6.8.2 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 3 - ) 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 3 - 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 off-
shore 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 10 000 km^2 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 sedi-
ments from this area suggest that increased sedimentation of algal material began
the 1960s. The increased agricultural NO 3 - 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 man-
agers, particularly where inputs in one country impact a neighbour.
6.8.3 Human effects on total ocean minor element budgets?
In the preceding sections we showed that riverine nutrient inputs impact regional
seas. We also stated in Section 6.5.4 that the huge reservoirs of nutrients in the
deep ocean mean that increases in, for example, the concentrations of NO 3 - in
riverwaters due to human activity have little effect on oceanic NO 3 - concentra-
tion, assuming that NO 3 - is effectively mixed throughout the ocean volume. We
can now demonstrate that this is the case using some simple reasoning.
The average NO 3 - concentration in the oceans is 30mmol l-^1. The total oceanic
NO 3 - inventory is calculated by multiplying this concentration by the volume of
the oceans, i.e.:
30 10¥ --^6 ()mol NO l 3 -^12 ¥¥1 37 10.^11 ()l =¥41 10^5 mol NO 3 - eqn. 6.20
The Oceans 235
