Environmental Engineering Reference
In-Depth Information
The concentrations and total loads of nitrogen, phosphorus and silica de-
livered to the coastal ocean influence the productivity of the phytoplankton
community, the types of phytoplankton that are most likely to grow and the
flux of phytoplankton-derived organic matter [12, 36, 45]. Phytoplankton not
incorporated into the food web and fecal material generated via the food web
sink into bottom waters where they are decomposed by aerobic bacteria. The
source of the organic matter for this respiratory activity is mostly from marine
phytoplankton growth stimulated by riverine-delivered nutrients, and not from
the carbon in the Mississippi River [16]. The bacterial respiration in the lower
water column and seabed results in a decline in oxygen concentration when
oxygen is depleted faster than it can be replaced by vertical diffusion through
the stratified water column.
2.2 Variability of Gulf of Mexico Hypoxia
The distribution of hypoxia on the Louisiana/Texas continental shelf reflects
the interaction of physical and biological processes dominated by riverine
inputs. Decreases in nutrient concentrations away from the Mississippi and
Atchafalaya rivers are paralleled by decreases in surface water chlorophyll
(Fig. 4) [34, 35]. Gradients away from the rivers result from mixing of the
buoyant plumes and biological uptake and regeneration of nutrients. Dissolved
inorganic nitrogen (DIN, nitrate+nitrite+ammonium) is high near the Missis-
sippi River delta and composed primarily of nitrate. Down current from the
delta, ammonium is the dominant DIN form and in sufficient supply to support
primary production. Phosphate is uniformly present, and silicate concentrations
are high near the delta and remain sufficient for diatom growth across a large
area. There is an optimal distance and depth for the formation of hypoxia based
on strength of the stratification, light conditions, nutrient availability and rates
of primary production and flux of organic matter to the seabed. This is qualita-
tively illustrated in the compilation of 17 mid-summer surveys (1985-2002, Fig.
5) that shows the frequency of hypoxia is highest down current (west) from the
freshwater and nutrient discharges from the Mississippi and Atchafalaya rivers.
Empirical models strongly correlate the size of the mid-summer hypoxic area
with the riverine nitrate load over several months prior to the mapping cruise
[42, 49].
Spatial Extent.
The hypoxic water mass is distributed across the Louisiana
shelf west of the Mississippi River and onto the upper Texas coast (mapped in
mid- to late July) [37]. Hypoxia extends from near shore to as much as 125
km offshore and in water depths up to 60 m. The size averaged 13,000 km
2
over the period 1985-2004, with a range from negligible in 1988 (a summer
drought year for the Mississippi River basin) up to 22,000 km
2
(Fig. 2). Hypoxia
may form in two distinct areas west of the Mississippi and Atchafalaya River
