– Study by researchers from University of Hyderabad, CSIR-NIO, Vizag and King Abdulla University of Science and Technology, Saudi Arabia
A new study, which appeared in Frontiers in Marine Science, section Marine Biogeochemistry on 4 Nov. 2024, sheds light on the complex interplay between riverine discharge and marine ecosystems in the coastal central Bay of Bengal, revealing that the Godavari River, India’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. While the impact of major rivers like the Ganges and Brahmaputra on the Bay of Bengal has been extensively studied, this latest research underscores the importance of relatively smaller River Godavari, which can also have a significant impact on the intensification of ODZ.
Read the paper at: https://www.frontiersin.org/journals/marine-science/articles/10.3389/fmars.2024.1419953/full
Dr. Sreejith, now a scientist at the Council for Scientific and Industrial Research – National Institute of Oceanography (CSIR-NIO) Goa, largely carried out this work at the Centre for Earth, Ocean and Atmospheric Sciences, University of Hyderabad (UoH) during his tenure as a Research Associate under the mentorship of Prof. K. Ashok and Prof. Sreenivas P This work was accomplished in collaboration with Dr. Feba Francis from the UoH, Dr. V.V.S.S. Sarma of the CSIR-NIO, Visakhapatnam, and Prof. Ibrahim Hoteit of the King Abdulla University of Science and Technology, Saudi Arabia.
The outcome from the work shows that the Bay of Bengal, a vital fishing ground, is prone to seasonal hypoxia, a condition characterized 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. The study found that during the monsoon season, when the Godavari River experiences peak discharge, large amounts of organic matter from dead trees/plants or soil are carried into the Bay. In addition to this, river water also carries significant amounts of nutrients to the coast that enhance phytoplankton production. Both 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 and 200 m depths is microbially consumed, leading to severe depletion in oxygen levels (referred to as hypoxia) in the Bay. This has potential implications for the fish catch several 10s of kilometers from the coast where the spread of river discharge from Godavari was observed, and intensive fishing is normally carried out. Since Godavari is rainfed, any decrease in monsoonal rainfall due to interannual climate drivers such as the ENSO, Indian Ocean Dipole, etc. will decrease river runoff, which in turn may improve the oxygen levels in the ODZ leading to improve fishery catch.
This work involved an analysis of river discharge data from the River Godavari recorded at the Dowlaiswaram barrage, World Ocean Atlas 2018 data, and critically, high-resolution biogeochemistry recorded Array for Real-Time Geostrophic Oceanography (ARGO) buoys in the Bay of Bengal off Godavari River mouth region. The ARGO buoys are state-of-the-art buoys that collect vertical profiles of various oceanic parameters from the surface through 2000 m deep by slowly sinking and resurfacing over 10 days. These buoys record valuable data such as temperature, salinity, dissolved oxygen, chlorophyll, etc., at a very high resolution, which, as can be seen, varies vertically and over time.
The authors emphasize the importance of understanding the complex interplay between riverine input, ocean circulation, and biological processes in driving hypoxia, which is mainly natural. Since these rivers are rainfed, the decrease in rainfall or weakening of monsoon intensity due to interannual climate drivers such as the ENSO, Indian Ocean Dipole, etc., may decrease river runoff that may improve the oxygen levels in the ODZ leading to improve fishery catch. Having said that, the findings from this study, however, suggest the importance of considering the impact of exacerbated human activities, such as river damming and water diversion, on coastal ecosystems. Furthermore, both natural and human induced climate change can affect the monsoonal rainfall and therefore river runoff. By gaining a deeper understanding of the processes involved, scientists and policymakers can work towards mitigating the negative effects of hypoxia and preserving the health of the Bay of Bengal.
The authors caution that the results need further reconfirmation through dedicated data campaigns because the ARGO buoys were about 50-100 km away from the Godavari River discharge recording point. To be sure, the influence of other nearby rivers, such as the Krishna and Mahanadi rivers, cannot be entirely ruled out. Dedicated data campaigns are needed to confirm the findings from the study, and examine in detail the implications for the fish catch.
This study was mainly funded by the Institution of Eminence (IoE) scheme, University of Hyderabad, for the project (UOH/IoE/RC1/20-015) entitled “Sub-seasonal through Interannual Variability of river discharge and its Impact on phytoplankton biomass in the Bay of Bengal”.
