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 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 the 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, 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 at the Kolkata institute told Mongabay-India.

The Sundarbans study

Spread out across 10,000 square km, the Sundarbans, the largest single-track mangroves in the world, lies in the Ganga-Brahmaputra-Meghna delta and is shared by 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 the 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 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, said 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 Ganga-Brahmaputra-Meghna outflow.

The Indian Sundarbans archipelago acts as the “nursery” for nearly 90% of the aquatic species of the 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 the 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.

Map of the study area. Photo by researchers
Map of the study area. Photo by researchers

“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. [The] 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.

Sagar Island is battling soil erosion, breach of embankments and loss of landmass and rising sea levels. (Photo credit: Anwesha Ghosh)
Sagar Island is battling soil erosion, breach of embankments and loss of landmass and rising sea levels. (Photo credit: Anwesha Ghosh)

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.

The monsoon play

Estuarine ecosystems, that form a transition zone from rivers to seas, receive nutrients from a variety of sources, including riverine discharge.

Ghosh says that if you can track the microbial milieu and see what they are up to, then you can monitor the health of the ecosystem.

Bacterioplankton are indispensable regulators of global element cycles owing to their unique ability to decompose and remineralise dissolved organic matter.

“You can track any biological parameter but bacteria are very good because they are present in huge numbers and they are very precise. The precision level in bacteria is tremendous. Besides, they provide information on the elemental cycling, which is key to sustainable fisheries,” Bhadury said.

But the bacterioplankton communities have a whole lot of issues to grapple with in the Sundarbans – seasonal changes in surface water temperature, heavy local rains during monsoon, continuous flow of freshwater from Ganga-Brahmaputra-Meghna riverine systems, daily intrusion of saline water from the Bay of Bengal and dynamism of dissolved nutrients.

Seasonal influences such as increased freshwater flow during monsoon can flip the script on the stability of estuarine microbes and thereby on nutrient cycling.

“Increased freshwater flow alters prevailing environmental factors in coastal ecosystems and may lead to changes in bacterioplankton communities,” said study co-author Ghosh.

“This area gets freshwater from the local rainfall from the south-west monsoon and also from the rainfall received in the upper reaches of the Ganga, Meghna and Brahmaputra rivers. All of these rivers have a long course and get a lot of rain in the upstream and if the water volume increases upstream, it also goes up in the estuary,” Ghosh observed.

Monsoons have two kinds of effects. First, more nutrients become available due to river run off, which comes from both upstream of the river and also from local inputs of nutrients. But at the same time, the nutrients are diluted largely due to increase in volume of water.

This increase also causes a flushing action that could decrease the overall time the organism needs to interact and break down nutrients into simpler forms

Further, as sediment is shaken loose and washed out due to the rains, a lot of nutrients in different forms enter the water from the land.

“This change in mix of nutrients also influences the bacterial community structure. In addition, a lot of soil bacteria from the sediment shake-up is also pumped into the estuary water. This change in environment hampers the activity of the soil bacteria,” Ghosh explained.

All of this happens very fast during the monsoons.

“But post-monsoons, these effects nullify and the system becomes more stable. The steady supply of freshwater decreases and nutrients become more homogenous. Around December, the East India Coastal Current reverses pattern and brings in a lot of saline water into the Sundarbans. Additionally, the surface water temperature decreases,” Ghosh said.

So a bacterial population that is more marine in nature starts to emerge due to enhanced salinity.

“We have shown that the system becomes completely dominated by the Vibrio [typically marine] in December,” Ghosh said.

“In a stable estuarine environment where water stands for a longer time, if you look at the bacterial population, it has signatures of marine and freshwater and it has its own signature. This is because the plankton has enough time to get used to this intermediate salinity,” said Ghosh.

In contrast, when there is a diverse bacterioplankton community structure, the cycling of nutrients is very different when you have the system dominated by a specific set of organisms because this set of organisms would be using very specific nutrients. This hits the availability of food for the fish, she added.

Sagar Island is the largest island in the Indian Sundarbans archipelago. Photo credit: Anwesha Ghosh.
Sagar Island is the largest island in the Indian Sundarbans archipelago. Photo credit: Anwesha Ghosh.

The study said: “Drastic increase in sediment associated bacteria such as members of Firmicutes and Desulfovibrio was found in monsoon, hinting possible resuspension of sediment-dwelling bacteria into the overlying water column. Principal component analysis revealed dissolved ammonium and dissolved nitrate to account for maximum variation observed in the bacterioplankton community structure.”

Overall, the study showed that a “strong interplay exists between environmental parameters and observed variability in bacterioplankton communities as a result of precipitation which can ultimately influence processes and rates linked to coastal biogeochemical cycles.”

A recent study that sheds light on water-related risks in the Sundarbans and policy alternatives sheds light on concerns due to changes in salinity in the estuaries of the Indian Sundarbans.

The study states: “Owing to higher freshwater flows in the Hooghly estuary, salinity has decreased in that zone. As a consequence, fish species of low commercial value, particularly hilsa [Tenualosa ilisha], have increased significantly, but the commercially important taxa have become less abundant [Ghosh, 2015; Sinha, Mukhopadhyay, & Mitra, 1997]. The situation is different in the central portion of the Sundarbans. Salinity has increased in that area because connections to meltwater sources have been eliminated by heavy siltation of the Bidyadhari Channel [Banerjee,2013].”

This article first appeared on Mongabay.