Rivermouth alteration of agricultural impacts on consumer tissue δ¹⁵N

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.