***Third paragraph is modified
Katelyn Chapman
Sarah George-Waterfield
English 105
2 September 2015
Feeder 1.1
Topic: The effects of climate change-driven ocean acidification (elevated carbon dioxide
levels) on bioerosion in coral reefs.
Citation: Wisshak, Max M. "Ocean Acidification Accelerates Reef Bioerosion." PloS
One 7.9 (2012): 1-8. Plosone.org. 18 Sept. 2012. Web. 2 Sept. 2015.
Ocean acidification is one of many climate-change-driven issues facing the Earth
today. Along with global warming, ocean acidification has the potential to (and has to
some extent already) cause a domino effect of increased bioerosion and trophic cascade
in our Earth’s coral reefs.
The coral reef biome is one of the most sensitive to temperature, salinity (the level
of salt present in a volume of water), and pH (the acidity of a volume of water) changes.
As is widely known in relation to global warming issues, an increase of carbon dioxide
has been released into our atmosphere through mostly human action such as coal power
plant and automobile emissions. This massive increase in atmospheric carbon dioxide has
caused a relative increase in the amount of carbon dioxide absorbed into the Earth’s
oceans. This increase in carbon dioxide levels in the ocean water has lead to a net
decrease in ocean water pH from the ideal, marginally basic pH in coral reefs (pH score
slightly above 7) to a slightly acidic pH (pH score slightly below 7). This decrease in
water pH (also known as an increase in water acidity) detrimentally affects coral reefs by
causing “weakened calcification” in corals and other organisms that produce carbonate
skeletons (1). Coincidentally, the increase in ocean acidification appears to create more
favorable conditions of bioeroding organisms such as certain sponge species that further
the de-calcification of corals, and therefore, the destruction of coral reefs. The decline in
coral health has already caused a decline in reef health overall as certain species of coral
are classified as keystone species—species that, when lost, cause a rapid and detrimental
decline in an ecosystem that is known as trophic cascade.
A recent experiment has tested the effect of decreasing pH levels in coral reefs on
the biodegrading sponge Cliona orientalis in Australia’s Great Barrier Reef. The purpose
of this experiment was to attempt to determine whether the increasing acidity of ocean
water leads not only to weakened calcification of corals and subsequent coral death but
also to the increase biodegradation of coral due to increased activity of biodegrading
organisms. Four cores of live Cliona orientalis sponges were placed in separate simulated
natural reef environments. The first tank was exposed below current oceanic carbon
dioxide levels, the second tank was exposed to current oceanic carbon dioxide levels, the
third tank was exposed to elevated carbon dioxide levels (and therefore a lower pH level),
and the fourth tank was exposed to strongly elevated carbon dioxide levels (and therefore
a strongly acidic pH level). The nutrient levels in the water, the temperature, and a 12
hour light/12 hour dark diurnal rhythm were kept the same for all four tanks as the
experimental controls. The sponges were left in their respective tanks for 10 days, after
which “bioerosion rates were determined gravimetrically by applying the buoyant weight
method at the beginning and the end of the experiment after 10 days of exposure” as “this
method allows an accurate determination of substrate weight loss” (4). **I would put this
paragraph relatively soon after the introduction paragraph as it explains the experiment
itself.
The results of the study were that “bioerosion rates significantly increased with
rising CO2” (5). In the moderately elevated tank the bioerosion rate corresponded to a
seventeen percent increase in bioerosion when compared to the current rate of bioerosion
in the reef from which the sponge was taken (5). In the strongly elevated tank the
bioerosion rate was sixty one percent higher than the current bioerosion rate in the reef
(5). These results support the hypothesis that increased acidity of ocean water (aka
decreased pH levels due to increased carbon dioxide levels) correlates strongly to the
increased activity of coral-killing sponges, and therefore suggests that ocean acidification
is causing the accelerated degradation of the Earth’s coral reefs.
Ocean acidification: a term used to describe significant changes to the chemistry of the
ocean. It occurs when carbon dioxide gas (or CO ) is absorbed by the ocean and reacts
2
with seawater to produce acid.
Bioerosion: the biological breakdown of calcareous reef materials.
Biome: A complex biotic community characterized by distinctive plant and animal
species and maintained under the climatic conditions of the region, especially such a
community that has developed to climax. Ex: Tundra, Coral Reef, Prairie, Desert
pH scale: a numeric scale used to specify the acidity or alkalinity of an aqueous solution.
Solutions with a pH less than 7 are acidic and solutions with a pH greater than 7 are
alkaline or basic.
Coral Calcification: the process by which corals produce calcium carbonate (CaCO3).
Salinity: dissolved salt content of a body of water.
Acidity: the amount of acid present in a solution, often expressed in terms of pH. A high
pH score (a number greater than 7) indicates low acidity, a low pH score (a number lower
than 7) indicates high acidity.
Trophic Cascade: an ecological phenomenon triggered by the addition or removal of top
predators (keystone species) that involves reciprocal changes in the relative populations
of predator and prey through a food chain, which often results in dramatic changes in
ecosystem structure and nutrient cycling.
Diurnal Rhythm: Repeating once each 24 hours. (Night and Day cycle).