• V V S S SARMA

Articles written in Journal of Earth System Science

• A revisit to the regulation of oxygen minimum zone in the Bay of Bengal

Occurrence of intense oxygen minimum zone (OMZ) is known in the Bay of Bengal (BoB), but it has been recently reported to have become more acute and is at its tipping point. Here, we show that the intensification of OMZ to acute condition is a random and short-term rather than perennial phenomenon based on re-evaluation of old and recent information in the BoB. Short-term modifications in dissolved oxygen (DO) in the OMZ are caused by balance among physical forcings: salinity stratification, occurrence of cyclonic (CE), and anticyclonic eddies (ACE). Our analysis reveals that ‘acute OMZ’ is only a transient phenomenon in the Bay since the dynamic periodic physical forcings, particularly ACEs, do not allow it to become a dead zone.

• Phosphate removal through adsorption on suspended matter in the Bay of Bengal: Possible implications to primary production

Low primary production in the Bay of Bengal (BoB) could not be explained as this region receives nutrients from atmospheric deposition, rivers, eddies, and mixing due to depression/cyclones. In addition to nutrients, BoB also receives significant amount of suspended particulate matter (SPM) from the major rivers and their concentrations are in an order of magnitude higher than elsewhere in the open ocean region. Here we show that the removal of phosphorus (P) through adsorption on SPM may be a potential mechanism to decrease in primary production in the northern BoB. Significant fraction of P removal (5–50% of total dissolved inorganic phosphate) in association with SPM was observed in the BoB. The magnitude of removal of P through SPM is linearly related with dissolved inorganic phosphate (DIP), particulate organic carbon (POC) in the water column suggesting that P is removed in association with organic matter. The fraction of P removed from that of DIP showed inverse relation with salinity, and linear relation with SPM concentration suggesting that SPM brought by river discharge removed P from the water column. The P removed by SPM in the mixed layer showed inverse relation with mixed layer integrated primary production in the open sea region but its impact is negligible in the coastal waters. The laboratory experiment by measuring primary production in the presence of different quantities of SPM concentrations confirmed decrease in primary production due to removal of phosphate in the BoB.

$\bf{Highlights}$

$\bullet$ Significant fraction of phosphate adsorbs on to the suspended particles and it is bio-non-available.

$\bullet$ Phosphate severely controls primary production in the northern Bay of Bengal.

$\bullet$ The removal of phosphate is more in the coastal waters, it does not control primary production due to higher input than removal.

$\bullet$ High N/P ratios is caused by phosphate removed on association with suspended matter.

• 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 130, 2021
All articles
Continuous Article Publishing mode

• # Editorial Note on Continuous Article Publication

Posted on July 25, 2019