Highlights
- Nutrient limits the phytoplankton growth in the Bay of Bengal due to stratification.
- Picoplankton dominated the phytoplankton community in the Bay.
- An increase in phytoplankton biomass was observed in response to ocean acidification (OA) in nutrient-rich regions.
- Decrease in phytoplankton biomass, bacteria and microzooplankton abundance was found in the nutrient-poor regions.
- Nutrient limitation overshadow the ocean acidification impact in the Bay of Bengal.
Abstract
The perennial rise in atmospheric carbon dioxide (CO2) from anthropogenic activities increased oceanic carbon uptake, thereby lowering seawater pH. Rapid ocean acidification (OA) by 2 to 3 times than other regions of the same latitudinal belts was reported in the Bay of Bengal (BoB) due to the deposition of atmospheric pollutants, in addition to the dissolution of atmospheric CO2. Therefore, the impact of OA on phytoplankton composition is hypothesized to be greater in the BoB than elsewhere in the globe. On the other hand, the BoB experiences intense oligotrophic conditions due to strong stratification driven by nutrient-poor freshwater discharge from major rivers, which is the primary bottleneck for phytoplankton growth, rather than carbon. It is hypothesized that nutrient limitation may overshadow OA’s impact on plankton in the BoB. To test this hypothesis, six microcosm experiments were conducted in the western BoB between 13 and 20°N with different nutrient levels in the surface waters. The pH of the surface water was adjusted by 0.2 units below the ambient pH using CO2 gas bubbling to simulate the projected OA scenario for the year 2100. Following pH adjustment, incubations were carried out for three days. Picoplankton dominated the phytoplankton community (∼96%) at all stations, with prochlorophytes as the dominant group. Among the phytoplankton, procholorophytes, diatoms, and prymeniophytes increased due to OA across all stations, while other groups of phytoplankton were declined. The increase in phytoplankton biomass was observed in the nutrient-rich stations, while a decrease in phytoplankton biomass, bacteria and microzooplankton abundance was found in the nutrient-poor stations, suggesting that nutrient availability dominated over the OA. Under a business-as-usual scenario, an increase in stratification due to melting Himalayan glaciers is projected, leading to strong stratification and reduced nutrient inputs through vertical mixing in the future in the BoB. Under such a scenario, nutrient limitation may possibly overshadow the OA impact in the future, despite a rapid decline in pH driven by increased CO2 dissolution and deposition of atmospheric pollutants.
Prabhakaran A., Sarma V.V.S.S., Murukan K., Treesa H., Joseph I., Sreelakshmi M. H., Poovancheri N., Sagar N., Loganathan J. & Srinivas T.N.R., 2026. Interplay between nutrient limitation and ocean acidification: plankton community shifts in the oligotrophic western Bay of Bengal. Continental Shelf Research 301: 105729. doi: 10.1016/j.csr.2026.105729. Article (subscription required).
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