Ocean acidification is frequently interpreted as the evil twin of climate change. For about three centuries, there has been an unnatural spike in carbon dioxide concentrations in our oceans. There’s more of it in the atmosphere than there was in the last 20 million years. This comes from human-based sources mostly. Fossil fuel refineries, fossil-based power plants, industries, land-use changes, and transportation are the main outlets of greenhouse gases, particularly carbon dioxide gas. These unbridled carbon dioxide emissions are responsible for ocean acidification.
Oceans cover two-thirds of the earth's surface and absorb about one-quarter of the carbon dioxide. Oceans help sustain life on earth. They are the home to oxygen-generating phytoplankton, kelp, and algae. They also provide nutrients and maintain food webs as well as support the fish and tourism industry, absorb atmospheric carbon dioxide, and regulate the climate.
What is Ocean Acidification?
The toxic concentrations of carbon dioxide have increased the average temperature of the planet and the oceans. The concentrations of carbon dioxide are stretching to record limits, with about 22 million tons of CO2 released per day. This has also affected the oceanic ecosystem and biodiversity.
“The suite of slow chemical processes in which the excessive carbon dioxide that sinks in water decreases the pH of the marine waters, making them acidic is called ocean acidification”.
This carbon dioxide, when dissolved, produces carbonic acid. This acid has detrimental effects on marine populations, especially with calcium carbonate exoskeletons, fishes, and coral reefs.
Influx of Carbon Dioxide
As the oceans are natural sinks for carbon dioxide, the process is also naturally compensated with a buffering effect. The rivers draining into the ocean carry rock minerals with them that neutralize the pH levels. It is also utilized by small photosynthetic plants and crustaceans in the oceans. Unfortunately, the flux between atmospheric and oceanic concentrations is long disturbed.
The major criminal for ocean acidification is the burning of fossil fuels. Today, our economies rely on fossil fuels and it is the major contributor to the elevated carbon concentrations. The 100 –year-long residence time of carbon dioxide makes it an important radiative force of climate change. Since it lingers around longer, it will continue to trap more heat, thanks to the greenhouse effect, and cause more acidification of the oceans even if we zero our emissions today.
Among other sources, the energy and transportation sectors are the main contributors. About 25% of the total emissions of the U.S. are attributed to transportation. Industrial procedures also contribute to the cause. The cement industry, for example, accounted for around 8% of gas released globally in 2015.
Deforestation is of concern too when talking about ocean acidification. It decreases the number of plants consuming gas. When these trees are burned, they release the carbon dioxide previously trapped inside their biomass. Tropical deforestation alone adds more carbon dioxide to the environment compared to the transport sector on the whole. The more carbon dioxide in the atmosphere, the more the ocean absorbs it and increases acidity.
In the coastal areas, tree-logging directly replaces acidic soil into waterways, gradually contributing to ocean acidification. This will have long-term consequences on marine biodiversity, productivity, and humans as well.
Ocean acidification changes the water chemistry and has negative effects on calcifying populations like corals, kelp, and oysters. It does this in two ways;
The carbon dioxide that sinks into bodies of water doesn’t remain as floating carbon dioxide. It dissolves and forms carbonic acid, or you can say carbonated water. Carbonic acid (H2CO3) then again swaps its hydrogen ion with water molecule; making bi-carbonate (HCO3-) and hydronium ions. The logarithmic concentrations of these hydronium ions are what make the marine waters acidic.
For millions of years, the oceans have maintained a slightly alkaline state, with an average pH of 8.2. However, it has now decreased by 0.1 units; from 8.2 to 8.1. This is equivalent to about a 30% increase in acidity, i.e. oceans are absorbing 30% more carbon dioxide. And it is projected to increase further by 120% at the end of the century.
The bicarbonates in acidic waters consume calcium to form aqueous calcium bicarbonates. When dissolved, this calcium that had previously accumulated in bottom sediments would not be available for calcification of marine species. Extremely low levels may even dissolve their skeleton completely. The carbon dioxide emissions and subsequent ocean acidification have gained such momentum within a century that calcifiers (calcium depositing) are struggling to adapt.
Ocean Acidification Impacts on Marine Life
Marine life is at a great risk with acidification. An Oxford Journal concluded that the synergistic relationship of ocean acidification and other anthropogenic environmental stressors is responsible for potential changes in the chemistry of the Marine ecosystem.
The calcium carbonate skeletons of marine organisms like scallops have started to soften. It means that they have to spend more energy trying to thicken their skeletons. This acidity change is stressing them out. They aren't able to put that energy into their biological activities, like reproduction.
Fish are losing their sense of smell and are unable to detect prey. The growth and development of marine species, trophic productivity and food security, and oceanic circulations are also influenced.
Ocean Acidification and Phytoplanktons:
Phytoplanktons and algae form the basis of the aquatic webs. Zooplanktons and crustaceans feed on them and, in turn, they are consumed by fish and corals. The acidification of the oceans is disrupting the complex food webs of aquatic ecosystems.
Phytoplanktons have a skeleton composed either of silicate or carbonate. The former is resistant to acidification, however, it causes carbonate-phytoplankton skeletons to weaken or even dissolve. This is very troubling for single-celled coccolithophores; 50% contributor of pelagic phytoplankton biomass. They’ve been around since the “Last Glacial Maximum” and are found in upper sunlit layers of the sea. In their ecological roles, they help sequester carbon dioxide and produce oxygen. Their population varies inversely with carbon dioxide concentrations. Carbon dioxide concentrations affects the calcification rate.
Ocean acidification might decrease the salmon population by affecting the free-swimming Pteropods (sea butterflies) that are the food source for juvenile salmon. Research showed that placing Pteropod shells in seawater with low pH projected for 2100 dissolved the shells within 45 days. This is alarming for salmon-based communities, like in Alaska. Alaskan culture as well as their economy share close knots with salmon. Any changes in the salmon populations would drastically affect their livelihood.
Effects of Carbonate Dissolution on Calcifying Creatures
The fact has been stated already that about 30% of the global carbon dioxide emissions sink into the ocean. In the current emission scenario, the concentrations are beyond the natural buffering limit of the ocean. The primary effect of acidification--under-saturation of calcium carbonate--is borne by crustacean creatures.
Acidic environments do not favor calcium carbonate formation. The mineral carbonate is the building block of the skeletons and shells of calcifying marine organisms. The hydrogen ions produced as a result of the dissolution of carbon dioxide have more affinity for carbonates than mineral ions. Thus, it forms bicarbonate instead of calcium carbonates.
Under-saturation of the mineral affects the organisms’ ability to produce and maintain their shells. Conversely, acidification or high hydrogen ion concentrations, favor the dissolution of calcium carbonate shells of marine organisms.
Because this acidification has occurred at such a quick pace, these creatures are not adapted to the low pH habitat. They have to spend more energy in the formation and thickening of body skeleton and shells under acidic conditions. This also affects the growth and development of the larvae and the overall reproduction rate of the species. These effects ripple throughout the marine trophic chain.
Coral reefs often referred to as the Rain Forest of the Ocean for their diversity, support one-quarter of the ocean species. This billion-dollar economy is severely damaged by ocean acidification. Decreased calcium carbonate levels are limiting the development of coral exoskeleton and warm waters are turning them monochromatic. Elevated water temperatures due to increased CO2 are also influencing a symbiotic association between corals and algae.
Algae grow on healthy coral reefs and provide them with food and with their bright colors. However, the corals expel the algae in warmer or acidic waters thus jeopardizing the whole coral reef-building system. The role of coral reefs in protecting coastal communities from storm waves and erosion is also compromised.