Publication describes stream phosphorus dynamics of minimally impacted coastal plain watersheds

Human activities can sometimes mobilize nutrients such as phosphorous and have potential implications for freshwater ecosystems. In an effort to manage human impacts to aquatic ecosystems, the Florida Department of Environmental Protection (FDEP), under the direction of the U.S. Environmental Protection Agency, recently adopted region-based numeric nutrient criteria for phosphorus and nitrogen in streams.

These numeric nutrient criteria identify concentrations above which a stream is impaired and subject to basin management activities to mitigate excess loading. Effective management of nutrients in streams, however, requires understanding of nutrient concentrations and their natural variation in streams minimally affected by humans.

Recently, a study by the University of Florida quantified spatial and temporal variation in stream phosphorous dynamics in minimally impacted, low-relief watersheds in Florida. The study was conducted with support from NCASI, Rayonier Corporation, Plum Creek Corporation, the Florida Forest Service, and the National Institutes of Food and Agriculture. 

Results from the study are available in a paper by Robert T. Hensley, Daniel L. McLaughlin, Matthew J. Cohen, and Paul H. Decker that recently appeared in Hydrological Processes. The authors report finding substantial natural variation in in-stream phosphorous concentrations within geographic regions and over time.

The abstract for the paper follows.

“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.”

For more information, contact Dr. Erik Schilling, who managed NCASI involvement in the study.

Contact Information 

Reference 

Hensley, R.T., D.L. McLaughlin, M.J. Cohen, and P.H. Decker. 2017. Stream phosphorus dynamics of minimally impacted coastal plain watersheds. Hydrological Processes 31:1636–1649. https://doi.org/10.1002/hyp.11132