Uptake and regeneration of inorganic nitrogen in coastal waters influenced by the Mississippi River: Spatial and seasonal variations

Abstract

The Mississippi and Atchafalaya Rivers introduce large amounts of nutrients to surface waters of the northern Gulf of Mexico. This paper reports the most complete data to date on inorganic nitrogen uptake and regeneration in a broad range of coastal environments influenced by the river water, along with information on nutrient concentrations and including pico- , nano-, and microplankton species composition. Nitrate in surface waters is greatly reduced near the river plume, at salinities between 5 and 25 PSU, where the largest variance in uptake rates was observed, and was coincident with peaks in surface chlorophyll. Despite the depletion of nitrate, nitrogen limitation was a rare event during the study, because of relatively high ammonium concentrations (>1 μmol NH4+ l-1) and regeneration rates. Two contrasting situations characterize the seasonal nitrogen dynamics in surface shelf waters. High nitrate input during the spring caused a large bloom in which the cells were well adapted to use nitrate. The dominant phytoplankton species were chain forming diatoms, also reported in sediment-trap studies in the area. Ammonium regeneration only accounted for a small fraction of the nitrogen requirements during the bloom. In contrast, the low flow of river water during summer resulted in low nitrate concentrations in surface water. In this case phytoplankton productivity was highly reduced and may depend greatly on 'in situ' ammonium regeneration.