Our NNOCCI associate, Dr. Scott Doney, along with his colleagues, has recently published an updated review of the effects of ocean acidification on marine ecosystems (see here for a previous review). Read on for a summary of their main findings, and to understand how to apply this information to your communications about ocean acidification to the public. The peer-reviewed article is divided into three sections, as we will mirror here: organismal responses, community and ecosystem effects, and risks to human communities.

To review, ocean acidification is a phenomenon caused by the dissolution of excess carbon dioxide into our oceans. “At seawater pH levels (~8), CO₂ added to seawater reacts with water to form bicarbonate and hydrogen ions”, the latter of which acts to lower seawater pH. For shell-forming marine organisms, this has consequences for the solubility of calcium carbonate that they use for shells and skeletons. For more information on the fundamentals of climate change, or the input of excess carbon dioxide into our atmosphere and oceans, refer to this NNOCCI publication.

Organismal responses

Generalizations on the sensitivity of organisms to ocean acidification are difficult across species, and even more so when considering variability in local environmental conditions. As the field of study has matured, we increasingly seek to understand not only present sensitivity but also future impacts of ocean acidification. We accomplish this with laboratory experiments mimicking future conditions or by studying populations living in naturally high-CO₂ environments. In some studies, such as with the purple urchin, we have learned that adaptation to future high CO₂ environments may be quicker than we previously estimated. Stories of hope help us communicate that it is not too late to slow the effects of climate change on our ecosystems. Doney et al. also emphasize the importance of incorporating multiple stressors, like pH, temperature, and oxygen availability, when designing experiments that help us understand ecologically-relevant phenomena. This means that ocean acidification alone is not the whole story, rather, experiments that incorporate multiple stressors are more likely to tell an accurate picture of our future. Lastly, it is important to experimentally investigate conditions that are relevant to the organism and ecosystem in question – for example, organisms living in naturally variable conditions are more likely to show resilience, but their environment may shift more quickly than others. Context-dependent science is imperative when we are investigating the effects of ocean acidification on organismal responses.

Community and ecosystem effects

Community structure will change under ocean acidification conditions. Algal communities, or those that photosynthesize, will increase in their productivity. However, consumers will not be able to match those increased productivity levels, leading to an imbalance in the food web. Some examples of this include seagrass meadows (increased seagrass and epiphyte growth), coral reefs (increased macroalgal overgrowth), and pelagic communities (increased harmful algal blooms). In seagrass beds, high natural variability in pH and other environmental conditions tells a story of potential resilience, where grazer organisms may keep up with increased primary productivity with grazing rates despite ocean acidification. In coral reefs, increased macroalgal growth combined with the dissolution of calcium carbonate structures could spell devastation for these communities. However, coral reefs will not disappear with ocean acidification, they will just shift in their community structure from slow-growing corals to fast-growing corals, which might reduce the structural complexity of the habitat for invertebrates and fishes. In pelagic communities, ocean acidification will increase the productivity of phytoplankton, which will provide more food, but this also includes the proliferation of toxic algal blooms. Overall, some vulnerable species across the food web will decrease in their productivity, and other resilient species redundant in their function (i.e., two corals that provide similar structure and/or food) will increase in abundance. These redundancy situations are good news for the overall health of our ecosystems, however, the risk of decreased global biodiversity is significant and we have a limited understanding of the indirect effects this might have.

Risks to human communities

Communication and research about ocean acidification are frequently framed in the economic, cultural, and ecosystem service contexts. When we communicate about ocean acidification, we still want to be careful about focusing too heavily on the economic benefits of conservation. Instead, strengthening our coastal communities that depend on the ocean for food, livelihood, and cultural resources is a better framework for the immense ecosystem services that the ocean provides. The marine mollusk shellfish fishery is a significant portion (9%) of world fishery production by value, and studies have worked to quantify the effects of ocean acidification on this important resource. Integrated assessment models (IAMs) are an important tool to consider multiple factors like climate change, management, and socio-economic factors on fishery productivity. For example, the Alaska-based southern Tanner crab fishery catch is projected to decrease by more than 50% in the next 20 years. In another model, total fisheries revenue in the Arctic region is projected to increase by 39% by 2050 because of warming and the subsequent movement of temperate fisheries. The cultural importance of coral reefs is considered in noneconomic models, where warming and ocean acidification pose significant risk to Southeast Asia. Since reefs are ecosystem engineers, slowing coastal erosion, filtering water, and protecting coastal communities from wave action, degradation of reefs is a significant threat to sustenance-based communities of lower socio-economic status worldwide. The same can be said for coastal submerged aquatic vegetation, including seagrasses and mangroves, with the added ecosystem service of carbon sequestration. 

Recommendations for the future

Decreasing our global carbon emissions must be a strong priority in protecting our ecosystems from the effects of ocean acidification. Local-scale adaptive management strategies will strengthen our community response, such as in effectively managing the shellfish fisheries and increasing sustainable shellfish aquaculture. We also need to invest in more targeted observation systems globally, and work together with climate change scientists worldwide to identify trends and ecosystem-scale changes.

About the author:

Dr. Richelle Tanner is a California Sea Grant Delta Science Postdoctoral Fellow at UC Davis, specializing in adaptive ecosystem management, marine evolutionary and environmental physiology, and climate change effects. She also serves as chair of the Science Partnerships Committee of the National Network for Ocean and Climate Change Interpretation.