Estuarine acidification

Estuarine acidification is a decrease in the pH of coastal marine ecosystems, specifically those of estuaries. pH change in estuaries is more complicated than in the open ocean due to direct impacts from land run-off and coastal current dynamics. Ocean acidification is the ongoing decrease in the pH of the Earth's oceans, caused by the absorption of carbon dioxide (CO₂) from the atmosphere^[1] (0.1 units over the last century).^[2] The ocean absorbs 30-40% of all CO₂ emitted to the atmosphere;^[3] this increase in aqueous carbon dioxide causes a decline in the pH of the ocean surface.^[2] As carbon dioxide combines with water, it releases protons (hydrogen ions), based on the following equation:

CO₂ + H₂O ↔ H₂CO₃ ↔ HCO₃ + H⁺ ↔ CO₃ + 2 H⁺

Causes of variable pH

Freshwater flow

An estuary is defined as "a water passage where the tide meets a river current". The pH of estuaries is highly variable because of freshwater flow from rivers and groundwater, as well as primary productivity (exacerbated by nutrient loading) and coastal upwelling. Fresh water from rivers typically has a lower pH than ocean water (~7 compared to ~8). Seasonal and annual changes in river flow entering an estuary can change the pH by whole units.

Photosynthesis and respiration

Primary production (plant growth) changes pH on a daily, seasonal, and annual basis. During photosynthesis, carbon dioxide is removed from the water, increasing pH. Organisms release carbon dioxide during respiration.^[4] This leads to a daily cycle of increased pH during daylight hours and a decrease in pH during the night, when respiration is dominant. Similarly, pH is higher during the winter when grazing is low compared to productivity.

Effluent

Many estuaries experience nutrient loading from runoff containing wastewater effluent or fertilizers, natural or artificial. Increased nutrients can stimulate primary productivity and alter the balance between primary productivity and respiration. This process can change pH by whole units within the estuary. Both these processes make it difficult to measure the overall change in pH associated with increased atmospheric carbon dioxide levels.

Currents

Areas with coastal upwelling such as the west coast of North America have experienced increases in acidification due to more acidic deep water upwelling into the estuary.^[5] This may have a detrimental effect on the survival of calcifying organisms^[2] because the organisms have a much more difficult time forming and maintaining their calcium carbonate shells.^[3]

Impact on marine life

A coccolithophore with many coccoliths (plates) formed from calcium carbonate

As the pH of marine systems decreases, it causes calcium carbonate (CaCO₃) to dissociate^[3] to keep in chemical equilibrium. Calcium carbonate is vital to calcifying organisms such as shellfish, corals, and coccolithophores (a type of phytoplankton). Acidification also harms micro-organisms in the environment. These organisms either directly provide humans with a food source or supports an ecosystem important to humans.

Research

Estuarine acidification is being intensively studied to understand the biological, chemical, and physical factors that affect pH in estuaries.

References

1. Ken Caldeira & Michael E. Wickett. "Oceanography: Anthropogenic carbon and ocean pH", Nature, 425, 365 (25 September 2003)
2. Orr, James C.; et al. (2005). "Anthropogenic ocean acidification over the twenty-first century and its impact on calcifying organisms.". Nature 437 (7059): 681–686
3. Richard A. Feely, Christopher L. Sabine, Kitack Lee, Will Berelson, Joanie Kleypas, Victoria J. Fabry, and Frank J. Millero. "Impact of Anthropogenic CO2 on the CaCO3 System in the Oceans", Science, 16 July 2004: 305 (5682), 362-366
4. NOAA "Estuary Education"
5. Richard A. Feely, Christopher L. Sabine, J. Martin Hernandez-Ayon, Debby Ianson, and Burke Hales. "Evidence for Upwelling of Corrosive "Acidified" Water onto the Continental Shelf", Science, 13 June 2008: 320 (5882), 1490-1492