Since the industrial revolution, the concentration of carbon dioxide (CO2) in the atmosphere has increased due to the burning of fossil fuels and land use change; the ocean absorbs about 30 percent of this CO2 and over this time the pH of surface ocean waters fell by 0.1 pH units. As the pH scale is logarithmic, this change represents an approximately 30 percent increase in acidity. When CO2 is absorbed by seawater, a series of chemical reactions occur resulting in the increased concentration of hydrogen ions.
The pH of seawater is typically between 7.5 and 8.4. Seawater is slightly alkaline due to dissolved basic minerals and the natural buffering from carbonates and bicarbonates in the water. The pH value of seawater is naturally as low as 7.8 in deep ocean waters as a result of degradation of organic matter in these waters, but it can be as high as 8.4 in surface waters in areas of high biological productivity. The pH of UK inshore waters typically ranges between 8.0 and 8.4, making them slightly alkaline. The pH of wastewater treatment plant (WWTP) effluent can be lower than that of natural water bodies due to the presence of organic acids and other compounds.
An increase in pH causes seawater to become more acidic and carbonate ions to be relatively less abundant. Carbonate ions are an important building block of structures such as seashells. Decreases in these ions can make the building and maintaining of shells and other calcium carbonate structures such as oysters, clams, sea urchins, corals and calcareous plankton more difficult. Changes in ocean chemistry can also affect the behaviour of non-calcifying organisms, certain fish's ability to detect predators is decreased in more acidic waters.
A GOSE Foresight report notes that the integrity of some UK species and habitats of conservation importance (including sites designated as Marine Protected Areas) could be affected by future changes in pH and temperature. Ocean acidification research has demonstrated that shell-forming organisms such as oysters, mussels, and scallops are particularly vulnerable.
The Solent is a complex estuarine system consisting of various rivers, tidal channels, and seawater. The pH of water varies depending on factors such as tides, weather conditions, and human activities. A study on water quality trends in the Solent, analysing data between 2000 and 2020, revealed a variation in pH across study sites, which reflects the effluents of the various activities happening within the Solent. Overall it showed the water within the Solent has become less acidic, changing from pH 8 in 2000 to pH 8.15 in 2020. The pH values at WTW sites were significantly lower than those elsewhere. There is lots of oyster restoration work happening around the Solent and in locations with a higher pH there is the possibility that oyster shell formation could be impacted. Analysis of pH in the ports of Southampton and Portsmouth revealed wildly fluctuating ranges between the years of 2000 and 2007. This occurred until a more stable fluctuating trend occurred at about pH 8.1 from 2007 until 2015.
Anthropogenically driven drivers for pH values include fertiliser runoff and its associated eutrophication. This can lead to reductions in pH through the breakdown of ammonia into nitrite via reactions with bicarbonate. This is concerning in the Solent due to its nitrification status.