Contrasting patterns in pH variability in the Arabian Sea and Bay of Bengal.

Continuous understanding of the ongoing ocean acidification (OA) is essential for predicting the future impact of OA on marine ecosystems. Here we report the results of open ocean time-series measurements (19 cruises) of seawater pH in total hydrogen ion scale (pHT) and associated parameters in the Arabian Sea (AS) and the Bay of Bengal (BoB). During southwest monsoon (SWM), the pHT within the 30 to 100 m water column shows the maximum difference between the two basins with BoB pHT being lower (up to ~0.39 units) than AS which could be due to freshwater influx from rivers, mixed layer dynamics, and cold-core eddies. However, during Spring inter-monsoon (SIM), the pHT of BoB follows the trend of AS. A contrasting finding is that the lowest pHT occurs at 350 to 500 m in the BoB while it is ~1000 m in the AS. The pHT within the 150 to 1500 m layer of these two basins shows lower values by 0.03 (±0.02) in the BoB as compared to the AS. The possible reasons for the low pHT within the BoB oxygen minimum zone (OMZ) could be due to intrusion of western Pacific water in the BoB, freshwater influx from rivers, variations in OMZ of the two basins, higher temperature (~2°C) within the OMZ of the AS, and denitrification in the AS. The pHT in both the basins (500 to 1000 m) is lower than in the North Atlantic and higher than in the North Pacific waters; however, the pHT in the 200 to 500 m is lower in the BoB than in all these basins. This study highlights the under-saturation of calcium carbonate at very shallow depths (~ 100 m) in the BoB, indicating that the plankton in the BoB are facing a major risk from OA compared to the AS and need further investigation.

Seasonally varying biogeochemical regime around the coral habitats off central west coast of India.

The Western Indian Continental Shelf (WICS) experiences upwelling during the Southwest Monsoon (SWM), leading to deoxygenation and acidification of subsurface waters. The region has patchy growth of corals, e.g. in the Grande Island and Angria Bank. Measurements made during the late SWM of 2022 reveal that the shelf waters around the Grande Island were subject to varying environmental conditions, viz. lower temperature (21.3-26.1°C), oxygen (0-4.9 mL L-1) and pHT (7.506-7.927). Complete anoxia was associated with sulphide build-up to a maximum of 5.9 µmol L-1 at 17 m depth. An additional episodic condition (high temperature, low oxygen and pH) also occurred associated presumably with a plankton bloom in April 2017. Hence, unlike the offshore coral site Angria Bank, waters around the Grande Island experiences extreme changes in physico-chemical conditions (e.g. Ωarg ∼1.2-1.8 during October 2022) seasonally as reported here. The biogeochemical conditions are however not as intense (Ωarg = 0.6) as observed along the eastern boundary upwelling system of the Pacific Ocean.

Malformation in coccolithophores in low pH waters: evidences from the eastern Arabian Sea.

Oceanic calcifying plankton such as coccolithophores is expected to exhibit sensitivity to climate change stressors such as warming and acidification. Observational studies on coccolithophore communities along with carbonate chemistry provide important perceptions of possible adaptations of these organisms to ocean acidification. However, this phytoplankton group remains one of the least studied in the northern Indian Ocean. In 2017, the biogeochemistry group at the Council for Scientific and Industrial Research-National Institute of Oceanography (CSIR-NIO) initiated a coccolithophore monitoring study in the eastern Arabian Sea (EAS). Here, we document for the first time a detailed spatial and seasonal distribution of coccolithophores and their controlling factors from the EAS, which is a well-known source of CO2 to the atmosphere. To infer the seasonality, data collected at three transects (Goa, Mangalore, and Kochi) during the Southwest Monsoon (SWM) of 2018 was compared with that of the late SWM of 2017. Apart from this, the abundance of coccolithophores was studied at the Candolim Time Series (CaTS) transect, off Goa during the Northeast Monsoon (NEM). The most abundant coccolithophore species found in the study region was Gephyrocapsa oceanica. A high abundance of G. oceanica (1800 × 103cells L-1) was observed at the Mangalore transect during the late SWM despite experiencing low pH and can be linked to nitrogen availability. The high abundance of G. oceanica at Mangalore was associated with high dimethylsulphide (DMS). Particulate inorganic carbon (PIC) and scattering coefficient retrieved from satellites also indicated a high abundance of coccolithophores off Mangalore during the late SWM of 2017. Interestingly, G. oceanica showed malformation during the late SWM in low pH waters. Malformation in coccolithophores could have a far-reaching impact on the settling fluxes of organic matter and also on the emissions of climatically important gases such as DMS and CO2, thus influencing atmospheric chemistry. The satellite data for PIC in the EAS indicates a high abundance of coccolithophore in recent years, especially during the warm El Nino years (2015 and 2018). This warrants the need for a better assessment of the fate of coccolithophores in high-CO2 and warmer oceans.

Impact of physical processes on oxygen loss and production of hydrogen sulphide and methane in a tropical freshwater reservoir.

Carbon neutrality of tropical reservoirs is a big concern in recent years as some estimates project high methane emission from these reservoirs. While there are studies available on the impact of physical processes (stratification and mixing) on the biogeochemistry of tropical reservoirs, not much information is available on the inter-annual variability in the low-oxygen conditions and production/accumulation of hydrogen sulphide (H2S) and methane (CH4) during summer. This paper presents time series data based on monthly in situ observations from a tropical reservoir (Tillari, Maharashtra) situated in the Western Ghats in India. Sampling was carried out for temperature, dissolved oxygen (DO), H2S, and CH4 at a fixed location from March 2010 until June 2014. The reservoir experiences stable stratification during summer (March to June) with complete loss of oxygen and production of H2S (max. ~ 9 µM) and CH4 (max. ~ 185 µM) in the profundal zone. During the summer stratification, the hypolimnion acted as a pool of CH4 with integrated values ranging between 3502 and 41,632 mg m-2. However, the intensity and duration of anoxia varied during different years, influencing H2S and CH4 production. Mixing in the reservoir was observed between July and September in association with the monsoonal runoff, which increased the DO concentrations in the sub-surface layers. Besides, complete mixing was observed between December and February due to winter convection. This, however, was found to play an important role, as weaker mixing in the preceding year was associated with severe oxygen loss in the profundal zone during the following summer with a production of H2S and CH4. In contrast, more robust mixing during winter led to moderate low-oxygen conditions with less production of these gases in the subsequent summer. Based on our observations and considering a large number of reservoirs in the tropics, we hypothesise that with the present trends of global warming and less cold winters, low-oxygen conditions in the profundal zone may become more severe in the future with positive feedback on H2S and CH4 production during summer.