How does rising atmospheric CO2 affect marine organisms?

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Fish Diversity is Unaffected by Ocean Acidification at a Natural CO2 Vent Site

Paper Reviewed
Mirasole, A., Signa, G., Gianguzza, P., Bonaviri, C., Mazzola, A. and Vizzini, S. 2020. Fish assemblages cope with ocean acidification in a shallow volcanic CO2 vent benefiting from an adjacent recovery area. Marine Environmental Research 157: 104851,

Many people are concerned about the potential impacts of so-called ocean acidification on marine life. They worry that rising levels of atmospheric CO2 will reduce the alkalinity of the ocean's surface waters by approximately 0.3 pH unit over the next century, which pH decline they claim will negatively impact all sorts of marine organisms, including fish.

In a real-world test of this hypothesis, Mirasole et al. (2020) examined the community structure and biodiversity of fish assemblages inhabiting Cymodocea nodosa seagrass meadows exposed to high pCO2 (low pH) conditions near a natural CO2 vent in the Mediterranean Sea, comparing their observations with that obtained at two control sites of normal seawater pH some 250 and 500 meters away.

The study area was located near Vulcano and Lipari Islands 24 km off the northeast coast of Sicily, Italy. Seawater pH values ranged from 7.8 at the underwater CO2 vent to 8.19 and 8.22 at the two control sites. At each location the authors performed underwater visual censuses along transects across two periods (September-November of 2014 and May-July of 2015), gathering data on fish type, size, number and age and seagrass shoot density and leaf canopy height. All data were subjected to appropriate statistical analyses to elucidate relationships between physico-chemical seawater variables, seagrass features and fish diversity. It was the authors' hypothesis that fish biodiversity would be higher at the control sites than at the underwater CO2 vent site that served as a natural analog of future predicted impacts of ocean acidification.

So what did the study reveal?

In the words of the authors, fish diversity, in terms of species richness and abundance "did not show a unique spatial pattern [nor] significant relationships with pH." More specifically, they report that "although a few species were more abundant at the low pH site than in both controls, we did not find the expected reduction in fish biodiversity or the homogenization of the community structure." Rather, the findings highlight "a well-structured nekton-benthic fish assemblage in the acidified site, probably driven by higher resources availability close to the CO2 vent." Consequently, they add, their work indicates that "fish do not avoid the low pH site, possible driven by the boosted food resources (low-order invertebrates) that allow to maintain a well-structured assemblage also in terms of trophic structure and diversity."

Given the above observations, Mirasole et al. write in the conclusion of their paper that "nekton-benthic fish assemblages from CO2 vents seem to be able to cope with ocean acidification under the CO2 emission scenarios forecasted for the end of the century, by forming well-structured fish assemblages." Further, they add "because of their behavioral and physiological features (mobile habitus, capacity of acid-base regulation) and indirect effects at CO2 vents (i.e., greater food availability), fish can balance the potential higher energetic cost to live under high pCO2/low pH environments without major changes in community structure." And that summation is great news and shows predictions of widespread fish decline and even extinction due to rising atmospheric CO2 are wildly incorrect.

Posted 3 July 2020