Riverine nitrogen and carbon exports from the Canadian landmass to estuaries

Dissolved total nitrogen (Nt) and total organic carbon (TOC) exports were measured from 30 catchments and regions draining 76 % of the Canadian landscape in order to estimate reactive N and organic C runoff losses to estuaries and the conditions that control them. N exports from the catchments were lower than measured in most of Europe and the United States due to significantly less agricultural activity and atmospheric deposition, especially in northern Canada. We produce statistical models using a number of geographical, climatic, agricultural, and population factors in order to predict N and C losses from the remaining regions. Using measured and extrapolated data, we estimated that the Canadian landscape exports 884 and 18,210 ktons of Nt and OC per year. Area normalized exports ranged from 29.4 kg km for the northern Mackenzie River to 299 kg km for the semi-agricultural Saint John. Area normalized OC exports ranged from 495 kg km in the high Arctic to 7,295 to the wetland dominated Broadback River in northern Quebec. N exports were best predicted by the latitude of the catchment centroid, mean slope, population density, runoff and % of the catchment as agricultural land. The best model for predicting TOC exports needed only slope and runoff. The Nt/OC ratio in the rivers unsurprisingly was highest in the southern portion of the country where anthropogenic activities were concentrated.