Reviews and Syntheses: Ocean acidification and its potential impacts on marine ecosystems

Ocean acidification, a complex phenomenon that lowers seawater pH, is the net outcome of several contributions. They include the dissolution of increasing atmospheric CO₂ that adds up with dissolved inorganic carbon (dissolved CO₂, H₂CO₃, HCO₃⁻, and CO₃²⁻) generated upon mineralization of primary producers (PP) and dissolved organic matter (DOM). The aquatic processes leading to inorganic carbon are substantially affected by increased DOM and nutrients via terrestrial runoff, acidic rainfall, increased PP and algal blooms, nitrification, denitrification, sulfate reduction, global warming (GW), and by atmospheric CO₂ itself through enhanced photosynthesis. They are consecutively associated with enhanced ocean acidification, hypoxia in acidified deeper seawater, pathogens, algal toxins, oxidative stress by reactive oxygen species, and thermal stress caused by longer stratification periods as an effect of GW. We discuss the mechanistic insights into the aforementioned processes and pH changes, with particular focus on processes taking place with different timescales (including the diurnal one) in surface and subsurface seawater. This review also discusses these collective influences to assess their potential detrimental effects to marine organisms, and of ecosystem processes and services. Our review of the effects operating in synergy with ocean acidification will provide a broad insight into the potential impact of acidification itself on biological processes. The foreseen danger to marine organisms by acidification is in fact expected to be amplified by several concurrent and interacting phenomena

Blue carbon stock of the Bangladesh Sundarban mangroves: what could be the scenario after a century?

The total blue carbon stock of the Bangladesh Sundarban mangroves was evaluated and the probable future status after a century was predicted based on the recent trend of changes in the last 30 years and implementing a hybrid model of Markov Chain and Cellular automata. At present 36.24 Tg C and 54.95 Tg C are stored in the above-ground and below-ground compartments respectively resulting in total blue carbon stock of 91.19 Tg C. According to the prediction 15.88 Tg C would be lost from this region by the year 2115. The low saline species composition classes dominated mainly by Heritiera spp. accounts for the major portion of the carbon sock at present (45.60 Tg C), while the highly saline regions stores only 14.90 Tg C. The prediction shows that after a hundred years almost 22.42 Tg C would be lost from the low saline regions accompanied by an increase of 8.20 Tg C in the high saline regions dominated mainly by Excoecaria sp. and Avicennia spp. The net carbon loss would be due to both mangrove area loss (~ 510 km2) and change in species composition leading to 58.28 Tg of potential CO2 emission within the year 2115

Air – water carbon dioxide exchange dynamics along the outer estuarine transition zone of Sundarban, northern Bay of Bengal, India

111-116Air-water CO2 flux was measured from the gradient of fCO2 (water) and fCO2 (air) in the Hooghly-Matla estuary to offshore transition zone. An average gas transfer velocity of 10.2 cm h-1 was evaluated in this region. fCO2 (air) ranged between 275.08 µatm to 459.07 µatm and fCO2 (water) varied from 149.1 µatm to 299.2 µatm. Wind velocity and bathymetry (depth) were observed to be the main controlling factors for exchange of CO2 between the atmosphere and water phase. Seawater is found to have a greater influence at the marine end of the estuary. Apart from physical mixing in this freshwater-sea water interacting zone, the study of fCO2 and pH reflects a possible influence of biological activity as well. A maximum efflux rate of 24.56 µmol m-2 h-1 and influx rate of -41.61 µmol m-2 h-1 is determined during high wind velocity. The overall region surveyed was found to behave as a sink during the study period with an average (daytime) influx rate of -14.03 µmol m-2 h-1

The present state-of-the-art of blue carbon repository in India: a meta-analysis

Abstract The present study collated data on the Indian blue carbon repository (mangroves, seagrasses, and salt marshes) from peer-reviewed literature on carbon stock assessment. This meta-analysis indicated that the blue carbon ecosystems of India could have a collective carbon stock of 67.35 Tg C (mangroves, seagrass, and salt marsh accounting for 67 Tg C, 0.0630 Tg C, and 0.0049 Tg C, respectively). Several studies have ubiquitously measured the spatial extent of mangroves (~ 4991 km2) and seagrasses (~ 517 km2) in India; however, the salt marshes (290–1398 km2) have contradictions in estimates. The green payments against the blue carbon ecosystems of India can be as high as ~ 9.6 billion US $, whereas the social cost of carbon sequestered by these ecosystems can vary between 0.47 and 5.43 billion US$. The present study also identified the key research areas that require priority to minimize the uncertainties in blue carbon stock assessment to foster a robust ecosystem-based approach for climate change adaptation in the country. The study identified that less than half of the total mangrove habitats of India are yet to be sampled leaving a scope of substantial uncertainty in nationwide blue carbon estimates. The spatial extent of India’s salt marshes is another aspect that needs to be delineated with a higher confidence level

Carbon Biogeochemistry of the Estuaries Adjoining the Indian Sundarbans Mangrove Ecosystem: A Review

