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GEO-6 Regional Assessment for North America


Water off the North American Pacific coast already has a low carbonate saturation state. When surface winds blow the top layer of water out from coastal regions, deeper water with higher acidity can well up, and harm shellfish. Periodic upwelling of carbon-dioxide-rich water has already happened on the US west coast, where larval oyster survival has been very low. There has been a reduced natural set of juvenile oysters in some Pacific coast estuaries where the commercial shellfish industry relies on natural reproduction of oysters. Researchers showed that impaired shell production by larval oysters was due to the increased acidity of the water (Barton et al. 2012). Hatchery staff now monitor ocean pH levels and time water intakes to ensure that oysters are exposed to less acidic water.


Ocean acidification effects are not restricted to shell production. Effects have been seen on behaviour and development of a number of marine animals. Exposure of eggs and larvae of a common estuarine fish to elevated carbon dioxide severely reduces survival and growth (Baumman et al. 2012). Behaviour is also altered in many animals, especially that related to the olfactory system. Fish in acidic water in the lab or living next to natural seeps, where carbon dioxide is released by volcanic activity, lose their natural fear of the odour of predators and become attracted to them. But predatory behaviour can also be impaired. Munday et al. (2014) found that shark attraction prey odour decreases in seawater with elevated carbon dioxide levels.


Eutrophication is an additional source of carbon dioxide in coastal waters (Sunda and Cai 2012). As organic matter decays and uses up oxygen, it also releases carbon dioxide, which speeds up acidification of coastal waters and estuaries in particular. Combined effects of hypoxia and acidification intensify the negative effects (Gobler et al. 2014). While the long-term solution to ocean acidification is reducing emissions of carbon dioxide through international agreements, other short-term approaches are feasible locally. Since excess nitrogen (eutrophication) contributes to elevated carbon dioxide levels in coastal waters as well as reduced oxygen, it is possible to mitigate acidification by reducing effluents and runoff of nutrients.


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2.5.3 Water quality: key threats and their implications


Contaminants


Toxic chemicals that were recognized decades ago as threats are generally declining slowly. Toxicants can disrupt metabolic, regulatory or disease defence systems, and compromise reproduction and even survival. Early life stages, gametes, fertilization, embryonic and larval development, are most sensitive. The hormonal control of reproduction can be affected by many contaminants, now called endocrine disruptors (Colborn et al. 1993). Exposures during early life stages may cause effects that appear later, sometimes many years later. Thus, long-term delayed effects and indirect effects are important.


Metals


The US National Oceanic and Atmospheric Administration (NOAA) survey of metals in coastal bivalve molluscs, the mussel watch programme, found no overall national trends (Kimbrough et al. 2008). There were 27 sites that had significant decreases, many in southern California, while nine showed significant increases. Levels were highest in urban and industrialized areas. Copper and mercury are among the most toxic metals to aquatic life. The fish olfactory system is especially sensitive to copper (Hansen et al. 1999; Hansen et al. 2004) and it is also especially toxic to algae and molluscs; it is used as an algicide and molluscicide. Mercury is a highly toxic metal that is found both naturally and as a contaminant. Its risk is determined by the form of mercury present in the environment. Methylmercury is the most toxic form (AMAP 2016; Legrand 2010), and inorganic mercury can be converted to methylmercury by bacteria in marine sediments. In addition to being far more toxic than inorganic forms of the metal, methylmercury is also biomagnified up the food chain.


Certain types of chemical contaminants, including methylmercury and chlorinated hydrocarbons tend to biomagnify through food webs, so the top carnivores have


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