Short- and Long-term Impacts of Hurricane Katrina on Lake Pontchartrain Ecosystem: Observation and Model Investigations.

Eighteen water quality surveys were carried out along six transects in Lake Pontchartrain in a period of 1.5 years following Hurricane Katrina in late August, 2005, which caused the flooding of the city of New Orleans. High levels of fecal indicator bacteria (i.e. E. coli and enterococcus) were observed in surface waters of the lake in the first few weeks after the storm.  The high counts in waters from the interior of the lake returned to pre-hurricane levels within two months of the flood induced by Katrina.  Spatial distribution of indicator bacteria in the lake waters showed remarkable gradients with the highest counts at stations near the mouth of the 17^th Street Canal and lowest at the furthest stations 5 km from the mouth of the canal.  Hypoxia was observed near the mouth of the 17^th StreetCanal while the concentration of dissolved oxygen (DO) increased to more than 5 mg L^-1 in the interior of the lake on Sep 19, 2005.  On Sep 28 and beyond, DO was mostly much higher at all stations. The quick return of condition to normal DO was probably due to an efficient wind-induced mixing of the shallow lake water, a process verified by our subsequent numerical experiments using a finite volume coastal ocean model.  It is found that strong wind stress events can produce a lake-wide circulation that facilitates the rapid mixing and dispersion of flood water, thereby minimizing its negative impact.  In summary, biological and physiochemical tests have indicated that floodwaters in New Orleans from Hurricane Katrina did not pose a significant threat to ecosystems from the standpoint of water quality.  Dewatering of the city did not cause large-scale impacts on surrounding bodies of water, albeit limited and short-term impacts were observed in Lake Pontchartrain, which received most of the floodwater pumped out of the city.  This showed a great potential of natural attenuation of pollutants in the lake ecosystem, which appears to be associated with wind-induced mixing process in the lake.