making use of the available nutrients and supported by regenerated production. The bloom dynamics is explained for pre-bloom, bloom and post-bloom conditions with measurements on nutrients and plankton assemblages. The Noctiluca bloom (mid-March) is succeeded by Trichodesmium (April–May), in the stratified nutrient depleted, abundant light environment and propagated southwards. Observed increasing trends in the SSHA over the period indicate strengthening of stratification and hence altered production patterns in the NEAS.

The central and northeast Arabian Sea (AS) has an intense and thick oxygen minimum zone (OMZ) and denitrification zone. It is comparable with the strongest OMZ of the north-equatorial Pacific Ocean. Denitrification in the AS is revisited using a set of cruise observations collected during February–March of 2009 by the Centre for Marine Living Resources, India. The region possesses one of the most robust N* depleted water reaching as low as -20 μ mol l−1 at depths (~600 m). In AS, the oxygen depletion is mainly due to sluggish circulation, weak lateral and vertical ventilation. The biological respiration in oxygen deficit condition depletes nitrate and further modifies the Redfield ratio at intermediate depths (200-600 m) from 16 N:1P to 8 N:1P. Further, analysis of phytoplankton groups present at the surface identifies the presence of Trichodesmium erythraeum, a nitrogen fixing species. The denitrification rate is estimated as 25.3 ± 7.0 Tg N yr−1. It is consistent with past estimates, and no dramatic change in denitrification in the AS was noticed. The study also highlights the potential for further research on N2 production from the Arabian Sea Oxygen Minimum Zones in a changing climate. Plain language summary Nitrate is an essential compound for phytoplankton photosynthesis. In most ocean waters, the ratio between N and P is a constant (16:1), called the Redfield ratio. Various processes can modify the Redfield ratio in the ocean, such as denitrification, nitrification, and anammox. Denitrification is a process through which the microbial processes remove the fixed nitrogen in the absence of enough dissolved oxygen in the water column (oxygen minimum zone; OMZ). In the Arabian Sea, a vast region of OMZ formed due to the combined effects of biological respiration, sluggish circulations in the intermediate depths, and strong thermocline, which prohibits the supply of oxygen from deep waters. Our study revisits the denitrification in the Arabian Sea OMZ region during the winter to spring transition season in 2009 using a set of onboard measurements. Due to this denitrification, an intense modification in the Redfield ratio in the northeast Arabian Sea region's intermediate depths is noted as 8 N:1P.

The paper describes the hydrography and vertical current structure along the shelf edge of South East Arabian Sea (SEAS) during summer and winter monsoons based on current profiles from moving Acoustic Doppler Current Profiler (ADCP). During summer monsoon, SEAS was characterized by upwelling with low saline water at the surface along the southern sector (8° N to 11° N). During winter, thermal structure was vertically homogeneous in the upper 80 m, and intrusion of low saline Bay of Bengal waters were found up to 14° N. In the southern sector, turbidity was more than the northern sector during winter and summer seasons. ADCP-derived current profiles during summer along 200-m isobath show dominant northward flow in the south, and southeasterly in the north as part of the West India Coastal Current (WICC). A comparison between ADCP current profiles and Ekman currents during summer indicates dominance of remote forcing (coastal Kelvin waves) over the local wind forcing in the 8-9° N sector whereas a combined influence of both remote forcing and wind in the 9-15° N sector. During winter, the direction of surface current reversed and was poleward generally except at the southern sector (7-8° N) where the flow was southwestward. Sector-wise comparison of ADCP and Ekman current showed less influence of wind on current fields throughout the sector except at south; wind has a major role in the current generation, whereas along the 8-15° N sector, the remote forcing dominates over the wind.

Understanding the forcing mechanisms that determine the coastal ecosystem dynamics has always been a challenging topic of research. The Arabian Sea (AS) coastal ecosystem in the western half of the northern Indian Ocean is influenced by the seasonally reversing atmospheric forcing and structured by distinct water masses along its depth. Although AS differs greatly from its eastern counter-half, Bay of Bengal (BoB) in the thermohaline characteristics, the inter-basin exchanges of the water masses between these ocean basins add complexity to its hydrography. Hence, the present study was attempted to understand the influence of the intrusion of the BoB water in structuring the zooplankton community of the coastal AS. In the mixed layer depth, a conspicuous spatial heterogeneity was noticed in the distribution and abundance of the zooplankton community contributed by the cumulative influence of the higher phytoplankton biomass and the advection of population through BoB water intrusion regulated by the coastal circulation in the southern part. The study also assessed the interaction of the zooplankton community with the specific water masses defined by distinct thermohaline characteristics along the vertical profile of the AS coastal waters. Three distinct water masses were identified within the 200 m depth of AS each categorized by discrete salinity and density features. The distinct groups of the dominant taxon, Copepoda identified through redundancy analysis demonstrate the species-specific preferences of the community to different water mass characteristics in the AS. This study addressing how hydrographic characteristics shape the macroecology of zooplankton in the coastal AS, will add new insight in understanding the dynamic ecosystems of the northern Indian Ocean.

