Estimating nutrient production from human activities in subcatchments of the Red River, Manitoba

Abstract

Human activities in the subcatchments of Lake Winnipeg's contributing basins are hypothesized to be an important cause of the increased nutrient loadings intensifying eutrophication processes in Lake Winnipeg, Manitoba, Canada. This study used a geographic information system (GIS) and available human activity data to describe the types and extents of nutrient producing human activities in subcatchments of the Red River Valley, Manitoba. The goal of the study was to estimate variation in the relative amount of nitrogen and phosphorus produced by human activities and thus potentially available for release to aquatic environments. Estimates of nutrient production indicated that agricultural activities, particularly through synthetic fertilizer application, were by far the largest potential source of anthropogenic nutrients to aquatic ecosystems in most subcatchments. Residential sources in these subcatchments contributed less than 1% of the total amount of nutrients produced. Assessment of spatial variation in nutrient production demonstrated that estimated nutrient production from fertilizer application in subcatchments on the west side of the Red River Valley was up to two orders of magnitude higher than that found in subcatchments on the east side. In contrast, high livestock densities in several eastern subcatchments of the Red River Valley produced more than five times the annual areal mass of nitrogen and phosphorus of most western subcatchments. Our findings demonstrate the importance of quantifying, a priori, the potential sources of nutrients to river tributaries so that future sampling sites can be stratified to include catchments where different nutrient producing activities are most extensive.