Effects of storm flows on phosphorus concentrations in a forested New Hampshire stream

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Shrestha, Anju
As the primary limiting nutrient in the freshwater aquatic ecosystems, excessive concentration of phosphorus (P) in streams and lakes can lead to eutrophication. Variations in bioavailability and solubility across P species inhibit inferences about downstream impacts of loading based on total P (TP) values alone, especially during heavy rain events when multiple sources are likely. To better understand storm contributions of P, this study measures hourly variability of TP, total dissolved P (TDP), total particulate P (TPP), dissolved organic P (DOP), and soluble reactive P (SRP) during three extended rain events at Livermore Cove Brook, a stream that feeds oligotrophic Squam Lake, New Hampshire, USA. The three events have different hydrological characteristics, with runoff ratios ranging from 0.08 to 0.19, corresponding to P loading. The maximum concentrations of TP during Events 1, 2 and 3 are 453.7, 45.0 and 83.7 �?g L-1, respectively. P levels in the stream responded quickly to the onset of each rain event, remaining high for only few hours during peak flow and subsiding as flows decreased. The hysteresis of different P species with unit discharge (UD) and percent of new water (%NW) in the stream reveals the evolution of P loading during storms. The concentrations of TP, TDP, TPP, DOP and SRP were higher on the rising limb of hydrograph than on the falling limb, with clockwise hysteresis in UD and %NW. Unlike the hystereses of TPP and DOP, the hysteresis of SRP looked different suggesting that SRP was from different source. TPP and DOP peaked an hour before the peak of UD and %NW suggesting that they were quickly mobilized and their sources were nearby stream, whereas SRP peaked an hour after the peak of UD and %NW suggesting that the SRP was slowly mobilized and its source was farther away from the stream. DOP was the dominant species of P in streamwater. Data suggested that there was large pool of DOP in base flow and nearby stream under the ground. Among the three events, Event 1 has the highest stream TP yield, 0.0075 kg ha-1. These results show that storms can create episodic peak loads of P, especially with high intensity precipitation. This is useful for managing P in streams and understanding the relationship between different P species, stream flows and storm events in New England.