Estuarine acidification

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Estuarine Acidification describes the changes occurring in the pH of coastal marine ecosystems. PH change in estuaries is more complicated than the open ocean due to direct impacts from land runoff and coastal current dynamics. Ocean Acidification describes the ongoing decrease in global surface ocean pH due to increasing atmospheric carbon dioxide concentrations[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 surface ocean.[2] As carbon dioxide combines with water it releases protons based on the following equation:

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

Effect on marine life

A coccolithophore with many "plates" or coccolithes formed from calcium carbonate

As the pH of marine systems decreases, it causes calcium carbonate (CaCO3) to disassociate[3] to keep the above equation in equilibrium. Calcium carbonate is vital to calcifying organisms such as shellfish, corals, and coccolithophores (a type of phytoplankton). Each of these organisms either directly provides humans with a resource (shellfish) or supports an ecosystem important to humans. Thus, understanding changes to the pH of estuaries is critical.

Causes of variable pH

Freshwater flow

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

Photosynthesis and respiration

Primary productivity changes pH on a daily, seasonal and annual cycle as well. During photosynthesis, carbon dioxide is removed from the water, increasing pH. Alternatively, organisms release carbon dioxide during respiration.[4] This leads to a daily cycle of increased pH during sunlit hours and a decrease in pH during the night time when respiration is dominant. Similarly, pH is higher during the winter months when grazing is low compared to productivity.

Effluent

Many estuaries experience nutrient loading due to wastewater effluent and/or natural and artificial fertilizer in runoff. Increased nutrients can stimulate primary productivity and alter the balance between primary productivity and respiration. This process can also change pH whole units within the estuary. Both these processes complicate our ability to measure an overall change in pH associated with increased atmospheric carbon dioxide levels.

Effect of currents

Coastal currents play a role in estuarine acidification as well. Areas with coastal upwelling (i.e. west coast of North America) have experienced increased acidification in recent years due to more acidic deep water upwelling into the estuary.[5] This may have a detrimental effect on calcifying organism survival rates[2] because the organisms have a much more difficult time forming and maintaining their calcium carbonate shells.[3]

Research

Estuarine acidification is currently being intensively studied in order to understand all of the biological, chemical and physical factors that are affecting pH in estuaries in order to better manage our resources.

See also

References

  1. ↑ Ken Caldeira & Michael E. Wickett. "Oceanography: Anthropogenic carbon and ocean pH", "Nature", 425, 365 (25 September 2003)
  2. ↑ 2.0 2.1 2.2 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. ↑ 3.0 3.1 3.2 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
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