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Recent studies have challenged the validity of the Redfield ratio. It is proposed that physical and biogeochemical processes govern the geographical variations in carbon : nitrogen : phosphorus (C : N : P) ratios. However, this proposal remains to be examined through concurrent observations of C : N : P ratios with physical and biogeochemical processes in various marine reservoirs. Here, we sampled the Arabian Sea for its C, N, and P content in organic and inorganic pools during the winter monsoon. We analyzed the role of convective mixing, eddies, and N2 fixation to explain the variation in observed elemental ratios. Convective mixing injected the cold water and enhanced the supply of N and P nutrients in the top layer (surface to 50–75 m) of the northern Arabian Sea. This led to a decrease in the N : P and C : P ratios in the particulate organic matter in the northern region, but C : N : P increased equatorward, averaging 245 : 32 : 1 in the top layer of the Arabian Sea. The variation in the elemental ratios in the top layer is best explained by the changes in water temperature. N2 fixation contributed negligibly to the N : P ratio of the export flux. The substantial decrease in N : P ratios of nutrients in the subsurface waters is most likely caused by the denitrifying conditions in the Arabian Sea. As the processes of convective mixing and eddies are are prevalent oceanic processes, our observations underpin that the interplay of these processes leads to changes in the elemental ratios globally. [ABSTRACT FROM AUTHOR]

We investigated the seasonal spreading and transport of buoyant plume in the shelf off Kochi using Finite Volume Community Ocean Model (FVCOM). The modelled river plume typically consisted of an offshore bulge and a coastal current. The spreading of the bulge extended up to a distance of 19 km from inlet during the summer monsoon to <10 km in the spring inter-monsoon. The Kelvin number varied between 0.1 and 0.9 which revealed that the plume exhibited both the features of small and large scale plumes, resulting in a highly complicated plume pattern. During the southwest monsoon the plume fringe twisted towards the south, while during the northeast monsoon it twisted towards north according to the reversal of monsoonal winds. The fresh water transport with respect to coastal currents varied in accordance with seasonal river discharge such that the value peaked in the wet season and dropped in the dry season. During the non-realistic (no wind) condition the plume initiated barotropic and baroclinic flow, after which it was acted upon by earth's rotation so that the plume propagated in the direction of Coriolis force (towards north), as geostrophic currents. The model run 'with wind' and 'without wind' condition revealed that in the shelf off Kochi the plume is transported in accordance with monsoonal winds/currents by nullifying the effect of earth's rotation. The categorization of plume influenced area and realization of the direction of plume transport can be used for interpreting the dynamically and potentially active zones in the shelf off Kochi. [ABSTRACT FROM AUTHOR]

Flow dynamics are a key part of the physical processes that drive the sediment transport in an aquatic system, affecting depositional processes and leading to landform evolution. The current asymmetry (CA) noted in Vembanad Lake (VL), India, have a particularly strong influence on the sediment budget and thus shape its geomorphology. In the present study the Coastal Ocean Model - Finite Volume Coastal Ocean Model (FVCOM) was used to compute the lateral current asymmetries and their implications for the long-term morphological changes in VL during different tidal phases. Model results were utilised to calculate the CA using two criteria: (1) Peak flood/ebb velocities with duration, and (2) Asymmetry Index (AI DV). AI DV values suggest the prevalence of ebb dominance over the entire system except at Cherai and Vallarpadam. However, several isolated pockets were noted on the right/eastern bank (shallow depth) of the system where the peak flood velocity exceeded the ebb, favouring accretion. Contrary to the concept of ebb asymmetry, mean velocity and duration asymmetries were prevalent in the asymmetry index (AI DV); hence we adopted peak flood/ebb velocity and its duration asymmetries to improve our understanding of the sediment transport dynamics. Typical flood asymmetry was identified on the right bank and at many of the lateral stations in VL, which switched to the modified ebb asymmetry during substantial river discharge. Analysis of CA enables understanding of the process that influences selective morphological changes, as explained by the model results. This conclusion was well corroborated by results obtained over the past three decades (1973–2008) from analysis of Landsat satellite imagery. Flood/ebb dominance in VL is explicitly substantiated with regions of accretion/erosion derived from satellite imageries. A holistic approach encompassing modelling, observations, remote sensing, and GIS techniques were used to address the erosion-accretion process in VL. •Vembanad Lake (VL) experiences ebb-flood asymmetry laterally. •Several isolated pockets in the right bank of the VL where the peak flood velocity exceeds the ebb favoring accretion. •Modified asymmetries for longer period alter sediment dynamics and geomorphology of the VL. •Long-term erosion and accretion zones identified using Landsat imageries well corroborated with modelling results. [ABSTRACT FROM AUTHOR]

