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The influence of an in‐network lake on the timing, form, and magnitude of downstream dissolved organic carbon and nutrient flux

Within fluvial networks, lakes can be sinks or sources of dissolved organic carbon (DOC) and nutrients, yet the controls over sink‐source behavior remain unclear. We investigated the influence that an in‐network lake exerted on DOC and nutrient export. Our investigation consisted of: (1) injecting a conservative tracer to determine lake travel times and flow paths; (2) sampling lake inflow, outflow, and surrounding groundwater to determine water and nutrient budgets; and, (3) sampling internal lake profiles to ascertain in‐lake physico‐chemical patterns through time. Conservative tracer data indicated considerable in‐lake retention and combined with inflow‐outflow discharge measurements revealed a decoupling of kinematic and solute pulses. Nitrate (NO3) was the dominant form of dissolved inorganic nitrogen (DIN) at lake inflow whereas ammonium (NH4) became the dominant component at lake outflow. The lake was a sink for NO3‐N and PO4, but a source for NH4‐N, DON, TDN, and DOC. We observed hydrologic controls on DOC concentrations and export patterns, but redox controls on DIN dynamics. Our results indicate that lakes within fluvial networks can be sources of dissolved organic material and reduced nitrogen (NH4) while simultaneously being sinks for NO3‐N and PO4‐P. Determining controls on sink‐source behavior and the cumulative effect of lakes on DOC and nutrient budgets is a necessary first step toward improved understanding of the role of lakes in network‐ to regional‐scale dynamics.