Multi-annual variability of pCO2(aq) and air-water CO2 flux in the mangrove-dominated Dhamra Estuary draining into the Bay of Bengal (India).

Small estuaries often remain neglected while characterizing air-water CO2 flux dynamics. This study reports the seasonal, spatial, and multi-annual variability of carbon biogeochemistry, emphasizing air-water CO2 flux from a small tropical mangrove-dominated estuary (Dhamra Estuary) of the Bay of Bengal, based on the 9-year-long sampling survey (2013 to 2021). The sampling covered twelve pre-fixed locations of this estuary. A suite of biogeochemical parameters was kept within the purview of this study to deliniate the interrelationship between CO2 fluxes and potential factors that can regulate/govern pCO2(aq) dynamics. Air water CO2 exchange rates were calculated using five globally accepted empirical gas transfer velocity equations and varied in a range of - 832.5 to 7904 µmol m-2 h-1. The estuary was a sink for CO2 in monsoon season, having the highest average flux rates of - 380.9 ± 125.5 µmol m-2 h-1, whereas a source in pre-monsoon (38.29 ± 913.1 µmol m-2 h-1) and post-monsoon (91.81 ± 1009.8 µmol m-2 h-1). The significant factors governing pCO2 were pH, salinity, total alkalinity and dissolved inorganic carbon (DIC). This long-term seasonal study emphasizes the need to include small regional estuaries for more accurate estimates of global CO2 flux to upscale the global carbon budget and its controlling mechanism.

Interannual and seasonal variability and future forecasting of pCO2(water) using the ARIMA model and CO2 fluxes in a tropical estuary.

The seasonal and interannual variation in the partial pressure of carbon dioxide in water [pCO2(water)] and air-water CO2 exchange in the Mahanadi estuary situated on the east coast of India was studied between March 2013 and March 2021. The principal aim of the study was to analyze the spatiotemporal variability and future trend of pCO2 and air-water CO2 fluxes along with the related carbonate chemistry parameters like water temperature, pH, salinity, nutrients, and total alkalinity, over 9 years. The seasonal CO2 flux over nine years was also calculated using five worldwide accepted equations. The seasonal map of pCO2(water) followed a general trend of being high in monsoon (2628 ± 3484 µatm) associated with high river inflow and low during pre-monsoon (445.6 ± 270.0 µatm). High pCO2 in water compared to the atmosphere (average 407.6-409.4 µatm) was observed in the estuary throughout the sampling period. The CO2 efflux computed using different gas transfer velocity formulas was also consistent with pCO2 water acquiring the peak during monsoon in the Mahanadi estuary (6033 ± 9478 µmol m-2 h-1) and trough during pre-monsoon (21.66± 187.2 µmol m-2 h-1). The estuary acted as a net source of CO2 throughout the study period, with significant seasonality in the flux magnitudes. However, CO2 sequestration via photosynthesis by phytoplankton resulted in lower emission rates toward the atmosphere in summer. This study uses the autoregressive integrated moving average (ARIMA) model to forecast pCO2(water) for the future. Using measured and predicted values, our work demonstrated that pCO2(water) has an upward trend in the Mahanadi estuary. Our results demonstrate that long-term observations from estuaries should be prioritized to upscale the global carbon budget.

The Interaction of Seasons and Biogeochemical Properties of Water Regulate the Air-Water CO2 Exchanges in Two Major Tropical Estuaries, Bay of Bengal (India).

The exchange of CO2 between the air-water interfaces of estuaries is crucial from the perspective of the global carbon cycle and climate change feedback. In this regard, we evaluated the air-water CO2 exchanges in two major estuaries-the Mahanadi estuary (ME) and the Dhamra estuary (DE) in the northern part of the Bay of Bengal, India. Biogeochemical properties of these estuarine waters were quantified in three distinct seasons, namely, pre-monsoon (March to May), monsoon (June to October), and post-monsoon (November to February). The significant properties of water, such as the water temperature, pH, salinity, nutrients, dissolved oxygen, chlorophyll-a (chl a), and photosynthetic pigment fluorescence of phytoplankton, were estimated and correlated with CO2 fluxes. We found that the ME acted as a source of CO2 fluxes in the monsoon and post-monsoon, while DE acted as a sink during the monsoon. The stepwise regression model showed that the fluxes were primarily driven by water temperature, pH, and salinity, and they correlated well with the phytoplankton characteristics. The chl a content, fluorescence yield, and phycobilisomes-to-photosystem II fluorescence ratios were major drivers of the fluxes. Therefore, for predicting air-water CO2 exchanges precisely in a large area over a seasonal and annual scale in the estuaries of the Bay of Bengal, India, critical key parameters such as water temperature, pH, salinity, chl a, and fluorescence yield of phytoplankton should be taken into consideration. However, the responses of phytoplankton, both in terms of production and CO2 capture, are critical research areas for a better understanding of air-water CO2 exchanges in coastal ecology under climate change scenarios.

Anthropogenic influence on the physico-chemical parameters of Dhamra estuary and adjoining coastal water of the Bay of Bengal.

Estuaries receive the anthropogenic pollutants of their watershed area. Dhamra estuary, on the east coast of India, is such an estuary that receives a huge amount of pollutants, and it will eventually pose a threat to the ecological sensitive areas in its vicinity. Therefore, a study was carried out on physico-chemical parameters and chlorophyll-a to delineate the sources of variation during pre-monsoon and post-monsoon seasons. Surface water sampling was carried out from 12 stations in the estuarine and coastal area. Factor analysis and cluster analysis were applied to seasonal data to understand the sources of variation. From the study, it was observed that the chemical parameters are severely affected by anthropogenic influences such as sediment dredging, aquaculture effluent, and waste from industry and sewage from upstream. In the long run, this will affect the nearby nesting ground of vulnerable Olive Ridley turtles, high bio-diverse mangrove forests, and saltwater crocodile habitat.