Impacts of Ocean Acidification on Marine Biodiversity

Volume 30 No. 2 of the Renewable Resources Journal features “Impacts of Ocean Acidification on Marine Biodiversity,” an article adapted from the Convention on Biological Diversity Technical Series No. 75. This article provides an overview of the most up-to-date information on the impacts of ocean acidification on biodiversity.

Introduction

Ocean acidification, often referred to as the “other CO₂ problem,” is a direct result of rising atmospheric CO₂ concentrations. By absorbing a quarter of the total human production of CO₂, the ocean has substantively slowed climate change. However, increased CO₂ concentrations, which leads to an increase in seawater acidity, has consequences for marine ecosystems. Unless CO₂ emissions are rapidly curtailed, seawater acidity is expected to increase to 170% of pre-industrial levels by 2100.

Ocean acidification has already increased by around 26% since pre-industrial times. The paleo-record shows that recovery from ocean acidification can be extremely slow. Following the Paleo-Eocene Thermal Maximum, for example, recovery took around 100,000 years.

Many scientific studies have demonstrated that a wide range of marine organisms are sensitive to pH changes of this magnitude, with effects on physiology, fitness, and survival. However, the effects of marine food webs, ecosystems, biogeochemistry, and ecosystem services are less certain.

Impacts on physiological responses

When external hydrogen ion levels substantially increase as a result of ocean acidification, organisms might need to use extra energy to maintain their internal acid-base balance. This increased energy demand can lead to reduced protein synthesis and reduction in fitness.

At the larval stage, shells are among the smallest and most fragile in the ocean, and they are potentially extremely vulnerable to decreased mineral saturation caused by acidification. Impacts include decreased larval size, reduced morphological complexity, and decreased calcification. Understanding how effects at early life stages can carry-over to influence growth and reproduction of the adult remains a significant challenge and knowledge gap.

Fish and some invertebrates also exhibit altered sensory systems and behavior when exposed to elevated levels of CO₂. Reef fish larvae lose their ability to discriminate between ecologically important chemical cues, such as odors from different habitat types, kin and non-kin, the smell of predators, and visual function. Adult reef fish also suffer impaired olfactory ability and altered behavior, with potential effects on predator-prey interactions, habitat selection, and homing to resting sites. A wide range of species appear to be affected, including commercially important species. Impaired behavior at all life stages occurs as a result of permanent exposure to CO₂ levels that are well within the range that could occur in the ocean this century.

Great Barrier Reef coral.

Impacts on benthic communities

Around half of benthic species have lower rates of growth and survival under projected future acidification. Benthic ecosystems comprise some of the key ocean communities that people rely on for food and ecosystem services. For coral, molluscs, and echinoderms, many studies show reduction in growth and survival rates. However, these responses are variable, and some species are resilient in low pH conditions. Many seaweed and seagrass species can tolerate, or may even benefit from, future acidification.

Impacts on pelagic communities

Plankton are taxonomically diverse. Plankton form a key component of the marine food chain and play an important role in chemical cycling. Non-calcifying phytoplankton can show increased photosynthesis and growth under high CO₂ conditions. Calcifying phytoplankton have more variable responses, both between and within species. The shells of planktonic foraminifera and pteropods are likely to experience decreased calcification. These decreases in shell thickness and size may also decrease the efficiency of future carbon transport between the sea surface and the ocean interior.

Impacts on biogeochemistry

Ocean acidification could alter many other aspects of ocean biogeochemistry, with feedbacks to climatic processes. Acidification may alter net primary productivity, trace gas emissions, nitrogen-carbon ratios in food webs and exported particulate matter, and iron bioavailability. The scale and importance of these effects are not yet well understood. However, the impacts of unmitigated ocean acidification have been estimated to represent a loss to the world economy of more than US $1 trillion annually by 2100.

Impacts on ecosystem services and livelihoods

Ecosystem services are the components of nature that help create human well-being and economic wealth. Many of the species that are likely to be negatively impacted by acidification are habitat-forming organisms providing shelter, food, and nursery functions to other marine species, including commercially important fish. They also contribute to coastal protection, recreation, and cultural benefits. Molluscs and crustaceans harvested for food are likely to be affected since they have calcareous shells and exoskeletons. Nevertheless, some commercially important species may be able to adapt, or be naturally resilient. The economic impact of acidification on the fisheries industry is relatively understudied. However, models suggest that there may be a substantial reduction in fisheries catch potential under future conditions, affecting the quantity, quality, and predictability of future harvest.

Conclusion

Ocean acidification represents a serious threat to marine biodiversity, yet many gaps remain in our understanding of the complex processes involved and their societal consequences. However, it is clear that substantive environmental perturbations, increased extinction risk for vulnerable species, and significant socio-economic consequences are highly likely. As the need for action to address acidification becomes more urgent, collaboration among governments and organizations in enhancing, sharing, and applying knowledge is vital.

Volume 30 No. 2 of the Renewable Resources Journal is available for free download.