Ocean acidification impacts on coastal ecosystem services due to habitat degradation

Abstract

The oceanic uptake of anthropogenic carbon dioxide emissions is changing seawater chemistry in a process known as ocean acidification. The chemistry of this rapid change in surface waters is well understood and readily detectable in oceanic observations, yet there is uncertainty about the effects of ocean acidification on society since it is difficult to scale-up from laboratory and mesocosm tests. Here, we provide a synthesis of the likely effects of ocean acidification on ecosystem properties, functions and services based on observations along natural gradients in pCO2. Studies at CO2 seeps worldwide show that biogenic habitats are particularly sensitive to ocean acidification and that their degradation results in less coastal protection and less habitat provisioning for fisheries. The risks to marine goods and services amplify with increasing acidification causing shifts to macroalgal dominance, habitat degradation and a loss of biodiversity at seep sites in the tropics, the sub-tropics and on temperate coasts. Based on this empirical evidence, we expect ocean acidification to have serious consequences for the millions of people who are dependent on coastal protection, fisheries and aquaculture. If humanity is able to make cuts in fossil fuel emissions, this will reduce costs to society and avoid the changes in coastal ecosystems seen in areas with projected pCO2 levels. A binding international agreement for the oceans should build on the United Nations Sustainable Development Goal to 'minimise and address the impacts of ocean acidification'.

Keywords: CO2 seeps; IPCC; Paris Agreement on climate change; UN Sustainable Development Goals; goods and services; marine biodiversity.

Figure 1.. Aragonite saturation state (Ω) as a function of pCO₂ and temperature.

Carbonate chemistry calculated using ‘seacarb’ [58] at salinity 35 PSU and total alkalinity 2250 µmol/kg seawater. The trajectories of Ω are overlaid between present-day pCO₂ (400 ppm) and IPCC RCP 8.5-year 2100 pCO₂ (∼950 ppm) for four temperatures, spanning tropical to polar regions.

Figure 2.. Changes in ecosystem state and benthic community composition in areas with present-day (400 µatm), and elevated levels of pCO₂ at volcanic seep sites (corresponding to IPCC emissions scenarios RCP 4.5 = 550 µatm and RCP 8.5 = 950 µatm).

Ecosystem state degrades because habitat complexity and biodiversity decline along gradients of increasing pCO₂. At tropical seeps, dead reef substratum (yellow) is eroded, structural complexity of living corals declines and algae proliferate. In the sub-tropics, there is a loss of hard coral and coralline algal cover (shown in pink); diverse macroalgal communities are replaced by turf algae. On exposed temperate coasts, coralline algae also decline as turf algae proliferate, with a loss of brown algal canopy cover. In each case, calcified invertebrates (represented here by black-spined sea urchins) become fewer and smaller as pCO₂ increases. For more information, see text in section ‘Effects of ocean acidification on ecosystem state’.

Figure 3.. Ecosystem properties, functions and services provided by coastal habitat-forming species and the communities that they support.

The loss of habitat-forming organisms and degradation of ecosystem state diminishes ecosystem services. Observations at CO₂ seeps worldwide show that ocean acidification results in reductions in habitat complexity, species richness and habitat coverage. This impairs ecosystem function and the goods and services available to society, such as coastal protection, recreation and food provision.