The Effects of Ocean Acidification on Marine Organisms and Food Webs

Ocean acidification (OA), a consequence of the increasing rate at which anthropogenic CO2 is being taken up by the oceans, stands as one of the major threats to marine biodiversity and ecosystem functioning during the Anthropocene. During the pre-industrial era, ocean surface pH dropped from around 8.2 to 8.1, an increase of 26% in H + concentration, and predictions for a further drop of 0.3 to 0.5 units by 2100, under a high-emission scenario. This review will examine the currently available evidence for the effects of OA on marine organisms over a wide range of trophic levels, from primary producers like 2025 phytoplankton and macroalgae up to apex predators, as well as its trophic impact on marine food web structure and function. Overall evidence from meta-analysis indicated a general reduction in calcification and reproduction and altered behavior of calcifying invertebrates, corals, and early life stages of commercially valuable fish under OA. Trophic transfer efficiency appears significantly reduced in acidified waters, even more so when combined with other global change drivers like ocean warming, hypoxia, and contamination. Model predictions under future scenarios suggest substantial drops in fisheries yield in many locations, including California Current, Puget Sound, and Southern Ocean, between 15 and 45% under the end-century scenarios. Microbial food webs are altered in their structure and functioning, leading to altered carbon cycling and nutrient availability. An integrated analysis of multiple stressors is clearly needed. The role of regionally-focused adaptive management responses, such as marine protected areas and reductions of non-climate stressors, must also be addressed to maintain ocean ecosystem services.