Phosphorus Availability in Western Lake Erie Basin Drainage Waters: Legacy Evidence across Spatial Scales

The Western Lake Erie Basin (WLEB) was inundated with precipitation during June and July 2015 (two to three times greater than historical averages), which led to significant nutrient loading and the largest in‐lake algal bloom on record. Using discharge and concentration data from three spatial scales (0.18–16,000 km2), we contrast the patterns in nitrate (NO3–N) and dissolved reactive phosphorus (DRP) concentration dynamics and discuss potential management implications. Across all scales, NO3–N concentration steadily declined with each subsequent rainfall event as it was flushed from the system. In contrast, DRP concentration persisted, even on soils at or below agronomic P levels, suggesting that legacy P significantly contributes to nutrient loads in the WLEB. These findings highlight the need to revisit current P fertility recommendations and soil testing procedures to increase P fertilizer use efficiency and to more holistically account for legacy P. Core Ideas Persistent P concentrations were measured from edge‐of‐field to basin scale. Persistent P concentrations after successive rainfall events are indicative of legacy P. Evidence of legacy P beckons for more comprehensive soil test metrics.