Estuarine benthic nitrate reduction rates: Potential role of microalgae?

Abstract

The ecological functioning of the Seine estuary is strongly affected by the input of nitrogen, especially in the form of nitrate, which contributes to the eutrophication of the Seine Bight (France). Elimination of nitrate by benthic denitrification in riparian zones or adjacent wetlands could significantly improve the water quality of the Seine estuary. The goal of this study was to investigate the potential for denitrification and the factors affecting these rates. To this end, we measured nitrate reduction and ammonium production rates using flow-through reactors in contrasted sediments collected along the Seine Estuary. Sediment and organic carbon characteristics (organic C, Corg:N ratio, bioavailable carbon, extracellular polymeric substances (EPS), chlorophyll a and phaeopigments and abundance of nitrogen transforming microorganisms were determined and related to the potential nitrate reduction rates. Nitrate reduction rates showed a large spatial and seasonal variation and showed a significant correlation with sediment phaeopigments, whereas overall microbial activity (ammonium production rates) were highly correlated to chlorophyll a and EPS fractions. Surprisingly, bacterial abundance was not correlated to nitrate reduction nor to ammonium production rates. The presence of microalgae appears to be an important driver for nitrate reduction rates in these riparian sediments and seems to have fueled the benthic nitrate reducing activity.