Covering approximately 10,000 km2 the Sundarbans in the Northern Bay of Bengal is the largest contiguous mangrove forest on earth. Mangroves forests are highly productive and diverse ecosystems, providing a wide range of direct ecosystem services for resident populations. In addition, mangroves function as a buffer against frequently occurring cyclones; helping to protect local settlements including the two most populous cities of the world, Kolkata and Dhaka, against their worst effects. While large tracts of the Indian Sundarbans were cleared, drained and reclaimed for cultivation during the British colonial era, the remaining parts have been under various protection regimes since the 1970s, primarily to protect the remaining population of Bengal tigers (Panthera tigris ssp. tigris). In view of the importance of such forests, now severely threatened worldwide, we trace the areal change that the Indian Sundarbans have undergone over the last two-and-a-half centuries. We apply a multi-temporal and multi-scale approach based on historical maps and remote sensing data to detect changes in mangrove cover. While the mangroves’ areal extent has not changed much in the recent past, forest health and structure have. These changes result from direct human interference, upstream development, extreme weather events and the slow onset of climate change effects. Moreover, we consider the role of different management strategies affecting mangrove conservation and their intersection with local livelihoods.

The Indian Sundarban Mangrove Forests: History, Utilization, Conservation Strategies and Local Perception

Studies on biogeochemical cycling of carbon in the Chilka Lake, Asia’s largest brackish lagoon on the east coast of India, revealed, for the first time, strong seasonal and spatial variability associated with salinity distribution. The lake was studied twice during May 2005 (premonsoon) and August 2005 (monsoon). It exchanges waters with the sea (Bay of Bengal) and several rivers open into the lake. The lake showed contrasting levels of dissolved inorganic carbon (DIC) and organic carbon (DOC) in different seasons; DIC was higher by ∼22% and DOC was lower by ∼36% in premonsoon than in monsoon due to seasonal variations in their supply from rivers and in situ production/mineralisation. The DIC/DOC ratios in the lake during monsoon were influenced by physical mixing of end member water masses and by intense respiration of organic carbon. A strong relationship between excess DIC and apparent oxygen utilisation showed significant control of biological processes over CO2 production in the lake. Surface partial pressure of CO2 (pCO2), calculated using pH–DIC couple according to Cai and Wang (Limnol and Oceanogr 43:657–668, 1998), exhibited discernable gradients during monsoon through northern (1,033–6,522 μatm), central (391–2,573 μatm) and southern (102–718 μatm) lake. The distribution pattern of pCO2 in the lake seems to be governed by pCO2 levels in rivers and their discharge rates, which were several folds higher during monsoon than premonsoon. The net CO2 efflux, based on gas transfer velocity parameterisation of Borges et al. (Limnol and Oceanogr 49(5):1630–1641, 2004), from entire lake during monsoon (141 mmolC m−2 d−1 equivalent to 2.64 GgC d−1 at basin scale) was higher by 44 times than during premonsoon (9.8 mmolC m−2 d−1 ≈ 0.06 GgC d−1). 15% of CO2 efflux from lake in monsoon was contributed by its supply from rivers and the rest was contributed by in situ heterotrophic activity. Based on oxygen and total carbon mass balance, net ecosystem production (NEP) of lake (−308 mmolC m−2 d−1 ≈ −3.77 GgC d−1) was found to be almost in consistent with the total riverine organic carbon trapped in the lake (229 mmolC m−2 d−1 ≈ 2.80 GgC d−1) suggesting that the strong heterotrophy in the lake is mainly responsible for elevated fluxes of CO2 during monsoon. Further, the pelagic net community production represented 92% of NEP and benthic compartment plays only a minor role. This suggests that Chilka lake is an important region in biological transformation of organic carbon to inorganic carbon and its export to the atmosphere.

