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Ocean Acidification and Human Health

Overview

Perhaps one of the most recent adverse effects of climate change to be explored is that of ocean acidification. Our oceans cover approximately 71 percent of the Earths surface and support a diverse range of ecosystems, which are home to over 50 percent of all the species on the planet.^[1] Oceans regulate climate and weather as well as providing nutrition for a vast variety of species, humans included.^[1] Covering such an extensive part of the planet has allowed the oceans to absorb a large portion of the carbon dioxide (CO₂) from the atmosphere.^[2] This process is part of the carbon cycle in which the fluxes of carbon dioxide (CO₂) in Earths atmosphere, biosphere and lithosphere are described.^[3] Humans have drastically added to the amount of carbon dioxide (CO₂) in the atmosphere through the burning of fossil fuels and the process of deforestation. Oceans work as a sink absorbing excess anthropogenic carbon dioxide (CO₂). As the oceans absorb anthropogenic carbon dioxide (CO₂) it breaks down into carbonic acid, a mild acid, this neutralizes the normally alkaline ocean water. As a result the pH in the oceans is declining. In the research surrounding global climate change we are only just beginning to realize that our oceans can sequester a finite amount of CO₂ before we start seeing impacts on marine life that could lead to devastating losses. Acidification of our oceans has the potential to drastically alter life as we know it - from extreme weather patterns and food scarcity to a loss of millions of species from the planet - all of these consequences hold the potential to directly affect human health.

Chemistry

The mechanism by which CO₂ is absorbed into the ocean is basic chemistry. CO₂ combines with water H₂O to form carbonic acid (H
₂CO
₃) then eventually dissociates into carbonate (CO²⁻
₃) and hydrogen ions (H⁺
). The free hydrogen ions (H⁺
) lower the pH of the surrounding waters making it acidic. The mechanism is shown here

CO_{2 (aq)} + H₂O ${\displaystyle \leftrightarrow }$ H₂CO₃ ${\displaystyle \leftrightarrow }$ HCO₃⁻ + H⁺ ${\displaystyle \leftrightarrow }$ CO₃²⁻ + 2 H⁺.

According to our records since the pre-industrial age pH has already dropped approximately 0.1 pH unit, or 30 percent because the pH scale is logarithmic.^[4] If we continue with busniess as usual it is expected that by mid-century pH could drop another o.3 pH units - at this rate our oceans would be two and a half times as acidic then previous levels. ^[4]

Temperature

Decreasing pH and rising water temperatures due to global warming and increased greenhouse gas emissions work synergistically. When the temperature rises, the chemical reaction above proceeds at a faster rate therefore, the water becomes more acidic as it warms. Conversely, warmer water is unable to hold as much CO₂ therefore, it releases more into the atmosphere, in turn, making the atmosphere warmer further warming the oceans water. ^[5]

Impacts on Marine Life

Acidification has multiple implications on marine life such as physiological sensitivities, reduced metabolism, decreased oxygen uptake and reproductive success.^[6] Looking at it from a bottom up approach, the simpler organisms are considered first moving up the food chain culminating with the ultimate apex predator, man.

Coral

Coral reefs appear to be both negatively and positively affected by pH and temperature changes. Corals in currently warmer waters appear to break down and die off as a consequence of lower pH and higher water temperatures, whereas corals in cooler waters appear to become hardier and grow faster due to pH changes and temperatures rising.^[7] This change in coral community composition will allow formerly cold water corals to slowly move into areas previously occupied by warm water species.^[7]This shift in regional locality will likely be a slow, laborious process and the mean time could actually lead to more pronounced ‘dead zones’ throughout the oceans. Acidification and temperature increase leads to ‘dead zones’, because it allows for eutrophication to occur. Blooms of algae and phytoplankton explode removing oxygen during their eventual death and decomposition.^[8] The Southern Ocean is an area of particular high risk.^[8] A decrease in coral reef cover leads to less viable fish habitat and a breakdown in the food chain, further exacerbating ‘dead zones’.^[7] In 2008, it was estimated that global fisheries dependent upon species associated with coral reefs topped US$5.7 billion annually.^[8]

