Ocean acidification causes ecosystem shifts via altered competitive interactions

Abstract

Ocean acidification represents a pervasive environmental change that is predicted to affect a wide range of species 1,2 , yet our understanding of the emergent ecosystem impacts is very limited. Many studies report detrimental effects of acidification on single species in lab studies, especially those with calcareous shells or skeletons 3-5 . Observational studies using naturally acidified ecosystems have shown profound shifts away from such calcareous species 6-8 , and there has been an assumption that direct impacts of acidification on sensitive species drive most ecosystem responses. We tested an alternative hypothesis that species interactions attenuate or amplify the direct effects of acidification on individual species 9-12 . Here, we show that altered competitive dynamics between calcareous species and fleshy seaweeds drive significant ecosystem shifts in acidified conditions. Although calcareous species recruited and grew at similar rates in ambient and low pH conditions during early successional stages, they were rapidly overgrown by fleshy seaweeds later in succession in low pH conditions. The altered competitive dynamics between calcareous species and fleshy seaweeds is probably the combined result of decreased growth rates of calcareous species, increased growth rates of fleshy seaweeds, and/or altered grazing rates 13 . Phase shifts towards ecosystems dominated by fleshy seaweed are common in many marine ecosystems 14-16 , and our results suggest that changes in the competitive balance between these groups represent a key leverage point through which the physiological responses of individual species to acidification could indirectly lead to profound ecosystem changes in an acidified ocean.We deployed recruitment substrates in zones of extreme low, low and ambient seawater pH caused by shallow CO 2 vents at two replicate sites (Supplementary Fig. S1). The pH zones are caused by spatial variability in CO 2 venting 6 , resulting in decreased pH and carbonate ion concentrations and increased dissolved inorganic carbon at ambient temperature and alkalinity (Supplementary Fig. S1 and Table S1). The carbonate chemistry in the ambient pH zone is comparable to current conditions in the temperate surface ocean in the Mediterranean, whereas the low pH zones are comparable to predictions for the acidification of the near-future surface ocean (in the year 2100; ref. 17). The carbonate chemistry in the extreme low pH zones is not predicted in the near future but provides an endmember scenario for understanding acidification impacts. Previous studies have highlighted a reduction in diversity and the abundance of calcareous species in the low and extreme low pH zones 6,7,18 , but the processes underlying these patterns have not been investigated. Recruitment tiles were deployed at the beginning of the growing season for algae, and an independent

The other ocean acidification problem: CO 2 as a resource among competitors for ecosystem dominance

“…Studies of volcanic CO 2 vents reveal the potential for altered competitive balances between space-holders at projected near-future levels of CO 2 (750 ppm [46]). In particular, these studies reveal the potential for increased dominance of mat-forming macroalgae in low pH environments in temperate [47] and tropical zones [48].…”

“…Here, we compare the effects of CO 2 enrichment on matforming algae among several projects, combining findings from laboratory experiments and mesocosms with those of natural field studies. For field studies, we use observations of benthic communities around volcanic seeps that have been shaped by decades-long exposure to fine bubble streams of CO 2 seeping from the seabed, which cause local alterations in the seawater chemistry [47,48]. These seep sites complement and improve the interpretive value of laboratory experiments.…”

“…Our comparisons focus on benthic communities exposed to similar elevations in CO 2 concentrations that are within the forecasted range of change by the end of this century [46,67]. Relative to ambient conditions (8.0-8.1 pH T ), two levels of projected future pH were compared in warm temperate (Mediterranean [47]) and tropical (Papua New Guinea [48]) regions (table 1). The future pH conditions were chosen to represent the projected near-future conditions anticipated for the end of the twenty-first century ( pCO 2 approx.…”

Adaptation and acclimatization to ocean acidification in marine ectotherms: anin situtransplant experiment with polychaetes at a shallow CO2vent system

“…The control area (C) (San Pietro point, approx. 4 km from the vent area; figure 1) was situated in close proximity to the Benthic Ecology research unit (Villa Dohrn) and was selected because of the persistent high pH/low pCO 2 conditions and its accessibility [44]. Three stations (C1, C2 and C3-3 m depth) were established approximately 50 m apart from each other (figure 1).…”

Temperature modulates the response of the thermophilous sea urchin Arbacia lixula early life stages to CO2-driven acidification

“…Dupont et al, 2012) allowing estimating acclimation and adaptation potential, the modulating role of ecological interactions and other environmental parameters (Kroeker et al, 2013). Future research should also aim at the investigation of the effects of multiple stressors on sea urchin recruitment and adult survival.…”