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Influence of net ecosystem metabolism in transferring riverine organic carbon to atmospheric CO2 in a tropical coastal lagoon (Chilka Lake, India)

The paper utilized Landsat 5 TM and Landsat 8 OLI for analyzing land use/land cover change and its impact on land surface temperature in Sundarban Biosphere Reserve, India. Split window algorithm and spectral radiance model were used for determining land surface temperature from Landsat 8 OLI and Landsat 5 TM, respectively. The land use land cover change analysis revealed phenomenal increase in the waterlogged areas followed by settlement and paddy and a decrease in open forest followed by deposition and water body. The distribution of average change in land surface temperature shows that water recorded highest increase in temperature followed by deposition, open forest and settlement. Overlay of the transect profiles drawn on land use/land cover change map over land surface temperature map revealed that the land surface temperature has increased in those areas which were transformed from open forest to paddy, open forest to settlement, paddy to settlement and deposition to settlement. The study demonstrated that increase in non-evaporating surfaces and decrease in vegetation have increased the surface temperature and modified the temperature of the study area.

/pdf/analyzing-land-surface-temperature-distribution-in-response-3at6honcib.pdf

Analyzing land surface temperature distribution in response to land use/land cover change using split window algorithm and spectral radiance model in Sundarban Biosphere Reserve, India

In August 2018, the Indian state of Kerala received an extended period of very heavy rainfall as a result of a low-pressure system near the beginning of the month being followed several days later by a monsoon depression. The resulting floods killed over 400 people and displaced a million more. Here, a high resolution setup (4 km) of the Weather Research and Forecasting (WRF) model is used in conjunction with a hydrological model (WRF-Hydro, run at 125 m resolution) to explore the circumstances that caused the floods. In addition to a control experiment, two additional experiments are performed by perturbing the boundary conditions to simulate the event in pre-industrial and RCP8.5 background climates. Modelled rainfall closely matched observations over the study period, and it is found that this would this would have been about 18% heavier in the pre-industrial due to recent weakening of monsoon low-pressure systems, but would be 36% heavier in an RCP8.5 climate due to moistening of the tropical troposphere. Modelled river streamflow responds accordingly: it is shown the six major reservoirs that serve the state would have needed to have 34% more capacity to handle the heavy rainfall, and 43% had the deluge been amplified by an RCP8.5 climate. It is further shown that this future climate would have significantly extended the southern boundary of the flooding. Thus it is concluded that while climate change to date may well have mitigated the impacts of the flooding, future climate change would likely exacerbate them.

/pdf/the-2018-kerala-floods-a-climate-change-perspective-1b3imuxgu0.pdf

The 2018 Kerala floods: a climate change perspective

Climate change induced sea level rise (SLR) added with anthropogenically altered environment leads to rapid land dynamics in terms of erosion and accretion; and alteration in species diversity and productivity, more pronouncedly in sensitive ecosystems such as river deltas. Here, we tried to analyze the historical records to understand the SLR with respect to hydrological conditions, sedimentation and morphological processes. We analyzed the land transformation of few islands in Indian Sunderbans using maps and satellite images in increasing order of temporal frequency between 1924 and 2008, which revealed that both the erosion and accretion processes go hand in hand. Increase of downstream salinity due obstruction in upstream has led to decrease in transparency of water causing decrease in phytoplankton and fish, density and diversity in the central sector of Indian Sunderbans. Analysis of the above ground biomass of three dominant mangrove species (Sonneratia apetala, Avicennia alba and Excoecaria agallocha) revealed better growth in the western sector compared to the central sector. The study reveals the cumulative effect of climate change and anthropogenic disturbance on the diversity and productivity in World’s largest ecosystem; and advocates mangrove plantation and effective management of freshwater resources for conservation of the most vulnerable and sensitive ecosystem.

Climate change impacts on Indian Sunderbans: a time series analysis (1924–2008)

We present evidence that the Indian Sundarbans is experiencing the effects of climate change over the last three decades. Observations of selected variables, such as the surface water temperature, salinity, pH, dissolved oxygen, and transparency show significant long-term variation over a period of 27 years (1980-2007). Specifically, the temperature in these waters has risen at the rate of 0.5°C per decade, much higher than that observed globally or for the Indian Ocean. Increasing melting of Himalayan ice might have decreased the salinity at the mouth of the Ganges River, at the western end of this deltaic complex. At the same time, salinity has increased on the eastern sector, where the connections to the meltwater sources have become extinct due to heavy siltation of the Bidyadhari Channel. The long-term changes in dissolved oxygen, pH level, transparency and water quality are also examined. The ecological impact of such changes warrants future study.

Observed changes in water mass properties in the Indian Sundarbans (northwestern Bay of Bengal) during 1980-2007.