Oceanic calcifying organisms

Ocean acidification also leads to a reduction in the ability for calcareous organisms to build and maintain their shells, skeletons and other structures.^{[8][9][10][11]} The decreased pH renders them unable to fix calcium Ca₂ to carbonate (CO²⁻
₃) for production of calcium-based protection, and they become easy prey for predators, if they are able to survive the more acidic conditions in the first place.^[10] Many island states and developing nations depend upon such organisms (like mussels and oysters) for sustenance and income since they may occupy land that has little terrestrial agricultural value.^[8] The decrease in native species allows for non-native, invasive species to take hold and a shift from calcareous species to soft-bodied inverts takes place.^[9] This also affects the food chain from a bottom-up perspective.^[12]

Fish

Fish are not immune to ocean acidification either. Not only does the lower pH affect their food availability, it has also been shown to impair their senses. It affects their sense of smell, hearing, balance and ability to sense predators.^[8] Further, studies have shown that acidification has positive and negative impacts on fecundity, distribution range, growth and seasonal movements.^[7][11][13] Some fish, like the anemone fish, have been able survive the pH shifts and live to reproduce, provided the parents existed in the same conditions prior to offspring being born.^[7] More studies need to be conducted with a wider range of species to determine the full scope of implications associated with this phenomenon. In one study from Southeastern Australia, ocean acidification had the largest negative impact on total fish biomass, more so than either fishing or ocean warming alone.^[13] Overall, ocean acidification had the single largest negative effect on total biomass (top predators, fishes, benthic invertebrates, plankton, and primary producers).^[13] Taken together, the additive effects of more than one stressor at the community level resulted in decreased biomass in majority of the marine communities.

Human Health

The health of our oceans has a direct effect on the health humans. According to Small and Nicholls, they estimated that 1.2 billion people worldwide, lived in the near-coastal region (within 100 km and 100m of the shoreline).^[14] This data was collected in 1990 and therefore is a conservative estimate in modern terms. In the U.S. alone 53% of the population lives within 50 miles of the coastal shoreline.^[15] Humans rely heavily on oceans for food, employment, recreation, weather patterns and transportation.^[16] In the U.S. alone the lands adjacent to the oceans contribute over $1 trillion annually through these various activities not to mention pharmaceutical and medicinal discoveries.^[16] In all, the oceans are very important for our survival as a species.

Infiltrating Fresh Water and Extreme Weather

With degradation of protective coral reefs through acidic erosion, bleaching and death, salt water is able to infiltrate fresh ground water supplies that large populations depend on. ^{[17] [18]} Nowhere is this more evident than atoll islands. These islands possess limited freshwater supplies, namely ground water lenses and rain fall. When the protective coral reefs surrounding them erodes due to higher temperatures and acidic water chemistry, salt water is able to infiltrate the lens and contaminate the drinking water supply.^[17] In coastal Bangladesh it has been demonstrated that seasonal hypertension in pregnant women is connected with such phenomenon due to high sodium intake from drinking water.^[18] Reef erosion, coupled with sea level rise, tends to flood low lying areas more frequently during storm surges and weather events. Warming ocean waters generate larger and more devastating weather events that can decimate coastal populations especially without the protection of coral reefs.

Food Safety

Our insatiable appetite for seafood of all types has lead to overfishing and has already significantly strained marine food stocks to the point of collapse in many cases. With seafood being a major protein source for so much of the population, there are inherent health risks associated with global warming. As mentioned above increased agricultural runoff and warmer water temperature allows for eutrophication of ocean waters. This increased growth of algae and phytoplankton in turn can have dire consequences. These algal blooms can emit toxic substances that can be harmful to humans if consumed. Organisms, such as shellfish, marine crustaceans and even fish, feed on or near these infected blooms, ingest the toxins and can be consumed unknowingly by humans. One of these toxin producing algae is Pseudo-nitzschia fraudulenta. This species produces a substance called domoic acid which is responsible for amnesic shellfish poisoning.^[19] The toxicity of this species has been shown to increase with greater CO2 concentrations associated with ocean acidification.^[19] Some of the more common illnesses reported from harmful algal blooms include; Ciguatera fish poisoning, paralytic shellfish poisoning, azaspiracid shellfish poisoning, diarrhetic shellfish poisoning, neurotoxic shellfish poisoning and the above mentioned amnesic shellfish poisoning.^[19]

References