Document: MIC-3-73-8

Hydrologic variability, organic matter supply, and denitrification in the Garonne River ecosystem.

BAKER, M.A.* ¹, P.VERVIER ² and L.ROQUES ²

Utah State University, Logan, UT 84322USA ¹
Centre d'ecologie des systemes aquatiques contineaux, CNRS Toulouse France ²

Abstract:
Groundwater nitrate contamination has become a worldwide problem as increasing amounts of nitrogen fertilizers are used in agriculture. Alluvial groundwater is uniquely juxtaposed between soils and streams. Hydrologic connections among these subsystems organize nutrient cycling in watersheds. We hypothesized that spring floods would flush soil-derived dissolved organic matter (DOC) into groundwater, and this would in turn fuel denitrification in a nitrate contaminated portion of the Garonne watershed. In situ acetylene block assays and measures of system biogeochemistry and hydrology were used to test this hypothesis. During high flow (mid-April to early June) the water table rose an average of 30 cm in monitoring wells. This was associated with a slight increase in DOC concentration. Denitrification rates nearly doubled during the high flow period, averaging 2.91 ug N₂O/L/min during baseflow compared to 5.05 ug N₂O/L/min during spring flood. Higher denitrification rates during spring appeared to be related to a change in DOC quality. While there was no seasonal difference in specific UV index, low molecular weight organic acids made up a greater proportion of DOC during high flow. These molecules are likely derived from decomposition of soil organic matter and are an important energy source for anaerobic respiratory processes like denitrification. This study highlights the importance of floods as major organizers of terrestrial-aquatic ecosystem interaction.

Keywords: denitrification, river, groundwater, DOC, floods

This abstract is being presented at: 10:30 AM in session:
Oral Session #70: Aquatic Ecology.