A complex picture of a changing ocean
Human activities are increasingly altering the chemistry, physics and biology of the ocean. Some of these impacts are apparent such as coral reef die-offs while others are hidden from view as with the expansion of low oxygen waters in the deep ocean. The oceans absorbs a quarter of the carbon dioxide humans emit. The absorption of atmospheric carbon dioxide has increased the ocean’s acidity by 30% over the last couple centuries. The progressive decrease in ocean pH is known as ocean acidification (OA).
Our understanding of the effects of OA as a stressor (or driver of change) on vital marine resources has improved in recent decades following a rapid expansion in research and monitoring. However, the story is more complicated than a single stressor acting on an individual species. In the environment, whole ecosystems are exposed to a suite of other stressors related to OA. These stressors include increasing air and water temperatures, decreasing dissolved oxygen levels, freshwater input from melting ice and changes in precipitation, and increased stratification. In coastal areas, OA’s impacts are even further complicated by exposure to other indirect stressors such as changes in land use, nutrient runoff, contaminant input, fisheries, maritime transportation and energy development.
The key to addressing the impacts of OA on marine resources is to produce solutions to problems relevant to society through a transdisciplinary science approach. This approach requires collaboration and coordination among the science community, other science disciplines, industry, policymakers, and coastal communities. The solutions should consider the interactive effects and consequences of co-occurring stressors in ecosystem based management plans that address OA, mitigation and adaptation strategies, and subsequent public policy. Breitburg et al. (2015) highlight the need for future research that focuses on mechanisms by which multiple stressors affect individuals, populations, and ecosystems.
A short (and very general) summary of what we know
Of the many stressors that interact with OA, increased temperature, decreased oxygen and altered food webs are inseparably linked. They are all consequences of rising carbon dioxide concentrations in the atmosphere and impact oceanic primary production (the base of food webs). Although specific changes in temperature and oxygen can vary at different locations around the planet, the impacts of these stressors are noticed globally (Figure 1).
The following summary highlights some interactions among OA and its closely related stressors and associated physical and biological responses:
- More acidic conditions and increased temperatures can alter biological energy allocation among growth, maintenance, and defense for organisms. This, in turn, can influence predator-prey interactions, change the metabolic rate of animals (depending on sensitivity), or increase energetic demands within food webs. Further, increasing temperatures both directly reduce oxygen solubility and enhance upper ocean stratification, reducing ventilation and oxygenation of interior ocean and bottom waters.
- Like temperature, the coupling of increased carbon dioxide concentrations and decreasing dissolved oxygen levels can have strong effects on physiological processes. This combination can affect a wide range of species and the effects can vary with age, can be non-additive and may not be predictable from single-stressor responses.
- In coastal and deeper pelagic waters increased aerobic respiration (a process that consumes oxygen and produces carbon dioxide) related to changes in temperature and primary production is another driver of OA. Thus, dissolved oxygen and pH are positively correlated in environments ranging from coral reefs and eutrophic estuaries to deep ocean oxygen minimum zones.
- Wind-driven upwelling which, in some cases, bathes continental shelves in low oxygen and low pH waters, has intensified in some regions in response to changing environmental conditions. This consequence can impact both benthic and pelagic communities.
How species (and food webs and ecosystems) respond to OA is complex and is even more complicated when other stressors are introduced. For a given system, the magnitude and direction of OA’s effects depend on the species, characteristics of the food web, and other environmental variables. Generally, elevated carbon dioxide concentrations can elicit nutritional responses, restructure the phytoplankton community (which can have cascading effects on the food web), increase energy expenditures in consumers, and alter food acquisition and digestive processes.
Untangling a complex picture
It is impossible to experimentally test the impacts of all stressors on all species and ecological processes. Breitburg et al. suggest that a theoretical framework for predicting the effects of multiple stressors is needed to improve our understanding of when and where co-occurring stressors will cause large, threshold and non-additive effects. Research priorities need to focus on interpreting the mechanisms that influence the propagation of stressor effects across space, time and levels of biological and ecological organization.
A key to managing multiple stressors is to identify commonalities in solutions to address co-occurring stressors together. Often these solutions relate to long-standing environmental problems particularly in the coastal environment and can lead to conflicting economic interests among groups of stakeholders. This underscores the importance of transdisciplinary approaches discussed previously to address the impacts of OA and other environmental stressors.
Carbon dioxide accumulation in the atmosphere is changing the ocean environment. It’s a critical piece to the climate change story because of its cascading effects throughout the ocean. Our awareness and understanding of OA has improved, but there remains a gap in the literacy of multiple stressor interactions. Once we shed more light on these complex interactions, we can produce effective solutions to preserve the health of vital marine ecosystems.
Primary citation: Breitburg, D.L., J. Salisbury, J.M. Bernhard, W.-J. Cai, S. Dupont, S.C. Doney, K.J. Kroeker, L.A. Levin, W.C. Long, L.M. Milke, S.H. Miller, B. Phelan, U. Passow, B.A. Seibel, A.E. Todgham and A.M. Tarrant. 2015. And on top of all that… Coping with ocean acidification in the midst of many stressors. Oceanography 28(2):48–61, http://dx.doi.org/10.5670/oceanog.2015.31.