Personnel: Prof. Ugo Piomelli, Mr. Carlo Scalo
Location and size of dead zones in the world (from http://en.wikipedia.org/wiki/File:Aquatic_Dead_Zones.jpg)
The presence of dissolved oxygen (DO) in water is critical to the
survival of organisms in marine environments. Surface waters are saturated with
atmospheric oxygen, which is transported towards the bottom by turbulent
motions. A number of physical factors can limit this mixing process. For
instance, stratification in the water column damps turbulence, reducing mixing;
this inhibits the vertical DO flux, reducing the oxygen supply to deeper
un-stratified water layers. Other factors, of biogeochemical nature, may amplify
this problem, such as agricultural runoff containing excess nutrients, which
fertilize algal blooms. The algae eventually die and sink to the bed where, in
the sediment layer, DO-consuming bacteria feed on them. When the DO flux to the
sediments (and biological demand) is greater than the flux through the
stratification, then near-bed oxygen depletion can occur. The amount of DO
absorbed by the sediment largely governs oxygen depletion in stratified water
bodies; if the lack of oxygen persists marine life may suffer. Areas affected by
this phenomenon are known as ‘dead zones’, and can be found in many coastal
environments and lakes such as the Gulf of Mexico, the Baltic Sea and the Great
Lakes.
Being able to predict the distribution of DO across the water column would allow a process-oriented parametrization of the sediment-oxygen demand (SOD) based on resolvable flow features in the near-wall region (such as wall shear stress). Water-quality models currently employed for lake or rivers management would considerably benefit from such parametrization. However, a full understanding of the biological responses to the thermo-fluid dynamic conditions requires physical and numerical models that couple hydro-dynamic transport and the dynamics of water quality and aquatic organisms at all the important scales.
These are the features incorporated in the model we have developed, accounting for the characteristics of oxygen transport in a turbulent flow, transfer mechanisms the across the SWI and absorption in the sediment bed. We implemented and tested an existing bio-geochemical model for bacterial DO absorption in the sediment layer coupled with Large-Eddy Simulation (LES) of turbulent transport on the water side. This prediction tool is capable of reproducing experimental observations and can be applied to shed light on the modeling issues of near-wall oxygen depletion and transfer dynamics in water-bodies.
Contours of instantaneous oxygen concentration around the water-sediment interface (black dashed line, at y=0). White dashed line is the instantaneous diffusive sub-layer thickness.
