Nitrogen removal in a riverine network

C.C. Allaway and F.R. Pick. Department of Biology, University of Ottawa, Ottawa, Ontario, Canada K1N 6N5

Recent models based on meta-analysis show that nitrogen retention decreases with increasing water depth in rivers. However, these models have rarely been tested at the scale of an individual drainage basin. In this study, changes in total nitrogen (TN) and nitrate (NO3) were measured along twenty-nine river reaches within the South Nation River basin to determine which factors influence nitrogen retention and where most retention occurs. The South Nation River and its tributaries drain a large area of Eastern Ontario (~ 4000 km2), where land use is mainly agricultural (~ 57%). Individual reaches ranged in depth from ~0.15 m to 4 m and in discharge from 2 L/s to 30,000 L/s. Both NO3 and TN displayed a reduction in the retention rate with increasing depth, however, significant loading was also observed at several shallow sites (<50cm). Nitrate retention and loading across the reaches varied from complete removal to a loading greater than 100%. For TN retention/loading ranged from +40% to -40% per km of reach. At the scale of a single riverine network, it appears that nitrogen retention is not a simple function of stream depth. Models predicting nitrogen retention in rivers need to incorporate additional factors such as surrounding land use.