Climate and the Gulf of Mexico "dead zone"

My colleague Don Scavia and I have an article in the latest issue of Limnology and Oceanography about the effect of climate on the development of the seasonal “dead zone” in the Gulf of Mexico.

I’ve written a bit about this issue before on Maribo. The intensification of agriculture in the central US since the 1950s – huge increases in nitrogen fertilizer use, planting of more nitrogen-fixing soybeans, drainage of wetlands, installation of artificial drains under fields – caused a 2-3 fold increase in the amount of nitrogen the Mississippi River delivers to the Gulf of Mexico. The large influx of nitrogen now promotes the growth of a seasonal low oxygen or hypoxic zone each summer on the continental shelf of the northern Gulf of Mexico.

From the 1980s until quite recently, however, agricultural land use and land cover were relatively stable in contrast to the more dramatic changes in the previous three decades (the surge in interest for ethanol may end the relative stability). The one factor that changed the most, year to year, is the weather. Our study examines how this year-to-year variability in rainfall influences the amount of nitrogen flooding down the Mississippi in the spring and the extent of hypoxia in the Gulf.

The study finds that, absent any major changes in land use and land cover, the year-to-year variability in precipitation across the “Corn Belt” (in the previous November and December and in March, April and May) is the primary driver of the year-to-year variability in amount of nitrogen delivered by the Mississippi during the late spring (in May and June). Using this relationship, the study then examines how climate variability affects the potential size of the hypoxic zone and the implications for reducing nitrogen losses and the size of the hypoxic zone. During very wet years, a nitrogen reduction of 50-60% – close to twice the original recommended target – is necessary to reach the goal of minimizing the size of the hypoxic zone (< 5000 km2).

The results are a reminder of the importance of factoring climate variability into water quality or aquatic ecosystem policy, particularly given the changes in climate expected to occur in the coming decades.