Phytoplankton community structure responses to urban effluent inputs following Hurricanes Katrina and Rita
James L. Pinckney, James L. Wee, Aixin Hou, Nan D. Walker
The recent flooding in the city of New Orleans, Louisiana (USA), following Hurricanes Katrina and Rita offered a unique opportunity to observe how ecological processes in Lake Pontchar- train were altered following the large inputs of untreated urban wastewater delivered during pump- ing operations to de-water the city. The purpose of this study was to quantify the response and recov- ery of phytoplankton in Lake Pontchartrain following this major environmental pollutant loading event. Weekly to biweekly water samples were collected at 27 locations within a 12 km radius from a point-source discharge at the 17th Street Canal outfall from15 September to 12 December 2005 (17 to 105 d after the passage of Hurricane Katrina). High performance liquid chromatography (HPLC) measurements of photosynthetic pigments and ChemTax were used to quantify the relative abun- dances of diatoms, dinoflagellates, cyanobacteria, euglenophytes, chlorophytes, and cryptophytes. Phytoplankton blooms with a magnitude as high as 42 µg chl al–1were limited to a 40 d period fol- lowing Katrina and were concurrent with de-watering operations for New Orleans. Phytoplankton biomass returned to relatively low, constant levels over much of the study area after active pumping and discharge of the wastewater effluent. Overall, phytoplankton group diversity declined 1 mo after Katrina and remained relatively constant, while group evenness showed little change. Although the relative abundances of algal groups were variable, diatoms constituted over half of the phytoplank- ton biomass (median: 51% of total chl a) and were consistently the most abundant group over the study period. Other than a moderate bloom during pumping operations, Hurricanes Katrina and Rita had very little direct prolonged impact on phytoplankton biomass, evenness, and diversity in Lake Pontchartrain.