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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.

Stratified tropical oceanic systems are in general observed with Subsurface Chlorophyll Maxima (SCM), which was identified as light adaptation of shade-loving picophytoplankton groups. Apart from the light adaptation strategies of phytoplankton, the physical properties of water masses have significant role to hold the phytoplankton in particular layers. The present study gives theoretical explanation on the influence of fluid properties of water on the settling velocities of micro-nano phytoplankton groups, which contribute the SCM. The data and samples were collected from North Eastern Arabian Sea (NEAS) during March (Spring Inter Monsoon), by means of Bio-Argo floats, CTD and remote sensing. The analysis gives indications to the possible strengthening of SCM in the scenario of warming and enhanced stratification. The study conducted simulations using basic stoke’s equations on most abundant species of the area and found that the fluid density has a significant role in the settling of non-motile, suspended phytoplankton groups, irrespective of their cellular density. The simulations show strong decelerations at the same depths, in the upper part of the pycnocline but with varying settling velocity. A numerical expression derived based on curve fitting and multiple regression analysis substantiates the influence of vertical density on SCM. The sensitivity analysis (Global sensitivity Analysis) indicates warming trend in NEAS strengthening the stratification, which in turn influences the concentration in SCM and is capable of altering the primary production.

The phytoplankton standing crop was assessed in detail along the South Eastern Arabian Sea (SEAS) during the different phases of coastal upwelling in 2009. During phase 1 intense upwelling was observed along the southern transects (8°N and 8.5°N). The maximum chlorophyll a concentration (22.7 mg m −3) was observed in the coastal waters off Thiruvananthapuram (8.5°N). Further north there was no signature of upwelling, with extensive Trichodesmium erythraeum blooms. Diatoms dominated in these upwelling regions with the centric diatom Chaetoceros curvisetus being the dominant species along the 8°N transect. Along the 8.5°N transect pennate diatoms like Nitzschia seriata and Pseudo-nitzschia sp. dominated. During phase 2, upwelling of varying intensity was observed throughout the study area with maximum chlorophyll a concentrations along the 9°N transect (25 mg m−3) with Chaetoceros curvisetus as the dominant phytoplankton. Along the 8.5°N transect pennate diatoms during phase 1 were replaced by centric diatoms like Chaetoceros sp. The presence of solitary pennate diatoms Amphora sp. and Navicula sp. were significant in the waters off Kochi. Upwelling was waning during phase 3 and was confined to the coastal waters of the southern transects with the highest chlorophyll a concentration of 11.2 mg m−3. Along with diatoms, dinoflagellate cell densities increased in phases 2 and 3. In the northern transects (9°N and 10°N) the proportion of dinoflagellates was comparatively higher and was represented mainly by Protoperidinium spp., Ceratium spp. and Dinophysis spp.

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.