UoH study finds low oxygen levels in the water off the Godavari River mouth

The study showed that Godavari river intensifies Oxygen-Depleted Zones (ODZ) or dead zone near its river mouth in the Bay of Bengal.
| Photo Credit: V. RAJU

Bay of Bengal, a vital fishing ground, is prone to seasonal hypoxia, a condition characterised by low oxygen levels in the water off the Godavari River mouth region immediately after the summer monsoon. This phenomenon may severely impact marine life, including fish kills, migration, and habitat degradation, says a new study led by researchers from the University of Hyderabad (UoH).

The study found that during the monsoon season, when the Godavari experiences peak discharge, large amounts of organic matter from dead trees/plants or soil are carried into the sea. The river water also carries significant amounts of nutrients to the coast that enhance phytoplankton production, said an official release.

It showed that Godavari river, country’s third largest (largest monsoonal river in India), intensifies Oxygen-Depleted Zones (ODZ) or dead zone near its river mouth in the Bay of Bengal. The work involved an analysis of river discharge data from the river at the Dowlaiswaram barrage and high-resolution biogeochemistry recorded Array for Real-Time Geostrophic Oceanography (ARGO) buoys in Bay of Bengal.

The buoys collect vertical profiles of various oceanic parameters from the surface through 2,000 metres deep by slowly sinking and resurfacing over 10 days. They record data such as temperature, salinity, dissolved oxygen, chlorophyll, etc., at a very high resolution.

Sreejith, now a scientist at the Council for Scientific and Industrial Research – National Institute of Oceanography (CSIR-NIO) Goa, carried out this work at the Centre for Earth, Ocean and Atmospheric Sciences, UoH during his tenure as a research associate under the mentorship of Prof. K. Ashok and Prof. Sreenivas P.

It showed that organic matter brought by rivers and that forms locally through phytoplankton production sink to a depth where they are decomposed. In this process, the dissolved oxygen in the water between 40 mts and 200 mts depths is microbially consumed, leading to severe depletion in oxygen levels or hypoxia.

This has potential implications for the fish catching several kilometres from the coast where the spread of river discharge from Godavari was observed. Since Godavari is rainfed, any decrease in monsoonal rainfall due will decrease river runoff, which in turn may improve the oxygen levels in leading to improved fishery catch.

Findings from the study suggest the importance of considering the impact of exacerbated human activities, such as river damming and water diversion, on coastal ecosystems. The authors caution that the results need further reconfirmation through dedicated data campaigns because ARGO buoys were about 50-100 km away from the river discharge recording point. Influence of other nearby rivers, such as the Krishna and Mahanadi rivers, cannot be entirely ruled out.

The work appeared in ‘Frontiers in Marine Science, section Marine Biogeochemistry’ and was accomplished in collaboration with UoH’s Feba Francis, CSIR-NIO Visakhapatnam V.V.S.S. Sarma and King Abdullah University of Science and Technology, Saudi Arabia’s Prof. Ibrahim Hoteit, added the release.