Hidden impacts of ocean warming and acidification on biological responses of marine animals revealed through meta-analysis.

Conflicting results remain on the impacts of climate change on marine organisms, hindering our capacity to predict the future state of marine ecosystems. To account for species-specific responses and for the ambiguous relation of most metrics to fitness, we develop a meta-analytical approach based on the deviation of responses from reference values (absolute change) to complement meta-analyses of directional (relative) changes in responses. Using this approach, we evaluate responses of fish and invertebrates to warming and acidification. We find that climate drivers induce directional changes in calcification, survival, and metabolism, and significant deviations in twice as many biological responses, including physiology, reproduction, behavior, and development. Widespread deviations of responses are detected even under moderate intensity levels of warming and acidification, while directional changes are mostly limited to more severe intensity levels. Because such deviations may result in ecological shifts impacting ecosystem structures and processes, our results suggest that climate change will likely have stronger impacts than those previously predicted based on directional changes alone.

Ocean acidification and seasonal temperature extremes combine to impair the thermal physiology of a sub-Antarctic fish.

To predict the potential impacts of climate change on marine organisms, it is critical to understand how multiple stressors constrain the physiology and distribution of species. We evaluated the effects of seasonal changes in seawater temperature and near-future ocean acidification (OA) on organismal and sub-organismal traits associated with the thermal performance of Eleginops maclovinus, a sub-Antarctic notothenioid species with economic importance to sport and artisanal fisheries in southern South America. Juveniles were exposed to mean winter and summer sea surface temperatures (4 and 10 °C) at present-day and near-future pCO2 levels (~500 and 1800 µatm). After a month, the Critical Thermal maximum and minimum (CTmax, CTmin) of fish were measured using the Critical Thermal Methodology and the aerobic scope of fish was measured based on the difference between their maximal and standard rates determined from intermittent flow respirometry. Lipid peroxidation and the antioxidant capacity were also quantified to estimate the oxidative damage potentially caused to gill and liver tissue. Although CTmax and CTmin were higher in individuals acclimated to summer versus winter temperatures, the increase in CTmax was minimal in juveniles exposed to the near-future compared to present-day pCO2 levels (there was a significant interaction between temperature and pCO2 on CTmax). The reduction in the thermal tolerance range under summer temperatures and near-future OA conditions was associated with a reduction in the aerobic scope observed at the elevated pCO2 level. Moreover, an oxidative stress condition was detected in the gill and liver tissues. Thus, chronic exposure to OA and the current summer temperatures pose limits to the thermal performance of juvenile E. maclovinus at the organismal and sub-organismal levels, making this species vulnerable to projected climate-driven warming.

Ocean warming and acidification pose synergistic limits to the thermal niche of an economically important echinoderm.

To make robust projectios of the impacts of climate change, it is critical to understand how abiotic factors may interact to constrain the distribution and productivity of marine flora and fauna. We evaluated the effects of projected end of the century ocean acidification (OA) and warming (OW) on the thermal tolerance of an important living marine resource, the sea urchin Loxechinus albus, a benthic shallow water coastal herbivore inhabiting part of the Pacific coast of South America. After exposing young juveniles for a 1-month period to contrasting pCO2 (~500 and 1400 µatm) and temperature (~15 °C and 20 °C) levels, critical thermal maximum (CTmax) and minimum (CTmin) as well as thermal tolerance polygons were assessed based on self-righting success as an end point. Transcription of heat shock protein 70 (HSP70), a chaperone protecting cellular proteins from environmental stress, was also measured. Exposure to elevated pCO2 significantly reduced thermal tolerance by increasing CTmin at both experimental temperatures and decreasing CTmax at 20 °C. There was also a strong synergistic effect of OA × OW on HSP70 transcription levels which were 75 times higher than in control conditions. If this species is unable to adapt to elevated pCO2 in the future, the reduction in thermal tolerance and HSP response suggests that near-future warming and OA will disrupt their performance and reduce their distribution with ecological and economic consequences. Given the wider latitudinal range (6 to 56°S) and environmental tolerance of L. albus compared to other members of this region's benthic invertebrate community, OW and OA may cause substantial changes to the coastal fauna along this geographical range.