Nitrogen loading into an urban estuary: Lake Pontchartrain (Louisiana, U.S.A.)

Abstract

We constructed a nitrogen loading budget for the Lake Pontchartrain watershed located north of New Orleans, Louisiana (U.S.A.). Water quality measurements, discharge estimates, and literature values were used to establish the annual and seasonal variations in loading rates for total nitrogen and nitrate. The relatively stable annual loadings (million kg N) are about 10× that of the pre-settlement nitrogen loading, and come from atmosphere (1.3), the watershed (7.8), pumped urban runoff from New Orleans (1.0), and leakage through the Bonnet Carre flood control structure (0.5–0.9). Relatively minor additional amounts come from nitrogen fixation in the Lake. Occasional openings of the Bonnet Carre Spillway (for flood protection) could triple the annual average loading within 1–2 months. Proposed smaller diversions (for wetland restoration) could raise present N loadings by 50%. The results of water quality management, flood protection and wetland restoration may thus have conflicting effects on the Lake's phytoplankton community, which is primarily nitrogen limited. Lowering the total nitrogen loading, however, seems quite possible, especially given that the present loadings are almost all reducible through existing technology, especially sewerage treatment. The analysis demonstrates that the consequences of ecosystem restoration efforts, continued population growth and flood protection to estuarine nitrogen budgets are intertwined with each other, have a seasonal component, and are changing as policies evolve.