DISCERNING THE LIMITING NUTRIENT TO PHYTOPLANKTON GROWTH IN RIVERS AND BAYOUS ON THE NORTH SHORE OF LAKE PONTCHARTRAIN

Discerning the limiting nutrient to phytoplankton growth in surface waters on the North Shore of Lake Pontchartrain is critical to managing water quality in the Lake Pontchartrain Basin. This estuary supports a lucrative commercial fishing industry as well as provides a desirable destination for recreational fishing, swimming, and other water sports for a significant portion of southeast Louisiana. Increased pollution from rapid urbanization and intensive agricultural practices in recent decades has resulted in elevated nutrient concentrations in surface waters flowing into Lake Pontchartrain. Eutrophication (excess nutrients) can lead to extreme rates of phytoplankton growth at optimum nutrient ratios (C:N:P), which results in anoxic conditions in the water and can lead to wide spread fish kills. Research at the University of New Orleans has discovered that surface waters in some roadside storm water ditches and rivers on the north shore of Lake Pontchartrain are phosphorus limiting, in spite of the fact that the Lake Pontchartrain estuary as a whole is primarily nitrogen limiting. Prior investigations have determined that excess nitrogen loading from anthropogenic sources often increase nitrogen disproportionately to phosphorus, causing the limiting nutrient to phytoplankton growth to shift.

This study is designed to determine the limiting nutrient to phytoplankton growth in six different rivers and bayous that discharge into the northern waters of Lake Pontchartrain. Surface water samples have been collected from three sites along each river or bayou (south, mid, and north) every six weeks since the beginning of March 2007. Preliminary results show elevated ammonium concentrations at half of the sampling sites. 50% of the sites have ammonium concentrations greater than the U.S. Geological Survey average in St. Tammany surface waters (0.0365mg/l) and 5% of the sites have ammonium concentrations greater than the cutoff for treated wastewater effluent in St. Tammany Parish (5.0mg/l). Interestingly, the sites with the highest ammonium concentrations are in the south, closest to Lake Pontchartrain, which is brackish and should be nitrogen limiting.