Stream phosphorus dynamics of minimally impacted coastal plain watersheds

Understanding natural variation in stream phosphorus (P)concentrations over space and time is critical for understanding natural drivers of catchment behavior and establishing regulatory standards. Across minimally impacted benchmark streams (n=81) in Florida, spatial variation in mean total P concentrations was large, indicating the importance of geologic controls on catchment solute dynamics. While this variation was significantly predicted by geographic regions, within regions we observed nearly comparable cross-site variation, suggesting important finer-scale heterogeneity in baseline catchment chemistry. Within-site residual variation (unexplained by region or site) was as large as spatial variation, suggesting temporal variation in response to drivers such as flow may be critically important. To further explore timescales of P export variation, we collected long-term, high-frequency (subdaily) measurements of stream discharge (Q) and soluble reactive P (SRP) in 2 forested watersheds. We observed significant variation at annual, event, and diel timescales, all of which arise primarily from corresponding Q-variation. Over the entire period of record, we generally observed a strong dilution signal, with SRP concentrations declining with increased Q. Despite significant SRP variation, flow variation was far larger and, thus, dominated temporal control on downstream flux. Within-storm events, we observed strong and consistent clockwise SRP versus Q hysteresis, suggesting mobilization of proximal SRP stores. Diel variation exhibited mid-afternoon concentration minima, Q-controlled amplitude, and pronounced seasonal shifts in both magnitude and timing consistent with riparian evapotranspiration-regulating lateral inputs of P-rich groundwater. Such high-resolution temporal signals allow identification of solute sources and provide insights into geologic and hydrologic drivers of solute variation.