The number of dead zones has doubled every decade since the mid 1900s (Greenhalgh, 2015). The spread and increase in severity of these anoxic areas resulted from a number of factors, but future changes are expected to be heavily influence by climate change. There are several ways that climate change will impact dead zone formation and location, but temperature changes and sea level rise are the two impacts with the greatest degree of certainty.
Warmer water is able to hold less dissolved oxygen because gas solubility decreases as temperature increases, resulting in bubbles of oxygen at the surface (Greenhalgh, 2015). In addition, stratification of the ocean due to warming of the surface layer creates layers with differing temperatures and decreases the amount of mixing in the water column (Altieri and Gedan, 2014). This means the oxygen bubbles tend to sit at the surface are not distributed to deeper waters. Deep waters are also cut off from the atmosphere, a primary source of oxygen in the ocean (NSF, n.d.). These are the general trends expected as a result of warming, but all areas will not warm evenly, resulting in differing impacts in some areas. Higher latitudes are expected to see the worst dead zone changes due to warming because these areas will be experiencing the greatest overall rise in temperatures (Altieri and Gedan, 2014). Coastal areas will be impacted greater because they are shallower, and temperatures are more closely dictated by the air temperature there than the open ocean is (Altieri and Gedan, 2014).
The primary focus of sea level rise from climate change is the impact on wetland habitats. Wetlands are natural buffers to the nutrient runoff that can cause algal blooms and eventually lead to dead zones. These habitats help by filtering nitrogen and phosphorus from runoff before it reaches coastal areas (Zielinski, 2014). These ecosystems are being threatened as sea level rises (Zielinski, 2014). Declines in wetlands will result in more nutrients reaching coastal areas and contributing to algal blooms and dead zones. Sea level rise is also increasing the total volume of water susceptible to eutrophication because of the expanse in volume of shallow coastal regions (Altieri and Gedan, 2014).
There are several other less impactful and more nuanced factors of climate change that can influence algal blooms and dead zones. The first is the timing and length of algal blooms. Warming and other changes in seasonality are causing seasonal algal blooms to appear earlier into the season and stay longer, extending the period of time that eutrophication can occur and cause dead zone conditions from which the system may not be able to recover from during the off season (Altieri and Gedan, 2014). Another hypothesized impact is a hypothesis formed by a researcher named Bakun. Climate change is expected to increase the land-sea temperature imbalance in some areas due to the land heating more significantly than the ocean (Bakun et al., 2015). This would drive a greater pressure gradient in these areas that could potentially drive upwelling favorable winds resulting in greater nutrient conditions for algal growth (Bakun et al., 2015).
Climate change will also have significant impacts on ecosystem health. As temperature increases, an organism’s metabolism also increases requiring them to take in more oxygen (Altieri and Gedan, 2014). This fact, paired with an overall decline in oxygen supply, leads to worse anoxic conditions and overall lowered ecosystem resilience.
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Bakun, A., et al. “Anticipated Effects of Climate Change on Coastal Upwelling Ecosystems.” Current Climate Change Reports, vol. 1, no. 2, 7 Mar. 2015, pp. 85–93., doi:10.1007/s40641-015-0008-4.
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Zielinski, Sarah. “Ocean Dead Zones Are Getting Worse Globally Due to Climate Change.” Smithsonian.com, Smithsonian Institution, 10 Nov. 2014, www.smithsonianmag.com/science-nature/ocean-dead-zones-are-getting-worse-globally-due-climate-change-180953282/.