Articles written in Journal of Earth System Science

• Suppressed biological production in the coastal waters off Visakhapatnam, India under the impact of the very severe cyclonic storm Hudhud

Tropical cyclones generally enhance biological production due to the increase in nutrients input due to vertical mixing. In contrast, the very severe cyclonic storm (VSCS)Hudhud decreased primary production due to the strong stratification associated with torrential rainfall and high suspended load from the major city where the cyclone made landfall. The study region received nutrients from the cold core eddy and coastal upwelling in the offshore and inshore regions, respectively, during pre-cyclone period and the same was suppressed under the influence of cyclonic winds led to convergence by shoreward Ekman transport. The land run-off brought nutrients to the coast during cyclone Hudhud; however, their concentrations were less than that during other cyclones (Orissa supercyclone, Sidr and Phailin). Such low nutrient levels resulted from the VSCS Hudhud crossing the urban region (Visakhapatnam city) whereas other cyclones crossed the coast over fertile agricultural lands which led to high nutrients input associated with phytoplankton blooms. Therefore, the biological response to a cyclone not only depends on the intensity of the cyclone but also on the region of the land it crosses.

• Influence of phytoplankton pigment composition and primary production on pCO$_{2}$ levels in the Indian Ocean

The tropical Indian Ocean is a net sink for the carbon dioxide (CO$_{2}$) in the atmosphere and phytoplankton production plays a crucial role in CO$_{2}$ fixation and determines the direction of CO$_{2}$ flux at the sea-to-air interface. In order to assess the influence of phytoplankton composition and primary production on pCO$_{2}$ levels in the Indian Ocean, sampling was conducted at 25 stations during the spring intermonsoon period under the auspices of the Indian GEOTRACES program. The pCO$_{2}$ was significantly correlated with salinity due to the discharge of low pCO$_{2}$ water by major rivers to the Bay of Bengal (BoB). The stronger negative correlation observed between pCO$_{2}$ and major phytoplankton marker pigments, net primary production and oxygen saturation levels suggesting significant influence of biological processes on pCO$_{2}$ levels in the Indian Ocean. This study indicates that pCO$_{2}$ levels are strongly modulated by biological processes than hitherto hypothesized as solubility pump in the Indian Ocean.

$\bf{Highlights}$

$\bullet$ Surface pCO$_{2}$ levels are undersaturated in the Indian Ocean compared to atmosphere, except Arabian Sea.

$\bullet$ Oligotrophic conditions prevailed in the entire tropical Indian Ocean.

$\bullet$ Picophytoplankton (cyanobacteria) is the dominant phytoplankton in the Indian Ocean.

$\bullet$ Significant relation between phytoplankton groups and pCO$_{2}$ indicates strong biological control on surface pCO$_{2}$.

• Journal of Earth System Science

Volume 132, 2023
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• Editorial Note on Continuous Article Publication

Posted on July 25, 2019