Human effects on regional seas 2 the Gulf of Mexico

The Mississippi river system is one of the largest in the world and drains over 40% of the USA, discharging into the Gulf of Mexico through a large and complex delta near New Orleans (Fig. 6.31). The river system drains intensively farmed areas of the USA and nitrate (NO-) concentrations in the river doubled from the 1960s to the 1980s as a result of increased fertilizer use (Fig. 6.31). Since the 1980s NO- concentrations have remained at this high level (see also Section 5.5.1). Increased diatom growth in the riverwater has caused a decrease in silica concentrations (removed to the diatom skeletons) of more than 30% (see also Section 5.5.1).

The Mississippi drains on to the continental shelf of the northern Gulf of Mexico (Fig. 6.31). Here the freshwater flow combines with ocean currents to produce a stratified water column, isolating the shelf bottom water for much of the year. The nutrients from the Mississippi help fuel algal growth in waters offshore of the delta. After death, some of the algal cells sink into the bottom waters to be degraded by aerobic bacteria, thereby consuming oxygen (see Section 5.5). A 10000 km2 region of low oxygen develops mainly in the spring and summer in these isolated bottom waters (Fig. 6.31).

Records of preserved phytoplankton and organic carbon in dated shelf sediments from this area suggest that increased sedimentation of algal material began the 1960s. The increased agricultural NO3- inputs from the Mississippi are very likely responsible—at least in part—for the low oxygen concentrations, although other factors such as wetland loss (Section 6.2.4), changes in river discharge and changes in physical conditions within and around the delta probably also play a part. The discovery of these low oxygen regions in the Gulf of Mexico have led to modification of farming practices throughout the Mississippi drainage region in order to reduce nutrient inputs.

Both the Baltic and Gulf of Mexico examples illustrate that activities taking place hundreds or even thousands of kilometres distant from the oceans can have major impacts on coastal seas. This creates a problem for environmental managers, particularly where inputs in one country impact a neighbour.

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