Acidification (Coastal)

Coastal acidification is the decline in seawater pH and carbonate saturation in nearshore waters, driven by atmospheric CO2 uptake plus local nutrient and organic-matter inputs. Unlike open-ocean acidification, the coastal signal is amplified by eutrophication: respiration of decomposing algal blooms releases CO2 and lowers pH on top of the baseline forced by rising atmospheric CO2 near 420 ppm. Upwelling, riverine freshwater, and acidic runoff add further variability. The combined effect can push aragonite saturation state below 1 in estuaries and shelf waters, corroding shells of oysters, mussels, and pteropods. The EU Marine Strategy Framework Directive treats acidification under its eutrophication and contaminant descriptors.