Environmental Engineering Reference
In-Depth Information
the early 1990s [45], climate-driven changes in discharge now influence the
seasonal formation of hypoxia more than they did before 1990.
Physical and Biological Processes.
The relative influence of the physical
features of the system and the progression of biological processes vary spatially
and over an annual cycle and are directly linked with the dynamics of the
Mississippi and Atchafalaya discharges.
The physical structure of the water column is defined by water masses
that differ in temperature, salinity, or both. Fresh water from the rivers and
seasonally-warmed surface waters reside above the saltier, cooler and more
dense water masses near the bottom. The existence of a strong near-surface
pycnocline, usually controlled by salinity differences, is a necessary condition
for the occurrence of hypoxia, while a weaker, seasonal thermocline often
guides the morphology of the bottom water hypoxia [51] (Fig. 3). Stratification
goes through a well-defined seasonal cycle that is generally strongest during
summer and weakest during winter [33]. These changes are responsive to the
strength and phasing of river discharge, the changing frequency of cold front
passages, regional circulation and air-sea heat exchange processes.
Figure 3.
Left panel: example of upper water column halocline and oxycline. Right panel:
example of upper water column halocline and lower water column thermocline and oxycline.
Data source: N.N. Rabalais, LUMCON.
