A large area of oxygen deficient water (hypoxia) forms annually from spring through early fall along the northern Gulf of Mexico as a result of nutrient enrichment in a stratified coastal system, both factors being driven by the Mississippi River. The watershed to ocean ecosystem in this case is 3800 km in length (or, 20+ latitudinal degrees). The drainage area is 3.1 million km2 and the affected coastal area is ~30,000 km2, excluding external atmospheric inputs and a global economy. Twenty-five years of hydrographic data from the Gulf of Mexico, watershed information from the middle of the 1850s, water quality data from the turn of the century, and paleoindicators indicate shifts in the cycling of materials and nutrients both in the watershed and the Gulf of Mexico, linking the components of the ecosystem continuum.
Results/Conclusions
The shifts in biogeochemical cycles in the watershed began well before hypoxia became an established phenomenon in the early 1970s, with changes in land use and hydrology. But the shifts accelerated at a dramatic pace post World War II and expanded to the coastal ocean with the increase in population and industry, and particularly in the expansion of agribusiness and fertilizer applications. The increased amounts of reactive Nitrogen (rN) and stored Phosphorus within the watershed has doubled or tripled the delivery of these nutrients to the Mississippi River-influenced coastal ocean. These increases along with a fairly stable input of silicate have enhanced primary production in the area of hypoxia, leading to hypoxia in the lower water column, shifted nutrient ratios, affected phytoplankton community composition, relative sources of carbon, carbon cycling, trophic structure, and benthic ecosystem functioning, and diminished habitat quality for living resources. This ecosystem is not an isolated case, with many others worldwide, and the number is expanding.
