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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]

Research indicates that the Bay of Bengal contains a two-gyre circulation system, one cyclonic and the other anticyclonic. The chlorophyll maximum was found to be almost twice as high in the cyclonic region than in the anticyclonic region, although its depth is larger in the anticyclonic region.

Upwelling enhances pCO2 levels due to injection of carbon‐rich water to the surface despite the removal of carbon due to increase in primary production supported by enhanced nutrients. It is hypothesized that in the Bay of Bengal, upwelling may decrease pCO2 due to existence of low saline and pCO2‐poor waters in the subsurface layer. In order to test this hypothesis, a high‐resolution state‐of‐the‐art ocean biogeochemical model (Regional Ocean Modeling System) runs are examined at the sea east of Sri Lanka (SESL) where intense upwelling occurs during summer monsoon (May to August). Upwelling enhances pCO2 by 34 μatm, whereas decrease in surface temperature and increase in surface salinity reduce pCO2 by 24 μatm. The estimated net effect of upwelling is an increase in pCO2 by 10 μatm. In contrast, soft and hard tissues together contribute to a decrease in pCO2 by 21 μatm suggesting that the biological effect dominates over upwelling, resulting in a net decrease of pCO2 by 11 μatm in the SESL. This striking contrast between the increase in pCO2 due to physical dynamics (upwelling) and the removal of pCO2 due to biological processes is caused by shallow (deep) nitracline (dissolved inorganic carbon‐isoline) in the SESL. Key Points: Phytoplankton bloom, in response to upwelling, decreases pCO2 in sea east of Sri Lanka (SESL) during summer monsoonRelatively shallow nitracline and deep DIC‐cline is observed in the SESLRemoval of pCO2 due to biological production dominates increase of pCO2 due to upwelling in SESL [ABSTRACT FROM AUTHOR]

Carbon isotopic composition of dissolved inorganic carbon (δ 13 C DIC ) in the Cochin estuary, a tropical eutrophic estuary along the southwest coast of India, and the adjacent coastal Arabian Sea was measured to understand spatio-temporal variability in sources and processes controlling inorganic carbon (C) dynamics in this estuarine-coastal system. δ 13 C DIC in the Cochin estuary showed wide variation during three different seasons (premonsoon: − 12.2 to − 3.26‰; monsoon: − 13.6 to − 5.69‰; and postmonsoon: − 6.34 to + 0.79‰). Detailed mixing curve approximation modeling along with relationships of δ 13 C DIC with dissolved oxygen and nutrients suggest dominant role of freshwater mixing and degassing of CO 2 on DIC dynamics during wet seasons (premonsoon and monsoon). Excess CO 2 brought in by rivers and in situ production due to respiration in the Cochin estuary result into one of the highest pCO 2 observed in estuarine systems, leading to its degassing. During postmonsoon, a relatively dry period with high salinity, calcite precipitation was a major process with calcite saturation index > 1 at few locations. Relatively lower average surface values of δ 13 C DIC in the coastal Arabian Sea (premonsoon: + 0.95‰; monsoon: + 0.88‰; and postmonsoon: + 0.66‰) compared to the predicted open ocean value along with mixing curve modeling suggest dominance of respiration/organic matter (OM) degradation over primary productivity. Estuarine influence on coastal DIC dynamics was observed in nearshore region (~ 10 km), whereas evidence of upwelling was found at farther locations. [ABSTRACT FROM AUTHOR]