An enhanced understanding of nonpoint source (NPS) nutrient export to the lower Great Lakes is needed to inform land use and land management decisions within southern Ontario. However, this understanding is limited by a lack of long-term, temporally-intensive monitoring. To address this knowledge gap, we revisit six agriculturally-dominated subwatersheds in southern Ontario, which were intensively studied during the mid-1970s, to assess changes in hydrology and NPS nutrient contributions. We compared 1975–1977 to 2016–2018 stream runoff, nutrient export (kg/day∙km²), and flow-weighted mean concentrations (FWMCs) of total phosphorus (TP), total dissolved phosphorus (TDP), total nitrogen (TN), nitrates (NO₃^–+NO₂^–) and Total Kjeldahl Nitrogen (TKN). Relative to the 1970s, runoff increased at three of six watersheds (by ~20–35%) while TP and TDP export increased at five watersheds (by ~50–125%). The increases in TP and TDP FWMCs were lower relative to phosphorus export changes at the three watersheds with increased runoff, suggesting that hydrology is an important driver of phosphorus export at these sites. Interestingly, export of TN and nitrates increased while TKN export decreased at most watersheds. We further note a shift in the timing of nutrient export at most sites, with ~40–70% of export now occurring during the winter and fall seasons whereas ~40–85% of past export occurred during spring and summer. These findings support an enhanced importance of non-growing season nutrient export from agricultural watersheds since the mid-1970s and stresses the need for targeted best management practices specific to the fall and winter seasons.

需要进一步了解向五大湖下游的非点源 (NPS) 养分输出，以便为安大略省南部的土地使用和土地管理决策提供信息。然而，这种理解因缺乏长期、时间密集的监测而受到限制。为了解决这一知识差距，我们重新审视了安大略省南部六个以农业为主的子流域，这些子流域在 1970 年代中期进行了深入研究，以评估水文和 NPS 养分贡献的变化。我们比较了 1975-1977 年和 2016-2018 年的径流、养分输出（kg/day∙km ²）和总磷 (TP)、总溶解磷 (TDP)、总氮 (TN) 的流量加权平均浓度 (FWMC) ), 硝酸盐 (NO ₃ ^– +NO ₂ ^–) 和总凯氏氮 (TKN)。与 1970 年代相比，六个流域中有三个的径流增加（约 20-35%），而五个流域的 TP 和 TDP 输出增加（约 50-125%）。TP 和 TDP FWMC 的增加相对于径流增加的三个流域的磷输出变化较低，表明水文是这些地点磷输出的重要驱动因素。有趣的是，在大多数流域，TN 和硝酸盐的出口增加，而 TKN 的出口减少。我们进一步注意到大多数地点的养分出口时间发生了变化，现在约 40-70% 的出口发生在冬季和秋季，而过去约 40-85% 的出口发生在春季和夏季。