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9.18: Increasing CO2 Concentrations, Hypoxia, and Eutrophication

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    • Contributed by Miracosta Oceanography 101
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    Increasing CO2 Concentrations in the Atmosphere and Oceans

    CO2 concentrations and temperature have tracked closely of the last 300,000 years (Figure 9.37).
    The recent (if not alarming) increase in CO2 concentrations in the atmosphere is a result of human consumption of fossil fuel, burning forests, and other land use changes. How the Earth's ecosystems are responding to these changes is measurable, and many things are changing. Continental glaciers are melting faster (causing serious concerns about coastal flooding), and the chemistry of ocean water is slowly growing more acidic (endangering ocean species that secrete CaCO3 skeletal material).

    Carbon dioxide curve and temperature curve during ice ages.
    Figure 9.37. Global temperature and CO2 concentrations over the last 300,000 years.

    Hypoxia and Eutrophication

    Hypoxia is oxygen deficiency in a biotic environment. Eutrophication is caused by excessive amounts of nutrients in a body of water (lake, sea, or parts of an ocean) which causes a dense growth of plant life and death of animal life from lack of oxygen (hypoxia). Excessive amount of nutrients come from runoff from land, with agriculture and sewage being primary contributors. Hypoxia has become a major problem in many parts of the world where whole regions of the seabed are dead or dying because of lack of oxygen. Eutrophication is a serious problem in the northern Gulf of Mexico around the mouth of the Mississippi River delta (Figures 9-38 and 9-39). Density stratification in isolated ocean basins can lead to depletion of oxygen at depth at as microbial decay consumes free oxygen and then starts to break down sulfate ions (-HSO4) to hydrogen sulfide (H2S). The Black Sea is an example where thermohaline density stratification has lead to anoxic conditions at depth (Figure 9.36).

    Hypoxia in the Gulf of Mexico Fish killed by hypoxia
    Figure 9.38. Region impacted by eutrophication in the northern Gulf of Mexico caused by high-nutrient runoff in the Mississippi River system. Figure 9.39. Example of a fish kill caused by hypoxia in a coastal marine environment. Hypoxia increases in the Gulf as water warms up during the summer season.

    See Hypoxia and Eutrophication (NOAA)