NITROGEN ATTENUATION IN THE CONNECTICUT RIVER, NORTHEASTERN USA; A COMPARISON OF MASS BALANCE AND N₂ PRODUCTION MODELING APPROACHES

Two approaches were used to measure in-stream nitrogen attenuation in the Connecticut River during studies conducted in April and August 2005. For the first approach, a mass balance of nitrogen inputs and output for two study reaches (55 and 66 km), at spring high flow and at summer low flow, was computed based on total nitrogen concentrations and measured river discharges in the Connecticut River and its tributaries. For the second approach, dissolved N₂ concentrations were measured during summer low flow in a 10.3 km subreach of the northern 66 km study reach. Measured N₂ was compared to modeled N₂ concentrations (based on temperature and atmospheric gas exchange rates) to determine the measured “excess” N₂ that indicates denitrification (net loss to the atmosphere).

Mass balance results showed no in-stream nitrogen loss in either study reach during April 2005, and no nitrogen loss in the southern 55 km study reach during August 2005. In the northern 66 km study reach during August 2005, however, nitrogen loss was indicated by an output mass 18% less than the total nitrogen inputs to the reach. N₂ sampling, performed only in the northern reach, gave similar results with an estimated N₂ production (in-stream loss) of 126 µmol N m^-2h^-1 (a rate per unit streambed area). Over the 10.3 km subreach, this modeled nitrogen loss rate is equivalent to 2.2% of the nitrogen load in the river at low flow. This would indicate a 14% loss of nitrogen over the entire 66 km northern reach if conditions were approximately constant for this distance. This loss compares favorably with the mass balance results. The percent nitrogen loss measured in the northern reach in August 2005 may represent an approximate upper limit for nitrogen attenuation in the Connecticut River because denitrification processes are most active during warm summer temperatures and because the study was performed during the annual low-flow period when total nitrogen loads are small.