The phenomenon of the unprecedented increase in H⁺ ion concentration and changes in the aquatic carbonate chemistry (pCO₂, pH, alkalinity, and calcium carbonate saturation state) primarily due to the rapid uptake of atmospheric CO₂ in the seawater is known as Ocean Acidification. At present, the ocean is 30% more acidic than it was 200 years ago, and the rate of acidification is faster than ever. Compared to the oceanic waters, coastal waters are subjected to higher anthropogenic pressure largely due to its proximity to the terrestrial environment. The decrease in pH of the coastal waters ‘Coastal Acidification’ is emerging as an additional global threat to the health of coastal ecosystems. Ecosystem responses to environmental changes has poorly been studied particularly with reference to continuous, long term monitoring of coastal ecosystems. The key issues are

• Decrease in oceanic pH
• El Nino and La Nina – sea surface temperature anomalies
• Eutrophication –nutrient enrichment from land-based sources
• Change in sea water carbonate chemistry

The combination of eutrophication and coastal acidification is likely to have greater adverse effects on coastal ecosystems. Therefore, optimal management of coastal eutrophication suggests controlling both N and P, in part because P can limit primary production in some systems.

Selected coral reef ecosystems (Gulf of Mannar in the east coast of India, Lakshadweep Island) and lagoon ecosystems (Chilika and Pulicat) are been studied to depict future responses to nutrient enrichment, coastal acidification and formation of oxygen minimum zones (OMZ).