Ocean Acidification: Under the radar yet right under our nose

Except for a few summer months out of the year, that encourage a refreshing dip into the cool ocean or local pool, humans will spend the majority of their lives on land. We spend our time up here typically unaware of other life forms in other ecosystems, and the effect we have on them. Yes, global warming and its effects has made quite the appearance in many aspects of science these days, but to what degree have these increased CO2 levels actually had on other life forms? In particular, what is going on in marine ecosystems? We can go through a whole day without coming into contact with any marine organisms, and this may be a main reason we do not think about the unbelievable influences that our above-water-world has.

Our Earth and its atmosphere operate as one giant closed system. The components to this system, that are also integral parts of the carbon cycle, are the atmosphere above the ground and the aquatic environment below the ground. As a closed system the atmosphere will directly reflect the activity of the aquatic world and the aquatic world will directly reflect the atmosphere. Any toxins, namely CO2, we put into the atmosphere will eventually make it into the marine world.  

The most well know effects of the increased CO2 levels on marine ecosystems, also known as ocean acidification, have presented itself in the devastating form of coral bleaching. Healthy coral reef communities have a symbiotic relationship with a species of algae, known as zooxanthellae. The zooxanthellae assist the coral with their nutrient production from the photosynthetic cycle, and the coral provide the algae with a protected environment to live. When the conditions of their environment become unfavorable, that is a dramatic change in the temperature and pH of the water, the zooxanthellae will become unhappy with their living conditions. They will then start to die off and expel themselves from the coral reefs. Without the zooxanthellae corals will not be able to photosynthesize or produce any of the necessary nutrients, and in turn they will die and lose their elaborate coloring to a dull white gray. 



(photo credit: J. Hoogesteger)

But other then the most publicized aspects of global warming and ocean acidification, what is going on under the radar but right under our noses? In some populations of fish the ocean acidification has had effects on olfactory senses. Although the molecular mechanism by which this function is inhibited is not completely understood, there are many hypotheses surrounding this unsuspecting result of ocean acidification. The most general but widely accepted reason for this particular sense to be hindered is because it is a response to a chemoreceptor detection of CO2. Chemoreceptors detect CO2 levels in the blood and induce responses accordingly, specifically the olfactory sensory neurons will elicit the response for sent. Perhaps the constantly increasing levels of CO2 have caused the olfactory cues to wear out, and they can no longer transmit the signals for proper sent detection.

One specific case shows that in the Juvenile lemon damselfish species, Pomacentrus moluccensis, the olfactory inhibition could soon have a dramatic effect on their population sizes. They are the prey species to the brown dotty back, Pseudochromis fuscus, which will also be negatively effected by this oceanic change. Both species are reef fish that depend on the intricacies of a reef community to protect and provide a suitable habitat for spawning. Not only will their habitat, the coral reefs, be degraded to a bleached desert they will also lose their ability to sense one another resulting in a disruption of their predator-prey interactions.

                             


(photo credit: Brown Dotty Back and Lemon Damselfish) 

In a paper by Cripps et. all observations showing unusual predator-prey interactions sparked enough interest to perform an in-depth experiment to further explain this relationship. Their conclusions showed that in an increased CO2 infused environment the prey were not as quick to detect the sent of predator presence or to find appropriate shelter to hind. When the experiment was conducted in pure ocean water the prey had quicker responses to the predators and were able to locate a better hiding spot for themselves and their eggs. There was also an observed difference in the response of the predator to available prey. Predators were roughly 20% less likely to locate and attack injured prey, easier meals, in increased CO2 waters then in pure ocean salt waters. Both parties of the predator-prey relationship are being negatively effected by this change in marine composition.

A question that I had after reading Cripps experimental discussion was: could there be a net loss of 0 if both the predator and prey are both harmed by this acidification? The answer is yes, if, and only if, they were effected equally and did not have any impact on other organisms. However this is not the case, the damselfish is significantly more impacted then the dotty back by this CO2 level change. The damselfish have lost their ability to sense the oncoming predator, to hide in a safe location, to hide their eggs in a safe location, and many of the possible productive habitat locations on a coral reef. On the other hand, the predators have only lost the ability to smell harmed prey, but are still fully capable of seeing their prey, as well as some reef habitat area as well. If the damselfish decline to their minimum viable population size and eventually crash then the population of dotty back fish will also start to decline due to a lost food resource. They will eventually reach a minimum viable population size and crash as well, causing their predator to fall into the same downward spiral. With consistently increasing CO2 levels and this possible crash of prey then predator, from the bottom up, the consequences will be catastrophic through the whole food web.

What does this all mean? Beyond the obvious solution of reducing our CO2 emissions and hoping to slow the degradation of coral reefs, which would protect many marine organisms from further damage, there is a more effective way to fight the issue. Awareness. If more people are aware of the repercussions of their actions then they would be more inclined to try to adjust their lifestyles. Education and knowledge of human impacts should go beyond the blanket statement of ‘global warming,’ it should try to connect humans to the world we live in through the bigger picture. Who would think that increased CO2 could inhibit a fish populations' sense of smell? We are just as much a part of the world as any underwater organism, the only difference being we have the power to make changes that will effect all life. It has been predicted that in 2100 the levels of CO2 could reach a staggering 950matm, which is a 550matm increase from the current day pelagic levels (Cripps). Rather then fixing the current situation of rising CO2 levels, we could focus on the next generations to come and how they can make a difference in slowing this destruction. Knowledge is power.