Estuaries with poor flushing and longer residence time retain effluents and pollutants, ultimately resulting in eutrophication, a decline in biodiversity and, finally, deterioration of water quality. Cochin Estuary (CE), southwest coast of India, is under the threat of nutrient enrichment by the anthropogenic interventions and terrestrial inputs through land runoff. The present study used the FVCOM hydrodynamic model coupled with the Lagrangian particle module (passive) to estimate the residence time and to delineate site-specific transport pathways in the CE. The back and forth movements and residence time of particles was elucidated by using metrics such as path length, net displacement and tortuosity. Spatio-temporal patterns of the particle distribution in the CE showed a similar trend during monsoon and post-monsoon with an average residence time of 25 and 30 days, respectively. During the low river discharge period (pre-monsoon), flood-ebb velocities resulted in a minimum net transport of the water and longer residence time of 90 days compared to that of the high discharge period (monsoon). During the pre-monsoon, particle released at the southern upstream (station 15) traversed a path length of 350 km in 90 days before being flushed out through the Fortkochi inlet, where the axial distance was only 35 km. This indicates that the retention capacity of pollutants within the system is very high and can adversely affect the water quality of the ecosystem. However, path length (120 km) and residence time (7.5 days) of CE were considerably reduced during the high discharge period. Thus the reduced path length and the lower residence time can effectively transport the pollutants reaching the system, which will ultimately restore the healthy ecosystem. This is a pioneer attempt to estimate the flushing characteristics and residence time of the CE by integrating the hydrodynamics and Lagrangian particle tracking module of FVCOM. This information is vital for the sustainable management of sensitive ecosystems. [ABSTRACT FROM AUTHOR]

Alappuzha mud bank draws special attention among the twenty-mud bank locations reported along the Kerala coast by its remoteness from riverine sources. Among several hypotheses proposed for the formation of mud bank, the subterranean hypothesis was most accepted because of the occurrence of low salinity in the bottom layers. The present study provides evidence to show that occurrence of low salinity waters near the bottom in the mud bank region is an artifact of measuring technique employed for the measurement of salinity. The usual technique of conductivity based salinity determination completely fails in the presence of water laden with high amount of suspended sediment. Laboratory experiments were conducted to determine the response of electrode and conductivity cell sensor types to determine the salinity using a range of suspended sediment in the water column. Actual sediment samples from the mud bank region were utilized for the above studies. Based on field observations and experiments, we conclude that the low salinity was the manifestation of the presence highly turbid fluid mud formation in the mud bank region rather than the influence of fresh water. [ABSTRACT FROM AUTHOR]

