Ocean acidification is a chemical shift in seawater driven by rising atmospheric CO2 that reduces pH and the availability of carbonate ions. Richard A. Feely at the NOAA Pacific Marine Environmental Laboratory has documented how increasing CO2 uptake by oceans alters carbonate chemistry along continental margins, creating conditions that challenge organisms that build shells or skeletons. These chemical changes propagate through ecosystems because they affect both the base and the links of marine food webs.
Mechanisms linking acidification to food webs
The first pathway is direct physiological stress on calcifying organisms. Victor J. Fabry at the Scripps Institution of Oceanography summarized evidence that mollusks, corals, and some plankton reduce calcification and experience shell dissolution under lower carbonate saturation. This reduces the abundance and size of taxa that provide structural habitat or serve as prey. The second pathway is through altered consumer performance: Ken Caldeira at the Carnegie Institution for Science and colleagues have shown that acidification can change metabolic rates, sensory ability, and reproductive success in fish and invertebrates, leading to shifts in growth and survival. The third pathway is food quality transformation: changes in phytoplankton community composition and nutritional content under high CO2, documented in laboratory and field studies, can reduce the energy transfer efficiency from primary producers to zooplankton and higher consumers.
These mechanisms interact with other stressors. Ove Hoegh-Guldberg at the University of Queensland emphasizes that warming and deoxygenation commonly co-occur with acidification, creating compound impacts that magnify disruptions to trophic interactions. In productive upwelling regions, Feely’s observations show that natural low-pH waters already challenge organisms, so anthropogenic acidification can push these systems past ecological thresholds.
Ecological and social consequences
When foundational species decline, trophic cascades can follow. Loss of calcifying plankton such as pteropods—whose shell weakening has been observed in multiple studies—reduces food for plankton-feeding fish and seabirds, potentially altering recruitment and fishery yields. Changes in coral calcification, highlighted in assessments by the Intergovernmental Panel on Climate Change, degrade reef complexity that supports diverse food webs and coastal livelihoods. For coastal communities, particularly Indigenous peoples and small-scale fishers whose diets and cultural practices are tied to specific species, these shifts have direct social and economic consequences.
The spatial pattern of impacts is uneven. High-latitude and eastern boundary upwelling systems are often more susceptible because cold or nutrient-rich waters absorb CO2 more readily, creating regional hotspots of vulnerability. This territorial nuance matters for management: local fisheries monitoring and habitat protection can buffer some impacts, while global CO2 emissions determine the long-term trajectory.
Mitigation and adaptation options include reducing CO2 emissions to slow acidification, enhancing local ecosystem resilience through habitat restoration, and adjusting fishery management to account for changing productivity. Adaptive capacity varies, and effective responses require integrating chemical monitoring, ecological studies, and the lived knowledge of coastal communities. Credible, long-term observations and cross-disciplinary research remain essential to predict and manage how ocean acidification will reshape marine food web stability.