The term “dead zone” refers to an area where dissolved oxygen levels in the bottom layer of water are too low to support most marine life. This condition, known as hypoxia, occurs annually in the northern Gulf of Mexico, extending westward from the Mississippi River delta along the Texas-Louisiana shelf. The recurring dead zone forms due to excess nutrient runoff, primarily nitrogen and phosphorus, carried into the Gulf by the Mississippi and Atchafalaya Rivers. These nutrients fuel massive algal blooms, which sink, die, and are decomposed by bacteria, a process that consumes oxygen in the deeper water.
Scientific Measurement and Monitoring
The size of the hypoxic zone is determined during annual summer research cruises, typically conducted in July, by scientists. Researchers sample the water across the northern Gulf of Mexico using specialized probes that measure dissolved oxygen, salinity, and temperature at various depths. The threshold for defining hypoxia is a dissolved oxygen concentration of 2 milligrams per liter (mg/L) or less. The size is quantified and reported in square miles or square kilometers, representing the total area affected by oxygen-depleted water. Since the zone fluctuates daily due to weather events, scientists use a five-year running average of these annual measurements for policy and management purposes to assess long-term trends.
The Official Target Size Goal
The official goal for the acceptable size of the dead zone is set by the Mississippi River/Gulf of Mexico Watershed Nutrient Task Force (HTF). The HTF’s long-term objective is to reduce the five-year running average size of the hypoxic zone to 1,930 square miles (5,000 square kilometers). This specific numerical target was established to signify a measurable return to a size that is less harmful to the ecosystem and fisheries of the Gulf. The Task Force initially aimed to achieve this goal by 2015, but the target date was updated to 2035 following insufficient progress. Achieving this size requires a significant reduction in nitrogen and phosphorus delivered to the Gulf, and the HTF has established an interim milestone of a 20% reduction in nutrient loading by 2025.
Strategies for Nutrient Reduction
The primary strategy for managing the dead zone involves reducing the flow of nitrogen and phosphorus from the vast Mississippi-Atchafalaya River Basin, which drains 41% of the continental United States. Since the largest source of pollution comes from agricultural runoff, the action plan focuses on promoting best management practices (BMPs) across the 12 states involved in the HTF. These practices include managing the amount, timing, and method of fertilizer application to minimize nutrient loss. Other key strategies include:
- Increased use of cover crops, such as grasses or clovers, planted after harvest to recycle excess nutrients and reduce soil erosion.
- Restoring and constructing wetlands, which act as natural filters, removing nutrients from the water before it enters the river system.
- Improvements to municipal wastewater treatment facilities to reduce point source pollution.
- Exploring programs like nutrient trading to achieve cost-effective reductions from both agricultural and municipal sources.
Current Progress Assessment
Despite decades of effort, the Hypoxia Task Force has not yet met its long-term goal. Recent measurements show that the five-year running average size of the dead zone remains substantially larger than the 1,930 square mile target. For instance, a recent five-year average was measured at 4,298 square miles, which is more than double the acceptable size. While the size fluctuates annually based on factors like rainfall and nutrient loading, the long-term trend has not shown the consistent decrease needed to meet the 2035 goal. This lack of a downward trend indicates that current efforts to reduce nutrient loads have not been scaled up sufficiently across the entire watershed.