Microplastic pollution and the impacts they generate on the marine ecosystem and its biota is a major global concern of recent decades. The present study was conducted to evaluate the spatio-temporal distribution of microplastics in the surface waters, sediments, and their subsequent ingestion by the commercially important fishes of Alappuzha Mud banks, a transient ecosystem formed in the littoral zones of the southwest coast of India exclusively during the Indian summer monsoon. Sampling conducted over three periods, Pre-mud bank (Pre-MB), Mud bank (MB), and Post mud bank (Post-MB) extending over three depths (2 m, 5 m and 18 m), along the semi-circular patch of mudbanks revealed marked spatio-temporal variability in microplastic distribution. In both surface water and sediments, microplastic concentration was comparatively high during MB than in Pre-MB and Post-MB periods. Spatially, during MB, the microplastic concentration was high at 5 m where the dampening of waves occurred concomitant to the thick fluid mud formation. In contrast, during Post-MB, with the subsequent dissipation of MB's and less wave dampening, the microplastics aggregated at 5 m were transported to both inshore (2 m) and offshore (18 m), thus raising their concentration at these depths. Likewise, the microplastic ingestion was more in fishes caught during MB (41%) than Post-MB (30%) and Pre-MB (29%) periods indicating increased uptake corresponding to the higher incidences in their ambient environment. Microplastic ingestion was more among pelagic planktivores, S. gibbosa (38%), A. chacunda (20%) and R. kanagurta (13%) compared to the demersal fishes. White coloured fragments of size 1–5 mm of polypropylene were the dominant microplastic in the surface waters, sediment and fishes analysed. The present study indicates the critical role of wind speed, rainfall, wave patterns, and the fluid muddy environment in regulating the microplastics distribution in a transient ecosystem formed along the southwest coast of India. • Microplastic distribution in Mudbank showed conspicuous spatio-temporal variability. • Microplastics were high in water, sediment and fishes during Mudbank period. • Microplastics aggregated at 5 m depth due to wave dampening elicited by mud banks. • White coloured polypropylene fragments of size 1–5 mm were the most abundant one. [ABSTRACT FROM AUTHOR]

Mangroves are one of the world threatened ecosystems, which is protected and categorised under CRZ-I according to the Coastal Regulation Zone (CRZ) notification. The present study marked the decadal (1990–2017, Landsat series) change of mangroves and mapped the species distribution (2017, Sentinel-2) around Thane creek, Mumbai using SMLC with QGIS-SCP software. ADCIRC model was used to simulate the tidal flooding/ebbing towards mangroves, and the model was validated with the in-situ observation of tides and currents. From 1990 to 2017, the mangrove area increased from 79.14 km2 to 154.5 km2 and expanded towards landward and seaward regions. Between 2010 and 2017, there was a fluctuation in the mangrove area with ups and downs in their distribution caused by different climatic conditions and anthropogenic activities. The increase in built-up area was two times than mangrove area increase. The dominance of Avicennia marina species confirms the increased anthropogenic activity along the landward side. To understand the response of mangroves to the tidal dynamics, the model was simulated with two scenarios; with mangroves and without mangroves. In the absence of mangroves (without mangroves), Thane Creek demonstrated a perfect funnelling effect by increased tidal amplification with strong flood currents, resulting in severe coastal inundation and erosion. While with mangroves , tidal currents attenuated 80% of its magnitude and favoured the sediment deposition in mangrove areas, which further enhanced mangrove growth, thereby stabilising the coast. The present study has found that the mangroves in the Thane creek are in the saturation stage and reached the maximum growth towards the landward. The study envisages that the stakeholders should adopt a proper management plan for the restoration of mangroves. The highlight of the present study is the combination of satellite imagery and hydrodynamic modelling. Satellite imagery provides the estimated extent of the mangroves, and this information will be used when simulating the model, which provides maximum inundation and attenuation. The utilisation of LULC classification and ADCIRC model results would be helpful to examine the threatening built-up area due to the tidal inundation. The combination of satellite classification and hydrodynamic model findings will be decisive for management authorities towards the conservation and regeneration activities for a balanced mangrove-creek ecosystem. • Mangrove forest has almost doubled from 1990 (79.14 km2) to 2017 (154.5 km2) with regeneration towards landward and seaward. • The mangroves attenuate 80% of the tidal current magnitude and enhance sediment deposition. • Mangroves of Thane creek are in the saturation stage, with urbanisation limiting the growth of mangroves towards landward. • The dominance of Avicennia marina species indicates the severity of pollution and urbanisation. • LULC classification and ADCIRC model results would be helpful to examine the risk to the built-up areas due to tidal surges. [ABSTRACT FROM AUTHOR]