Ocean Acidification: The Other CO2 Problem

Many of Arctic responses to climate change explored in this blog, such as lake ecology regime shifts, terrestrial ecosystem alterations, melting of the Greenland Ice Sheet, and permafrost thawing, have focused on the warming effect of increased atmospheric CO2 levels as a driver for change. However, marine environments are also under threat from enhanced CO2 uptake in the ocean as a result of elevated atmospheric CO2 concentrations (Sabine et al., 2004). Worldwide, there has been an increase in ocean acidity of ~30% over the last 200 years, a rise that is mirrored in the Arctic Ocean (AMAP, 2013). With its low temperatures, sea ice retreat and large input freshwater input, the biological and chemical impacts of ocean acidification are expected to be most pronounced in the Arctic Ocean (Popova et al., 2014).

To what extent will the Arctic Ocean become acidified?

Aragonite reduction

The reaction of excess CO2 with water forms carbonic acid that acts to lower pH and decrease the concentration of carbonate ions (carbon undersaturation) which are used in the formation of the shells and skeletons of some organisms (e.g. corals, coccolithophorids, foraminifera, pteropods). A seasonal reduction in aragonite (one of two forms of calcium carbonate) has already been observed in Arctic surface waters, including the Canada Basin, Arctic shelves, and western Arctic Ocean. The transferral of anthropogenic CO2 to Arctic surface waters has accelerated because of significant reductions in sea ice. In addition, this increased influx of brackish water has led to further declines in the availability of Ca²⁺, decreasing alkalinity and moving us closer toward an aragonite tipping point (AMAP, 2013).

The inorganic carbon system in the Arctic Ocean (AMAP, 2013)

The role of primary production

Coastal and riverine erosion supply a large volume of organic carbon to the Arctic shelves. In some nearshore locations, the release of some of this terrigenous organic carbon store through oxidation to CO2 is not permitted by seasonal ice, however, metabolism still occurs. With some of the highest primary productivity rates observed in the world ocean, metabolism of organic carbon in Arctic shelf waters releases large amounts of CO2, which lowers pH. Upwelling of this CO2-rich water is likely to increase with reductions in sea ice, with these waters in the western Arctic Ocean then transferred to the Pacific Ocean (AMAP, 2013).

Alterations to the marine N cycle

The response of microbially-driven biogeochemical cycling to enhanced CO2 concentrations was investigated by Tait et al. (2014). Fixation of nitrogen gas by diazotrophs is the largest N source to oceans globally, while denitrification through microbial processes (reduction of oxidised N to N gas) and anammox (anaerobic oxidation of ammonium with nitrate) leads to losses of N from oceans. By altering microbially-induced ammonia oxidation sediment-water N fluxes are influenced by ocean acidification. Reductions in pH lead to enhanced nitrate uptake by sediments and increase ammonium release while decreasing release of nitrite. This could have a significant effect on primary production rates by altering the benthic supply of key nutrients.