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Biodiversity of Ecosystems Decreases in a High-CO2 Ocean

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Absorption of atmospheric CO2 in the oceans could have an impact on marine ecosystems.

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Climate change resulting from increased atmospheric carbon dioxide (CO2) and other greenhouse gases could also have other consequences.
Scientists are concerned that the absorption of atmospheric CO2 in the oceans could have an impact on marine ecosystems. This would result from absorption of atmospheric CO2, which causes a decrease in seawater pH and carbonate ion concentrations, while increasing CO2 and bicarbonate ion concentrations.

Studies suggest that calcareous species that rely on carbonate ions to build their shells or skeletons of calcium carbonate could suffer reductions in calcification and ultimately abundance in an acidified ocean. Furthermore, studies in naturally acidified ecosystems suggest that acidification can cause an entire reorganization of marine communities, but little is known about the ecological mechanisms underlying these changes and the consequences for ecosystem function.

Whereas acidification may slow the growth rates of some calcareous species, it may also increase the growth rates of noncalcareous algae due to increased concentrations of the chemical substrates used for photosynthesis. This could potentially alter competitive interactions between calcareous species and fleshy algae by slowing the recovery of calcareous species from disturbance and promoting a shift toward noncalcified, macroalgal-dominated communities.

A recent study looked at how ocean acidification affects the recovery patterns of benthic rocky reef assemblages, which comprise one of the most diverse and spatially complex ecosystems of the Mediterranean.

They used natural volcanic CO2 vents, found off the coast of Italy, because it releases predominantly CO2 with very small amounts of other gases, and creates gradients in pH and carbonate chemistry. They identified three zones that vary in pH. Ambient pH zones, that are comparable to current conditions in the surface waters in the Mediterranean Sea. Low pH zones that are most comparable to nearfuture (i.e., 2100) scenarios, whereas those in the extreme low pH zones correspond to more extreme scenarios based on high CO2 emissions or the more distant future (e.g., 2500).

For the methodology, to analyze the recovery patterns, fours plots (20 × 20 cm) were cleaned with wire brushes to remove all aquatic fauna and algae in each zone. Then, these plots were photographed in situ at 0, 1, 3, 6, 14, 20, and 32 months after the initial clearing and interpreted.

Interpretation shown that the differences were primarily due to a higher abundance of calcareous algae in ambient pH versus a higher abundance of fleshy algae in the low pH zones. In contrast, biofilm/filamentous algae and erect fleshy algae dominated the extreme low pH zones. In addition, there was a greater variability in the communities in the ambient and extreme low pH zones than those in the low pH.

Kroeker et al. results reveal how altered community dynamics in conditions representing future environmental scenarios reduce the natural variability in community structure, resulting in more homogeneous, simplified, and turf-dominated communities.

Also, an important distinction between the vent ecosystem and future scenarios of ocean acidification is that some species larvae can originate from nonacidified source regions, which would not be the case in an acidified ocean. Thus, if there are bottlenecks in species tolerance to acidification caused by reproduction or larval survival, these effects could be underestimated at the vent site.

Reference:
Kroeker KJ, Gambi MC and Micheli F. (2013). Community dynamics and ecosystem simplification in a high-CO2 ocean Proceedings of the National Academy of Sciences of the United States of America DOI: 10.1073/pnas.1216464110

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