Wednesday, July 7, 2010
I am going to start a post about fish by talking about soft drinks. Actually, I am using soft drinks or soda pop as an allegory for ocean acidification. Soda pop is a carbonated beverage. Carbonation occurs when carbon dioxide (CO2) is dissolved in an aqueous solution. So all those little bubbles that make your drink fizz are carbon dioxide gas. CO2 gas itself is colorless and slightly acidic. As such, when it is dissolved into a solution it increases the acidity of that solution. The more CO2 the more acidic (the lower the pH).
Let's take the next logical (and scientifically tested) leap. Atmospheric CO2 naturally dissolves into the Earth's large bodies of water, like the oceans, particularly in the shallows. If the atmospheric CO2 stays at a constant concentration then this keeps the ocean acidity at a relatively constant concentration. As atmospheric CO2 increases, so does the amount of CO2 that dissolves causing a decline in ocean pH. This process is called ocean acidification. If our current trajectory continues then atmospheric CO2 concentrations will exceed 500 ppm by the middle of the century and reach 730-1020 ppm by the end of the century. This will cause ocean pH to decline by 0.3-0.4 units, which is a lot considering the pH scale only goes to 14. This type of change is known to affect the ability of marine calcifiers form shells and skeletons, and not much is known on how it will affect other species such as fish.With species that have a planktonic larval phase, like many fishes, chemical cues are used to locate suitable adult habitat and avoid predators during the settlement process.
A study published in PNAS, takes a look at ocean acidification and its relationship to marine ecosystem function. The study used both laboratory and field experiments on clownfish (Amphiprion percula) to show that predicted levels of dissolved CO2 for this century are likely to alter the behavior of larval fish, decreasing their survival rates during recruitment into adult populations. Previous studies have shown that these fish lose their ability to distinguish chemical cues from preferred settlement habitat and predators when they are exposed to acidified seawater. This impairment increases the mortality among a life stage that already incurs naturally high mortality rates. With this in mind the authors designed an experiment where they tested the levels of CO2 at which this impairment occurs and if it actually affects mortality in natural populations of clownfish. They reared larval clownfishes under a range of CO2 treatments (a current day control of ~390 ppm, 550, 700, and 850 ppm) and then tested their behavioral responses to olfactory cues from predators. That was the lab part, they then repeated the experiments with wild-caught damselfish larvae (Pomacentrus wardi) to see if they had the same reactions. Finally, they transplanted settlement-stage damselfish to various natural reefs to test if exposure to elevated CO2 altered their behavior and increased the risk of mortality in these fish.
The researchers found that young clown fish are unable to sense the chemical cues that direct them towards favorable habitats and away from predators, meaning that they swim towards water containing predators' pheromones and away from the safety of the reef. Altered larval behavior was detected in water that contained as little as 700 ppm CO2 (a concentration that could be reached by 2100), and at 850 ppm CO2 the ability to sense predators was completely impaired. These types of results were shown to be similar in the damselfish as well as clown fish, adding to the known body of research about how high concentrations of dissolved CO2 adversely affect shellfish and crustaceans. In general, the larval fish exposed to high dissolved CO2 were found to be more active and exhibit riskier behavior, resulting in 5-9 times higher mortality from predation (mortality increases as CO2 concentration increases). This reduces the recruitment success of these species and affects the sustainability of fish populations.
This is the study:
Munday, Philip L., et al. (2010) Replenishment of fish populations is threatened by ocean acidification. Proceedings of the National Academy of Sciences: 107(29), 12930-12934. (DOI: 10.1073/pnas.1004519107)
And here's a story:
(image from sandyart.com/Clown_Fish)