By Sahana Ghosh | IANS
They may be microscopic in size, but bacterioplankton steer major carbon cycling and food webs in aquatic ecosystems. Bacterioplankton are the bacterial component of plankton that live in water bodies, including oceans and lakes.
Now, scientists at the Indian Institute of Science Education and Research (IISER), Kolkata are keeping an eye out for bacterioplankton and how they chow down and use carbon, in the world’s largest delta, the Sundarbans, to track changes in freshwater flow that could have implications for sustainability of coastal fisheries.
Explaining the premise for this microbe-based monitoring, they reason that when environmental parameters change (such as influx of fresh or saline water), it throws the existing bacterial community structure into turmoil.
This disturbance, in turn, shapes up how nutrients are broken down in the system and made available through the food chain to the organisms higher up in the web, such as fish.
Aiding the team in their observations is a time series, the Sundarbans Biological Observatory Time Series (SBOTS), which maps data monthly to pinpoint trends in the health of the Sundarbans ecosystem.
“Understanding elemental cycling (such as carbon cycling) is a good way to understand the health of the ecosystem. I believe this is the only mangrove time series in India if not in Asia,” Punyasloke Bhadury of the Centre for Climate and Environmental Studies (CCES) at IISER, Kolkata told Mongabay-India.
Spread out across 10,000 square km, Sundarbans, the largest single-track mangroves in the world, lies in the Ganga-Brahmaputra-Meghna (GBM) delta and is shared between India and Bangladesh.
This mangrove ecosystem encompasses over 102 islands in the Indian side (in the state of West Bengal) with a network of innumerable rivers, rivulets and creeks.
The study stations for the time series are located in the 223.4 square km Sagar Island, the largest compact island of Sundarbans that sits at the confluence of the Hooghly river (a branch of the Ganga) and the Bay of Bengal.
The shrinking island lies 6.7 metres above sea level and is battling soil erosion, breach of embankments and loss of landmass and rising sea levels.
“Based on our preliminary studies, we believe that freshwater flow in the Sundarbans (at Sagar) is now starting to change and these changes are now starting to show clearly through the level of changes happening at the bacterial community structures,” Bhadury said on the sidelines of the international Land-Ocean-Atmosphere workshop.
At this year’s workshop convened by Bhadury at the institute, it was discussed how understanding the land-ocean-atmosphere interactions aids in management of terrestrial and coastal resources nationally to maximise benefits to people’s livelihood as well as environmental protection.
A key example of such dynamics, informed Bhadury and colleague Anwesha Ghosh, is the land-ocean boundary-based coastal ecosystem in the Sundarbans, a region that experiences the third largest river discharge in the world through the Ganges-Brahmaputra-Meghna outflow.
The Indian Sundarbans archipelago acts as the “nursery” for nearly 90 percent of the aquatic species of eastern coast of India. In Sagar, fisheries rank second after agriculture in terms of livelihood.
Based on their research, the scientists say they now have some idea of how carbon cycling is taking place in the system and that the system in Sagar Island is now “probably transitioning to a marine system from an estuarine system.”
“The reason why we set up monthly monitoring since 2010 in the time series is so that we have long-term data to determine the exact changes,” explained Ghosh.
“What makes it more complex for the Sundarbans is that if you look at all the seven estuaries, each of them is very different. The freshwater inputs differ from the eastern part to the west. Eastern part of the Sundarbans are more protected (very minimal human interference) so the kind of nutrients you would see from anthropogenic sources is different than that you would see in the western part,” Ghosh said.
Sustainability practitioner Anurag Danda, who was not involved in the study, emphasised on the variability across the estuaries.
“Yes, at this location the system may be transitioning into a marine system but it may not be the case in Meghna estuary at the eastern extreme due to higher volume of freshwater flow,” Danda, senior advisor to WWF, told Mongabay-India.
Tracking freshwater flow is important, said Danda, as deltas are highly sensitive to changes in freshwater flows.
“Progressive layering of sediment over time results in the build-up of delta land mass that will continue to build seaward — as long as rates of erosion do not exceed rates of sedimentation. Reduced freshwater flows reduce the rate of sedimentation thereby impacting the physical integrity of a delta. We are already witnessing this on the south western sea-facing islands in the Sundarbans,” Danda told Mongabay-India.
Under rising sea level conditions, freshwater flows become even more important, he said.
Reduced freshwater flows result in the saline front being pushed further inland and reduction in mangrove area because of man-made barriers to successful migration of mangroves. The man-made freshwater agro-ecosystem further inland also witnesses falling productivity due to increased soil and water salinity, he said.
“In addition, there is a change in species composition on land and in water. This is not problematic by itself but the change could result in replacement of commercially important species by less important species,” he said.
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