ABSTRACT:

Riparian zones are key ecotones that buffer aquatic ecosystems through removal of nitrogen (N) via processes such as denitrification. How dams alter riparian N cycling and buffering capacity is however poorly understood. Here we hypothesize that elevated groundwater and anoxia due to the back-up of stream water above milldams may enhance denitrification. We assessed denitrification rates (using denitrification enzyme assays) and potential controlling factors in riparian sediments at various depths upstream and downstream of two relict US mid-Atlantic milldams. Denitrification was generally low and not different between upstream and downstream, although was greater per river km upstream considering deeper and wider geometries. Further, denitrification typically occurred in hydrodynamically variable, surface sediments where nitrate-N and organic matter were most concentrated. At depths below 1 m, both denitrification and nitrate-N decreased while ammonium-N concentrations substantially increased, indicating suppression of ammonium consumption or dissimilatory nitrate reduction to ammonium. These results suggest that denitrification occurs where dynamic groundwater levels result in higher rates of nitrification and mineralization, while another N process that produces ammonium-N competes with denitrification for limited nitrate-N at deeper, more stagnant depths. Additionally, nitrate-N-rich runoff from agricultural areas increases denitrification rates, while Na-rich runoff due to road salt application limits denitrification, highlighting the importance of synergistic interactions between land-use legacies. Ultimately, while it is unclear whether relict milldams are sources of N, limited denitrification rates indicate that they are not always effective sinks; thus, milldam removal – especially accompanied by removal of ammonium-N rich sediment terraces – may improve riparian N buffering.

ABSTRACT:

Dam removals are on the increase across the US with Pennsylvania currently leading the nation. While most dam removals are driven by aquatic habitat and public safety considerations, we know little about how dam removals impact water quality and riparian zone processes. Dam removals decrease the stream base level, which results in dewatering of the riparian zone. We hypothesized that this dewatering of the riparian zone would increase nitrification and decrease denitrification, and thus result in nitrogen (N) leakage from riparian zones. This hypothesis was tested for a 1.5 m high milldam removal. Stream, soil water, and groundwater N concentrations were monitored over two years. Soil N concentrations and process rates and δ15N values were also determined. Denitrification rates and soil δ15N values in riparian sediments decreased supporting our hypothesis but no significant changes in nitrification were observed. While surficial soil water nitrate-N concentrations were high (median 4.5 mgN L-1), riparian groundwater nitrate-N values were low (median 0.09 mgN L-1), indicating that nitrate-N leakage was minimal. We attribute the low groundwater nitrate-N to denitrification losses at the lower, more dynamic, groundwater interface and/or dissimilatory nitrate reduction to ammonium (DNRA). Stream water nitrate-N concentrations were high (median 7.6 mgN L-1) and contrary to our dam-removal hypothesis displayed a watershed-wide decline that was attributed to regional hydrologic changes. This study provided important first insights on how dam removals could affect N cycle processes in riparian zones and its implications for water quality and watershed management.