Acidification Adversely Affects All Aquatic Animals Appreciably

Coral reefs provide our oceans with more abundance of biodiversity than any other marine ecosystem.  Although they are merely small pockets within the vast expanse of ocean, they contain over 25% of the known species of fish worldwide.  Comprised of thousands of colonial polyps, corals secrete calcium carbonate (CaCO₃) that hardens to form the intricate and protective reefs that these polyps, and many other marine species reside.  This process is known as calcification.  Corals are only one of many marine organisms that use this process to protect from the elements and predation, be it in the form of shells to the construction of exoskeletons.  Marine calcification is necessary for the most important members of marine ecosystems.  It is influenced by the carbon cycle, and more importantly it is influenced by the amount of carbon dioxide (CO₂) circulating into ocean waters.  Even small changes in acidification as a result of increased CO₂ output will potentially cause significant structural changes in the marine food web, and more importantly cause a substantial decline in biodiversity of these fragile communities.

 

A simple diagram depicting the natural flow of carbon through the three carbon sinks.  Image courtesy of: http://en.wikipedia.org/wiki/File:CO2_pump_hg.png

It is finally becoming irrefutably evident that human activity in the past few centuries is a direct contributor to the rapid increase of carbon dioxide (CO₂) emitted into the atmosphere.  The subsequent damage to the environment as a result of increased CO­₂ output is not fully understood, but what is understood is that increased emissions into the atmosphere are directly linked to increased acidity of the world’s oceans.  To help conceptualize this, think of the earth as a closed system, and the atmosphere, the crust, and the oceans as its three primary components.  Each component will hold a portion of the total amount of CO₂.  As we extract the stores of CO₂ from the earth’s crust (via oil wells, coal mines, etc.), it becomes atmospheric CO₂ (after inevitable combustion).  The atmosphere will not hold all of this new CO₂, however, because balance must be maintained between the three components.  The ocean responds by absorbing the atmospheric CO₂ in a chemical reaction involving the release of hydrogen ions (H+), which helps to mitigate the climactic effects of increased atmospheric CO₂, but resultantly decreases the oceanic pH. 

Marine Calcification is dependent upon the pH of ocean waters.  The average pH of ocean waters is currently about 8.1, or slightly alkaline.  This value decreased from 8.25 since the pre-industrial era, which is significant considering that this is on a logarithmic scale where a pH of 8 is ten times that of a pH of 7.  The most apparent impact of this sharp decline in oceanic pH is coral bleaching.  Coral organisms are very susceptible to changes in aquatic conditions.  Increased acidity leads to a decrease in the rates at which corals can calcify their protective shells, which makes them more vulnerable to erosion.  Bleaching is a result of this reduced rate in conjunction with increased surface water temperatures, which cause added stress to the fragile polyps.  In 1998, a single bleaching event caused the loss of nearly one fifth of global populations of coral.  The current levels of acidity alone may not be enough to cause the destruction of coral reef habitats, but the synergistic effects of acidification and increased surface water temperature have proven to pose a substantial threat to these ecosystems.  Using business-as-usual models, global ocean temperatures are expected to increase by as much as 5°C, and global oceanic pH may see a drop to 7.8.  Considering that coral can only resist temperature changes between 1-2°C, our global reefs are at considerable risk of extinction is nothing is done.

The threat posed upon corals is further propagated by the effect of acidification on their food source, plankton.  Many species of plankton form exoskeletons out of CaCO₃, much like corals.  Models predict that within 100 years, global pH will inhibit the exoskeleton formation in these microorganisms so much so that they will not be able to survive.  The images above show the structural calcification of phytoplankton now (left) compared to structural calcification under predicted concentrations one century from now (right).  The images indicate that calcification is inhibited by acidification, and causes structural deformities in phytoplankton growth.  The most obvious difference in the two images is the spacing of calcified plates.  Under normal (current) conditions plates are evenly distributed, providing sufficient protection from the elements, but under conditions of increased acidification the plates do not form evenly and are perforated and broken.  This could prove to be devastating for phytoplankton and anything that feeds on them.

The visible effects of acidification are not the only effects on aquatic populations in the world’s oceans.  A number of studies are surfacing that attempt to evaluate the impacts of lowered pH on other aspects of organism physiology.  One study performed by researchers from the School of Marine and Tropical Biology examined the effects on olfactory senses of predatory reef fish common to the Great Barrier Reef off the coast of Australia.  The study involved comparing prey detection of the predatory fish,  the brown dottyback (Pseudochromis fuscus), in a control environment as well as two environments with increased levels of acidity (reflecting near and distant future estimates).  The study found that there were substantial declines in the predator’s awareness of prey, in this case the lemon damselfish (Pomacentrus moluccensis), in the elevated pH environments, in the prey’s detection of the predator, and in the prey’s ability to seek out safe locations to spawn.  The most troubling takeaway from this experiment is that elevated CO₂ levels in the atmosphere may lead to conditions that have varying impacts on different species.  The two species from the study were affected, but there was a two-fold effect on the prey species.  This will likely have a devastating impact on the predator-prey balance in real-world conditions, and may even lead to the extinction of the prey species.

With the significant impacts of acidification on predator-prey interactions in mind, the inevitable changes in the dynamics of interacting species is clearly at risk for substantial if not complete loss.  Phytoplankton are among the most essential organisms for life in the ocean, corals house thousands of species of fish, and myriad other species are adversely affected by the changes in pH for reasons other than decreased ability to calcify CaCO₃.  The consequences of acidification have been studied and examined in the wild.  There are potentially many more impacts that we have not accounted for, especially since the determination of the effects of acidification is relatively fresh in scientific community.  The risk that decreasing oceanic pH poses on the innumerable interspecific interactions in the world’s oceans merits a level of awareness not yet met by the general public.  Awareness is crucial, however, if we are to mitigate the consequences of acidification.  Otherwise we are rolling a dice, and the odds are not in our favor.