The ocean provides resources key to human health and well-being, including food, oxygen, livelihoods, blue spaces, and medicines. The global threat to these resources posed by accelerating ocean acidification is becoming increasingly evident as the world’s oceans absorb carbon dioxide emissions. While ocean acidification was initially perceived as a threat only to the marine realm, here we argue that it is also an emerging human health issue. Specifically, we explore how ocean acidification affects the quantity and quality of resources key to human health and well-being in the context of: (1) malnutrition and poisoning, (2) respiratory issues, (3) mental health impacts, and (4) development of medical resources. We explore mitigation and adaptation management strategies that can be implemented to strengthen the capacity of acidifying oceans to continue providing human health benefits. Importantly, we emphasize that the cost of such actions will be dependent upon the socioeconomic context; specifically, costs will likely be greater for socioeconomically disadvantaged populations, exacerbating the current inequitable distribution of environmental and human health challenges. Given the scale of ocean acidification impacts on human health and well-being, recognizing and researching these complexities may allow the adaptation of management such that not only are the harms to human health reduced but the benefits enhanced.

Variation in rates of herbivory may be driven by direct effects of the abiotic environment on grazers, as well as indirect effects mediated by their food. Disentangling these direct and indirect effects is of fundamental importance for ecological forecasts of changing climate on species interactions and their influence on biogenic habitat. Whilst elevated atmospheric CO2 may have direct effects on grazers with calcareous structures via ‘ocean acidification’, it may also have indirect effects via changes caused to their food. In our study we initially tested, and confirmed, that enriched CO2 altered per capita rates of grazing before assessing the relative importance of indirect and direct effects in driving this response. Our results eliminated the model of a direct effect of CO2 enrichment on the grazers themselves and supported the model of an indirect effect driven by a change in the food (i.e. turf algae). We suggest that this indirect effect manifested as grazers responded to the increased nitrogen content (i.e. %N) of algal tissue that resulted under CO2 enrichment. Understanding such indirect effects of modified environmental conditions provide important mechanistic links between climate conditions and the ecological processes they influence.Laura J. Falkenberg, Bayden D. Russell, Sean D. Connel