Effects of Agricultural Land Use Change on Nitrogen and Phosphorus in North Shore Lake Ontario Tributaries

Description

Row crop agriculture and associated land use practices including tile drainage and conservation tillage have been cited as a probable cause of re-emerging eutrophication in the
lower Great Lakes. In this thesis, I sought to quantify and evaluate the effect of agricultural land
cover and land use changes on total phosphorus (TP) and nitrate-nitrogen (NO3-N)
concentrations and export in north shore Lake Ontario tributaries. This included (a) a long-term
data analyses at 12 large watersheds (47 to 278 km²) using historical land cover and water quality
data (1971-2010), and (b) a space-for-time study examining 12 small sub-catchments (< 8 km²)
with majority (> 50%) row crop, pasture, or forest cover. Concentrations of TP were greatest in
urbanized watersheds and declined particularly during the first decades of the study period, while
NO3-N concentrations were greatest and steadily increased in agricultural catchments with
increasing row crop cover. The space-for-time approach revealed that TP concentrations were
similar across agricultural land uses and that export was most dependent on runoff. Meanwhile,
NO3-N concentrations and export were greatest in row crop catchments and were positively
related to row crop area. These results suggest that increases in row crop cover and associated
agricultural practices including increased nutrient amendments and tile drainage may be
responsible for increased NO3-N concentrations and export in northern Lake Ontario tributaries.
Author Keywords: agriculture, Lake Ontario, nitrogen, phosphorus, streams, Water quality