Terrestrial agricultural activities strongly influence riverine nitrogen (N) dynamics, which is reflected in the δ15N of riverine consumer tissues. However, processes within aquatic ecosystems also influence consumer tissue δ15N. As aquatic processes become more important terrestrial inputs may become a weaker predictor of consumer tissue δ15N. In a previous study, this terrestrial-consumer tissue δ15N connection was very strong at river sites, but was disrupted by processes occurring in rivermouths (the ‘rivermouth effect’). This suggested that watershed indicators of N loading might be accurate in riverine settings, but could be inaccurate when considering N loading to the nearshore of large lakes and oceans. In this study, the rivermouth effect was examined on twenty-five sites spread across the Laurentian Great Lakes. Relationships between agriculture and consumer tissue δ15N occurred in both upstream rivers and at the outlets where rivermouths connect to the nearshore zone, but agriculture explained less variation and had a weaker effect at the outlet. These results suggest that rivermouths may sometimes be significant sources or sinks of N, which would cause N loading estimates to the nearshore zone that are typically made at discharge gages further upstream to be inaccurate. Identifying definitively the controls over the rivermouth effect on N loading (and other nutrients) will require integration of biogeochemical and hydrologic models.