On 17th June 2013 the state of Uttarakhand in India (Latitude 28.72°N to 31.45°N and Longitude 77.57°E–81.03°E) received more than 340 mm of rainfall, which is 375% more than the daily normal (65.9 mm) rainfall during monsoon. This caused heavy floods in Uttarakhand as well as unprecedented damage to life and property. In this study we aim at assessing the performance of two deterministic forecast models, Global Forecast System (GFS/T574) and Unified Model (NCUM), run at NCMRWF, in predicting the heavy rainfall observed over Uttarakhand region of India during 17–18th June, 2013. Verification of the synoptic features in forecasts of the two models suggests that NCUM accurately captures the circulation features as compared to T574. Further verification of this event is carried out based on the contiguous rain area (CRA) technique. CRA verification is used in computing the total mean square error (MSE) which is based on displacement, volume and pattern errors. This verification technique also, confirms the better skill of NCUM over T574 in terms of forecast peak rainfall amounts, volume and average rain rate, lower MSE and root mean square error (RMSE) as well as having higher hit rates and lower misses and false alarm rates for different rainfall thresholds from Day 1 to Day 5 forecasts.

Forecasting the heavy rainfall during Himalayan flooding—June 2013

Chilika Lagoon (19°28′–19°54′N, 85°06′–85°36′E), on the Orissa coast, India, is one of the world's unique ecospheres. It is the largest brackish water lagoon with estuarine character. Because of its rich biodiversity and socioeconomic importance, it was designated as a “Ramsar site”—a wetland of international importance—in 1981. Interest in detailed analysis of the circulation, biotic, and abiotic factors affecting the lagoon and its limnology is a result of the opening of the new mouth on 23 September 2000 to resolve the threat to its environment from various factors—eutrophication, weed proliferation, siltation, industrial pollution, and depletion of bioresources. This paper describes the development of a two-dimensional depth-averaged hydrodynamic model for Chilika Lagoon and the resulting simulation of currents and salinity corresponding to (i) the Southwest and Northeast monsoon seasons and (ii) pre- and post–mouth-opening conditions. Numerical experiments are performed to understand the dif...

https://web.iitd.ac.in/~adrao/publications/2007_7.pdf

Numerical Simulation of Circulation and Salinity Structure in Chilika Lagoon

Modelling the effects of variable salinity on the temporal distribution of plankton in shallow coastal lagoons.

The quality of coastal waters can be markedly influenced by the growth and distribution of phytoplankton. In this study, a non-linear mathematical model is used to explain the seasonal variability of plankton in shallow coastal lagoons. A local stability analysis for dynamical systems is undertaken in order to estimate the range of values of model parameters. Numerical experiments and sensitivity tests with different parameters show that of the thirteen parameters in the model, the main parameter affecting the productivity is the growth rate of the phytoplankton, which depends on light and nutrients. The time of maximum planktonic growth, in particular, is mostly dependent upon the parameter values within the growth rate term. As a case study, the model is then applied to Chilika Lagoon (19 ∘ 28′N–19 ∘ 54′N and 85 ∘ 06′E–85 ∘ 36′E), the largest brackish water lagoon with estuarine character on the east coast of India. Interest in detailed analysis of the ecology of the lagoon and the various factors affecting it is due to the opening of the new mouth on 23 September 2000 to resolve the threat to its environment from various factors — eutrophication, weed proliferation, siltation, industrial pollution and depletion of bioresources. The model is tested for different sectors of the lagoon which have distinct characteristics in terms of depth, light penetration, nutrients and hence planktonic growth. A fine-tuning of the model parameters, with the help of the stability analysis discussed for a general model, was performed in order to validate the model results with the data available for the lagoon. Even in its simplest form, the model is able to reproduce most of the significant characteristics of the planktonic distribution in the lagoon — the bimodal oscillations, and major plankton peaks.

Anumeha Dube

Papers

Observed changes in water mass properties in the Indian Sundarbans (northwestern Bay of Bengal) during 1980-2007.

Forecasting the heavy rainfall during Himalayan flooding—June 2013

Numerical Simulation of Circulation and Salinity Structure in Chilika Lagoon

Modelling the effects of variable salinity on the temporal distribution of plankton in shallow coastal lagoons.

Mathematical modelling of the seasonal variability of plankton in a shallow lagoon