In this study, the influence of the spatial and temporal variability of upwelling intensity and the associated biological productivity observed during different phases of summer monsoon along the southwestern continental margin of India (SWCMI) on the δ13C and δ18O of the inorganic biogenic carbonate shells was investigated. Multispecies benthic bivalve shells (1–5 mm) separated from ten surface sediment samples of SWCMI (off 12°N, 10°N and 9°N) collected during the onset (OSM) and peak (PSM) phase of the summer monsoon of 2009 were analysed for δ13C and δ18O. Sea surface temperature along the study region indicates prominent upwelling in PSM than in OSM. A comparison of analytical and predicted values for δ18O in the bivalve shells confirmed their in situ origin during both the sampling periods. During PSM, the δ13C values in the benthic bivalve shells were more depleted in 13C than during OSM which recorded lower values of δ13C in dissolved inorganic carbon of bottom waters expected in the study region in PSM due to the upwelled waters, high surface productivity and the associated high degradation of the organic matter in the subsurface and bottom waters. However, this depletion of δ13C was not observed in benthic bivalve shells obtained from 10°N, since it is influenced by high export fluxes of carbon from the Cochin estuary since early monsoon months.

The processes of upwelling off the southern tip (the Kanyakumari coast) and the west coast of India are highly localised features with different forcing mechanisms, and they cannot be treated as a uniform wind-driven upwelling system. Off the Kanyakumari coast, upwelling is due to the south-west monsoon winds that are tangential to the coast. Along the west coast, the area between 8° N and 9° N represents the shadow zone to the influence of the remote forcing on the upwelling; the latter is forced by the longshore wind stress. Moderate to relatively intense upwelling occurs along the Kollam to Mangalore coast (9° N to 13° N) due to the combined action of the longshore wind stress, the coastally trapped Kelvin waves, and the offshore propagating Rossby waves. North of this area (13° N to 15° N), upwelling is weak due to weak wind stress and is closely confined to the coastal belt. This results from the suppressive effect of the southwards-flowing Arabian Sea high saline water on the process of upw...

Seasonal variation of the hydrography along the southeast Arabian Sea is described using data collected onboard FORV Sagar Sampada in September--October 2003 (later phase of Southwest monsoon, SWM) and March--April 2004 (Spring inter monsoon, SIM). During the later phase of the SWM, upwelling was in the withdrawal phase and the frontal structure was clearer in the northern sections (13 and 15 degrees N lat) indicating strong upwelling in the area. The driving force of upwelling is identified as the combination of alongshore wind stress and remote forcing with a latitudinal variability. Although a more prominent upwelling was found in the north, a maximum surface Chlorophyll-a was found in the south (10 degrees N). During the SIM, the area was characterized by oligotrophic water with relatively high Sea Surface Temperature (>29 degrees C) and low salinity (33.8 to 35.4). During March, the surface hydrography was found to be controlled mainly by the intrusion of low-saline waters from the south, while during September by the high saline water from the north. The presence of various water masses [Arabian Sea High Salinity Water (ASHSW), Persian Gulf Water (PGW), Red Sea Water (RSW)] and their seasonal variations in the region is discussed and their decreasing influence towards the south is noted during both periods of observation. During the SWM, the dynamic topography showed the equator-ward flow of the West India Coastal Current (WICC) at the surface and a pole-ward coastal under current at sub-thermocline depth. During the SIM, surface circulation revealed the WICC flowing pole-ward north of 13 degrees N, but equator-ward flow in the south, with a clockwise circulation around the Lakshadweep High.

Oceanic waters of the Northern Arabian Sea experience massive algal blooms during winter-spring (mid Feb-end Mar), which prevail for at least for 3 months covering the entire northern half of the basin from east to west. Ship cruises were conducted during winter-spring of 2001-2012 covering different stages of the bloom to study the biogeochemistry of the region. Phytoplankton analysis indicated the presence of green tides of dinoflagellate, Noctiluca scintillans (=N. miliaris), in the oceanic waters. Our observations indicated that diatoms are coupled and often co-exist with N. scintillans, making it a mixed-species ecosystem. In this paper, we describe an approach for detection of bloom-forming algae N. scintillans and its discrimination from diatoms using Moderate Resolution Imaging Spectroradiometer (MODIS)-Aqua data in a mixed-species environment. In situ remote sensing reflectance spectra were generated using Satlantic™ hyperspectral radiometer for the bloom and non-bloom waters. Spectral shapes of the reflectance spectra for different water types were distinct, and the same were used for species identification. Scatter of points representing different phytoplankton classes on a derivative plot revealed four diverse clusters, viz. N. scintillans, diatoms, non-bloom oceanic, and non-bloom coastal waters. The criteria developed for species discrimination were implemented on MODIS data and validated using inputs from a recent ship cruise conducted in March 2013.