Ocean Acidification’s Effect on our Oceans

Ocean acidification can be loosely explained as the gradual decrease in ocean pH, caused by the uptake of carbon dioxide from the earths atmosphere. Meaning, as more and more carbon dioxide is released into the atmosphere, the oceans pH decreases.

Most people have come to terms with the reality of Global Climate Change (no longer referred to as global warming, since climate change actually causes certain places on earth to become cooler), but there are surprisingly still those that believe it is all a lie.

Many people say “but isn’t it normal for the earths temperature to fluctuate? Hasn’t this happened before?” To address this assumption-yes, the earths temperature and CO2 does fluctuate globally. The overall problem with this theory, however, is that while the earths carbon dioxide levels do fluctuate, ever since the Industrial Revolution, there has been a very unnatural spike in global CO2 concentrations, shown in this graph provided by ReviseScience.com. The rates of carbon emissions are not decreasing, and it is something people need to acknowledge.

Many people are aware of Global Climate change, but most people are unaware of the secondary consequences of this process. One of these consequences is ocean acidification.

This process is the slow acidification of the ocean due to rising amounts of CO2 in the atmosphere that subsequently gets absorbed by the ocean. Half of the worlds oxygen is produced by small photosynthetic organisms in the ocean. Just like land plants, they use carbon dioxide and convert it into oxygen. According to Scripps Institute of Oceanography, the ocean is responsible for absorbing 26% of all carbon emissions, this translates to 2.5 billion tons of carbon dioxide. This process is vital to all respiring organisms on the planet.

According to ecology.com, the atmospheric CO2 at the Mauna Loa observatory has risen from 310ppm(parts per million) to 400ppm from 1960 to 2010.This is a very significant amount that has caused global processes to change. Because of this large rise in carbon dioxide in the atmosphere, the ocean has been absorbing more than it can use for photosynthesis. Because the ocean cannot use this carbon dioxide, but it is still contained within the water, it creates a more acidic environment-reducing the oceans pH. This is a harmful process for many reasons: animals that use calcium carbonate (all shelled organisms) are now being exposed to acidic conditions. As many people know due to the egg-in-vinegar experiment, acid dissolves calcium, which means it dissolves shell. This not only puts our favorite shellfish at risk — it harms the microorganisms that are vital to our survival. These small organisms are known as plankton. There are animal plankton (zooplankton) and plant plankton (phytoplankton). The latter are the ones that create half of our oxygen, one type is pictured below.

Calcium Carbonate phytoplankton: a Coccolithophore

The skeletons that make up these small plants come in two varieties: calcium or silicate. Silicate organisms are resistant to acidification, leaving the calcium carbonate phytoplankton vulnerable. This causes their skeletons to weaken and in some studies, they dissolve almost completely in high enough concentrations. This is very troubling for many reasons, one being that coccolithophores are responsible for the biggest biomass of phytoplankton in the ocean.

Coccolithophores are unicellular, eukaryotic phytoplankton. Sediment records showed that during the “Last Glacial Maximum” coccolithophore mass was high while CO2 levels were low, this occurs because when ice freezes it captures CO2 inside. This is shown in the graph below:

From this graph we can see that when the coccolithophore mass was at its highest, conversely CO2 levels were at the lowest. Displaying a inverse relationship between CO2 levels and the amount of coccolithophores. The last glacial maximum was about 20,000 years ago, when CO2 levels were around 200ppm and the coccolithophore mass was 13.5pg (1 picogram=10–12 grams). This contrasted with coccolith mass decreases during the deglaciation period (1.7–2,000 years ago) during which more CO2 enters the atmosphere from melting ice, causing CO2 levels to rise to 280ppm- a very large increase, and coccolith mass to decrease to ~7.5pg (almost cut in half). The correlation between coccolith mass and CO2 levels were observed at all latitudes in differing ocean basins. (Beaufort et al. 2011) This shows that no matter the location of the plankton, the main factor affecting the calcification of these organisms is the levels of CO2 in the atmosphere, and hence, the water. Specifically, the higher the CO2 levels, the lower the calcification rate and mass of these organisms.

There is one exception that has been found for these calcifying organisms. In Calcidiscus leptoporus, a species of coccolithophore, calcification rates are highest at present CO2 levels. The tested subjects were seen with deformed coccoliths and coccospheres at low and high pCO2 levels (M. Guinotte and Victoria J. Fabry, 2008). This shows that a species of coccolithophores has adapted to the rising CO2 levels. Providing evidence that not all calcifying organisms are negatively affected by ocean acidification. The evidence showing that a species of coccoliths have the ability to thrive in a more acidic environment is good news for the future. This shows possible hope for the future of calcifying organisms-if they are able to keep up with the ever increasing levels of CO2 in the atmosphere and oceans.

These studies are important to the future of the oceans because of their contribution to the ocean system as a whole. Globally, coccoliths produce most of the CaCO3 (calcium carbonate) in the oceans. This means they account for almost all of the “export flux” of CaCO3 to the deep sea from the surface waters (Fabry, V. J., and B. A. Seibel, 2008). This distribution of CaCO3 to the bottom of the ocean is important for sediment production and nutrient distribution. Lots of this CaCO3 dissolves on its way down to settle on the bottom, making it available to other organisms in the water to use for their own shell growth. It is hypothesized and has been observed that in an environment with increased pH, reefs transform from a calcium carbonate ruled system to one dominated by organic algae (Fabry, V. J., and B. A. Seibel, 2008). This could heavily decrease diversification in reef systems, which are at their base kept alive by reef building corals, another calcifying organism.

Coccolithophores appear to have a difficult future ahead. Though it may be years in the future, ocean acidification is starting to come into play in the present. These organisms experienced extreme decreases in calcification rates when put under conditions of a pH predicted to be a reality in the near future. One unexplainable species of coccolith has found a way to not only calcify but do so successfully at its highest rate in the oceans current pH, rather than the more basic pH of the past. This is a promising result and may lead to a better understanding of how calcifying organisms may persevere in the future.

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