LAUSR

Abstract Urbanization is generally recognized as the most widespread form of landuse/landcover change (LULC) within populated regions, including southern Ontario, and is often at the expense of surrounding agricultural land. However, changes in agricultural LULC within these peripheral regions should be considered when interpreting water quality changes in watersheds containing mixed LULC. The objectives of this study were to first, quantify changes in LULC within twelve Lake Ontario tributaries between 1971 and 2010, and secondly, to determine whether these changes co-occurred with changes in total phosphorus (TP) and nitrate‑nitrogen (NO3-N) concentrations in streams. Water quality data were obtained from the Ontario Provincial Water Quality Monitoring Network while historical land use was reconstructed from agricultural census reports, historical land cover maps, and modern remotely sensed datasets. Urban cover increased, although percent increases in urban cover were small in the most agriculturally dominated watersheds (3–8%). The area of agriculture declined across all watersheds, yet the proportion of agricultural land dedicated to crop (corn, soybean, and wheat) production increased, including in the most urbanized watersheds (e.g. Mimico 89% urban; 2009–11). Total P concentrations in streams were highest at the urbanized watersheds, particularly in the 1970s, before declining in recent decades. In contrast, NO3-N concentrations were highest (>1.5 mg/L; 2000–10) within the most agricultural watersheds (e.g. Gages 71% agriculture) and have increased over the same period of row crop expansion. Further research is needed to determine the mechanisms behind the potential relationship between expanding row crop cover and stream NO3-N concentrations in southern Ontario.