Effects of Storm Events on Phosphorus Concentrations in a Forested New England Stream

Phosphorus is considered the primary limiting nutrient in the freshwater aquatic ecosystems; thus, excessive concentrations of phosphorus (P) in streams and lakes can lead to eutrophication. Understanding the transport of P from minimally impacted watersheds provides a benchmark for quantifying human enhancement of P loss from the landscape. To better understand storm event transport of P from small watersheds of northern New England, we examined hourly variations in P in its multiple forms during three rain events at Livermore Cove Brook, New Hampshire, USA. The three storm events had different hydrological characteristics that resulted in different maximum total P concentrations. Streamwater P levels rose quickly at the onset of each event, remained high for a few hours during peak flow, and subsided as flow decreased. Dissolved organic P was the dominant species of P in the streamwater during baseflow and event flow. For each event, P concentrations were higher on the rising limbs of the hydrograph, resulting in clockwise hysteresis of the concentration-discharge relationships. Different behavior of the more inorganic, soluble reactive P hysteresis curve suggests a different source than the other P species. Both particulate and dissolved organic P were highest an hour before peak discharge and storm event water, suggesting quick mobilization of sources proximal to the stream. Soluble reactive P peaked an hour after discharge and storm event water, suggesting slower mobilization or more distal sources. Overall, these results show that storms create episodic peak concentrations of P that constitute a substantial flux of P to downstream lakes and wetlands. Consequently, efforts to manage P in New England lakes likely require greater attention to P transport during episodic storm events and their contribution to annual P loading.