The present study reviewed the carbon-biogeochemistry-related observations concerning CO2 and CH4 dynamics in the estuaries adjoining the Indian Sundarbans mangrove ecosystem. The review focused on the partial pressure of CO2 and CH4 [pCO2(water) and pCH4(water)] and air–water CO2 and CH4 fluxes and their physical, biogeochemical, and hydrological drivers. The riverine-freshwater-rich Hooghly estuary has always exhibited higher CO2 emissions than the marine-water-dominated Sundarbans estuaries. The mangrove sediment porewater and recirculated groundwater were rich in pCO2(water) and pCH4(water), enhancing their load in the adjacent estuaries. Freshwater-seawater admixing, photosynthetically active radiation, primary productivity, and porewater/groundwater input were the principal factors that regulated pCO2(water) and pCH4(water) and their fluxes. Higher chlorophyll-a concentrations, indicating higher primary production, led to the furnishing of more organic substrates that underwent anaerobic degradation to produce CH4 in the water column. The northern Bay of Bengal seawater had a high carbonate buffering capacity that reduced the pCO2(water) and water-to-air CO2 fluxes in the Sundarbans estuaries. Several authors traced the degradation of organic matter to DIC, mainly following the denitrification pathway (and pathways between aerobic respiration and carbonate dissolution). Overall, this review collated the significant findings on the carbon biogeochemistry of Sundarbans estuaries and discussed the areas that require attention in the future

Vegetation cover change analysis from multi-temporal satellite data in Jharkhali Island, Sundarbans, India

331-342Present study intends to quantify change of natural vegetation cover (mainly of mangrove forest) in Sundarbans Island between the time span of 2004-2010, when sustained efforts of a forestation and conservation has been in vogue. Vegetation indices like Normalized Difference Vegetation Index (NDVI), Global Environmental Monitoring Index (GEMI), Optimized Soil Adjusted Vegetation Index (OSAVI) and Transformed Difference Vegetation Index (TDVI) have been used to decipher the measure of vegetation cover in this island and its changes during the period. Radiometric normalization technique is used to nullify various imaging condition anomalies while comparing multi-temporal data for change detection analysis. TDVI has been found to be more effective in vegetation cover change detection in such deltaic environment. Present study shows an overall net increase of vegetation cover in the island as a result of sustained conservation and plantation efforts

Tide induced annual variability of selected physico-chemical characteristics in the northern Bay of Bengal (nBoB) with a Special emphasis on Tropical Cyclone-Phailin, 2013.

952-959Daily monitoring (in the day time) of sea surface temperature (SST), sea surface salinity (SSS), pH and weekly measurement of dissolved oxygen (DO) were carried out in the northernmost sector of the Bay of Bengal for the first time, covering roughly an area of ~750 sq. km throughout an annual cycle from 1st June, 2013 to 31st May, 2014. SSS, pH and DO maintained a higher and lower value during the highest high tide (HHT) and lowest low tide (LLT) hours respectively, throughout the year, except in some weeks of monsoon. But SST did not follow any such type of tide influence trend. The strong physical forcing and high energy tidal surges caused by the tropical cyclone (Phailin) increased the SSS, pH and DO by 33.11%, 3.73% and 40% respectively. In contrast, a massive cooling in SST by 15.38% was observed. Though SST returned to normal range within one week, DO and SSS took almost 10 days to reach equilibrium. However, tropical cyclone poses a short term strong impact on the physico-chemical properties and ecology of the shallow continental shelf coastal water

Light absorption characteristics of chromophoric dissolved organic matter (CDOM) in the coastal waters of northern Bay of Bengal during winter season

884-892Absorption spectra of chromophoric dissolved organic matter (CDOM) and other physico-chemical components were measured in the coastal water of northern Bay of Bengal (nBoB) lying adjacent to West Bengal coast, India, during October, 2014-March, 2015. Absorption coefficient [aCDOM(440)] of CDOM exhibited a considerable inverse linear relationship with salinity in the surface waters implying traditional mixing effect of marine and fresh water. A dominant terrestrial CDOM signal influenced by higher fresh water discharge from the river Hugli was seen prior of winter (October, 2014). However, during January, aCDOM(440) value again increased along with a concomitant increase in chlorophyll-a. However, total suspended matter (TSM) showed strong linear positive and consistent relationship with aCDOM(440) throughout the study period, ascertaining terrestrial source of CDOM. Moreover, prior of winter, study site receives more discharge from rivers with corresponding signals of lower slope value indicating less aged and high molecular weight of CDOM

Mechanisms and drivers controlling spatio-temporal evolution of pCO2 and air-sea CO2 fluxes in the southern Java coastal upwelling system

Upwelling systems are known for their complex dynamic processes spanning a wide variety of spatio-temporal scales, due to strong coupling between the ocean and atmosphere. One such upwelling system is found at the eastern boundary of the Indian Ocean along the southern coast of Java during the southeast monsoon (SM). This study examines the mechanisms and drivers involved in the spatial and temporal evolutions of surface pCO2 in this upwelling system over seasonal to inter-annual timescales using a coupled, high-resolution regional model. A decomposition analysis was used to quantify the changes in pCO2 in response to changes in surface-temperature (T), surface-salinity (S), dissolved inorganic carbon (DIC), and total alkalinity (ALK). The upwelling of deeper, cooler waters decreases the surface pCO2 by 50 ± 1.48 μatm from June to September. However, the presence of DIC-rich waters at significantly shallower depths increases the surface pCO2 by 27 ± 0.9 μatm. The surface pCO2 increases by 5 ± 0.50 μatm due to changes in salinity, which exerts less control than T-driven changes. Biological changes increase the surface pCO2 by 17.54 ± 2.74 μatm. The maximum seasonal amplitudes of the variations in pCO2 caused by seawater solubility and biology are in counterbalance during the SM. Therefore, the upwelling-driven combined effect of physical mechanisms dominates the effect of biological mechanisms in reducing surface pCO2 under these conditions. During the SM, the southern coast of Java acts as a seasonal CO2 sink (−1.36 ± 6.48 g C m−2 year−1), although consideration of the mean flux over 2006–2017 shows that the region acts as a source of CO2 (1.31 ± 5.03 g C m−2 year−1). Furthermore, following a negative Indian Ocean Dipole (IOD) year, a strong sink of atmospheric CO2 is observed during the SM. © 2023 Elsevier Ltd11Nsciescopu