Abstract
The current issue with global warming and rapid production of anthropogenic carbon dioxide (CO2) has not only been an issue on land but in the world’s oceans too. This uptake of CO2 by oceans has altered its chemistry and has consequences for marine organisms and habitats. From coral calcification rates to copepod egg hatching, ocean acidification affects many organisms, with some benefitting from the higher acidity conditions.
This article will look at the anticipated effects ocean acidification has on different marine organisms and habitats, and how some organisms have mechanisms to cope with the lower pH values. Ocean acidification is a topic of interest, but little is known about how it affects organisms and habitats as many studies have shown that it’s effects are accompanied and affected by thermal stress and other parameters. It is important to understand it’s true effects to possibly slow it down or reverse the effects.
Introduction
Oceans are a sink for carbon dioxide and contain about 50 times more CO2 than the atmosphere [1]. This increase in CO2 has resulted in a drop of 0.1 pH units since the industrial revolution, an estimated 30% increase in acidity in the oceans [2]. Ocean acidification threatens marine organisms and habitats and the effects of acidification impact the different species and habitats differently.
Corals form the framework for coral reefs and coral reef organisms. With lowered pH levels, there is a decrease in ambient carbonate ion concentrations, reducing the saturation of aragonite, suppressing the skeletal growth in Sclerectinian corals [3]. It was also found that certain Sclerectinian corals with higher tolerance were able to build resistance to lowered pH levels by up-regulating pH at their site of calcification which raises their saturation state and increases their calcification rates with little additional energy cost while the more sensitive species did not have this up-regulating capability [4]. A study conducted in 2012 comparing the results of short and long-term exposure of cold water corals Lophelia pertusa to high pCO2 levels showed that there was a decrease in the rates of calcification by up to 29% short term while there was evidence of increased calcification rates and net growth when the corals were exposed for 6 months [5]. This shows that certain coral species are able to acclimatize to the surrounding changing water conditions.
However, it was very recently discovered that these acidic conditions are also causing corals and the sediments that corals form on to dissolve. Calcium carbonate sediment dissolution is more sensitive to lowered pH levels compared to calcification rates. This will result in the loss of coral reefs and a habitat for many organisms if dissolving rates outweigh calcification rates [6]. With coral reefs on the decline, there is a loss of biodiversity as the degradation of the 3D framework results in the loss and change of ecological niches. Reef fishes declined by 75% in abundance due to reef degradation and some even went locally extinct. Larval settlement was also affected as organisms that settle on corals decreased while those that were not dependent on corals for settlement showed an increase in abundance [7].
Ocean acidification has a stronger impact on bleaching and productivity of the corals compared to calcification rates. An 8-week study conducted by Anthony et al in 2008 looking at the effects of lowered pH levels on Acropora spp., Porites spp. and crustose coralline algae (CCA) showed that Acropora and CCA were up to 50% bleached by the end of the study while Porites were only 20% bleached. Productivity levels however dropped close to 0 at low pH levels (7.6-7.7) for both Porites and Acropora [8].
A study conducted by Noonan et al in 2015 found that a decrease in pH resulted in enhanced photosynthetic activity and the concentrations of photosynthetic pigments in thermally sensitive corals Seriatopora hystrix, which led to an increase in productivity as this species was able to utilize the dissolved inorganic carbon. They also concluded that elevated pCO2 levels did not result in bleaching but rather it was the effect of an increase in sea temperatures [9].
Coral reef fishes such as O. doederleini and O. cyanosoma were found to have reduced aerobic activity in lower pH levels, with a 33% and 47% decline in aerobic scope respectively. Acidification also increased mortality rates of O. doederleini [10]. Other reef fishes such as the Pomacentrus amboinensis, in their pre-settlement stage, were unable to learn their predators’ odour, failing to evade them, which may suggest that lowered pH levels affect their cognitive thinking [11]. Clownfish larvae that had been reared in acidified waters were attracted to the smell of their predator, instead of trying to evade them, losing their ability to distinguish between prey and predator [12].
Bivalves such as the blood clams Tegillarca granosa were studied and on top of being unable to calcify, feeding activity and metabolism had been suppressed. It also led to extracellular acidosis. The overall effects saw a decline in fitness which may prevent them from escaping predators [13]. Blue crabs, lobsters and shrimp however produced thicker shells in increased acidity conditions, making them more resistant to predators [14]. The brittlestar Amphiura filiformis is able to upregulate calcification but muscle mass in the tentacles decreased. With a loss in muscle mass, the brittle star will be unable to collect food and undergo irrigation, ultimately affecting its feeding and respiration [15].
A case study on Calanus finmarchicus studied the effects of lowered pH on metabolism, egg production rates and egg hatching success, with a control pH of 8.2 and experimental treatment pH of 7.0. Although metabolism was affected, and egg production rates slowed down, there were no apparent treatment effects of lowered pH on metabolism and egg production rates but had a significant treatment effect (p<0.05) on hatching success, where only 4% of eggs hatched in elevated CO2 levels [16].
In 2013, Kroeker et al performed an experiment to see how various organisms responded to end of century pH levels and the results in Figure 1 showed that there was a significant negative effect from ocean acidification on survival, growth, calcification, development and abundance, with 27% reduction in survival and calcification. Coccolithophores, corals and molluscs saw up to 39% decrease in calcification while other calcifying organisms such as echinoderms and crustaceans were not affected [17].
Figure 1 shows the effect of future acidification values on different response variables for different type of taxa, with survival and calcification being most affected.
Figure taken from Kroeker et al, 2013.
Figure 2 shows how acidification varies with different taxa, where molluscs are more affected compared to echinoderms. Figure taken from Kroeker et al, 2013.
Different organisms are affected by ocean acidification differently as the different studies have shown. Some species are more tolerant to lower pH levels but may have detrimental effects on less tolerant species. Although many studies have been conducted, there are still uncertainties to how ocean acidification interacts with species and how it affects them as the different studies produced different results and reached separate conclusions. A lot of the studies conducted were not very recent and findings may have changed. There is also a gap in the knowledge of ocean acidification on non-calcifying organisms, which needs to be further explored. However, the effects of ocean acidification could be reversed with the help of antacid if it is further studied and looked into [18].