The Gulf of Mexico has a seasonal hypoxic zone, or “dead zone”, forming every year in late summer (Fig. 1). Dead zones refer to areas of low dissolved oxygen, which can harm plant and animal life. The dead zone was first recorded in the early 1970s where it occurred every 2-3 years, but now occurs annually. In addition, the size has been growing. This year, the NOAA had forecasted the summer dead zone to cover approximately 5,827 square miles, which is above the average 5,205 square mile 37-year period of record average.
The Gulf of Mexico is economically very important for the fishing industry. Only in 2016, commercial fishing in the cost generated between $17 billion and $57 billion and 200,000 jobs (NMFS 2016). Hypoxic conditions can majorly disrupt the coastal ecosystem. Exposure to hypoxic waters has been found to alter fish diets, growth rates, and reproduction (Rose, 2017). A report by the Union of Concerned Scientists estimated up to $2.4 billion in damages to fisheries and marine habitats every year (Boehm, 2020).
Dead Zone Development
The major cause of the hypoxic zone is due to the influx of nutrient-rich freshwater via the Mississippi and Atchafalaya River Basins, which provide 90% of the freshwater to the Gulf of Mexico. Nitrates and phosphates, sourced mainly from fertilizers used in farmlands, stimulate the growth of harmful algae blooms in the spring and summer. These algae blooms sink to bottom waters where they are decomposed by oxygen-consuming microbes. When the rate of oxygen consumption outpaces diffusion from the surface, hypoxic conditions are created.
Recent climatic trends have an impact in exacerbating oceanic dead zones through increased precipitation rates and warmer sea surface temperatures (SST). In the Gulf of Mexico, SST has increased by 1°C since 1970. Warmer waters hold less dissolved oxygen. In addition, global warming has also intensified extreme precipitation events due to increased rates of evaporation and moisture-holding capacity of the atmosphere. According to the NOAA, between 1991-2020, average annual precipitation has increased by +1.34” per decade. More precipitation can create more runoff. Higher discharge from the Mississippi River Basin delivers higher nutrient loads and freshwater into the gulf. Fresh and warmer water is less dense, which creates a stratified column of ocean water that resists mixing. This prevents oxygenated surface water from reoxygenating bottom water. As a result, oxygen depletion occurs more rapidly, and bottom waters can become more anoxic.
Recent Records
2017 experienced a record-high 8,776 square mile hypoxic zone. This was the result of an extremely wet spring in the Midwest. However, 2018 followed with a below-average dead zone due to persistent westerly winds pushing water to the east and allowing for upwelling to occur along the coast.
This year’s forecasted dead zone was done using stream gauge data from USGS for Mississippi and Atchafalaya river discharge and nutrient loading. In May 2024, nitrate levels were 7% below long-term averages, however, phosphorus loads and river discharge were 22% and 5% above long-term averages, respectively.
While severe weather events have become more common, contributing to short-period fluctuations in measured hypoxic zone areas, overall we see warming climates having a positive forcing effect on the development of the Gulf Dead Zone.
Working for a Solution
“Year-to-year fluctuations in the dead zone are driven primarily by weather in the Corn Belt, but the long-term trends are driven by nutrient loads from agriculture,” said Don Scavia, professor emeritus at the University of Michigan School for Environment and Sustainability. This means that the main target for remediation is through reducing the nitrates and phosphates that enter our waterways. The Gulf of Mexico Hypoxia Task Force has set a target to restrict the size of the Gulf Dead Zone to below 1,900 square miles by 2035. This is mainly done through nutrient-level reduction strategies.
One initiative set to reduce runoff contamination is the NOAA’s Runoff Risk Forecast. The forecast provides 24-hour information for runoff events, helping farmers determine the best time to apply fertilizer. Farming practices such as using cover crops, reduced tillage, and crop rotation are also applied.
References
Ritzel, B. (2014) (PDF) gulf of Mexico’s hypoxic Dead Zone, Gulf of Mexico’s Hypoxic Dead Zone.
Dead zones and climate change Virginia Institute of Marine Science. Available at: https://www.vims.edu/research/topics/dead_zones/climate_change/
NOAA forecasts above-average summer ‘dead zone’ in Gulf of Mexico (2024) National Oceanic and Atmospheric Administration. Available at: https://www.noaa.gov/news-release/noaa-forecasts-above-average-summer-dead-zone-in-gulf-of-mexico
Upper mississippi river basin Technical_Version_Upper_MS_River_Communicating_CC.pdf. Available at: https://www.noaa.gov/sites/default/files/2022-03/Technical_Version_Upper_MS_River_Communicating_CC.pdf
Yin, S. et al. (2022) Long-term trends of streamflow, sediment load and nutrient fluxes from the Mississippi River Basin: Impacts of climate change and human activities, Journal of Hydrology.
Larger-than-average Gulf of Mexico ‘dead zone’ measured (2021) National Oceanic and Atmospheric Administration. Available at: https://www.noaa.gov/news-release/larger-than-average-gulf-of-mexico-dead-zone-measured
Boehm, Rebecca. (2020). Reviving the Dead Zone: Solutions to Benefit Both Gulf Coast Fishers and Midwest Farmers, Cambridge, MA: Union of Concerned Scientists.
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