Transport and Transformation of Phosphorus in a Forest Stream Ecosystem

Abstract

A phosphorus budget was constructed to examine P retention and processing during 1 yr (1974—1975) in Bear Brook, an undisturbed headwater stream in the Hubbard Brook Experimental Forest, New Hampshire, USA. Year—to—year variation in the P mass balance was also estimated for a 13—yr period using a empirical model of the annual budget. In the model, fluvial inputs and exports of P were calculated using the 13—yr record of streamflow and regressions between P concentration and discharge developed from measurements made during 1974—1976. Precipitation and streamflow were average in the 1974—75 water year, and the relative importance of P input vectors during this year were: tributary streams (62%) > falling and blowing litter (23%) > subsurface water (10%) > precipitation (5%). Geologic export of P in stream water was the only export vector of consequence. Under these average hydrologic conditions, there was no annual net retention of P in the stream: annual inputs of 1.25 g P/m 2 were essentially balanced by exports of 1.30 g P/m 2. However, during most days of this year inputs exceeded exports: P accumulated, was processed in the ecosystem, and was exported during episodes of high stream discharge. Because of the pulsed nature of P flux, a mass balance provides an overestimate of the P entering functional pathways of a stream ecosystem. Over the 13—yr period (1963—1975), annual mass balances calculated with the model were variable; the ratio of P exports to pinus varied from 0.56 to 1.6 and was directly related to annual streamflow. Thus monthly transport patterns or annual mass balances generated from only 1 yr of record may lead to erroneous conclusions on stream ecosystem function. Although variability characterized most aspects of P dynamics in Bear Brook, processing of P is consistent. Inputs of dissolved P (DP, <0.45 μm) and coarse particulate P (CPP, >1 mm) exceeded exports, while exports of fine particulate P (FPP, 0.45 μm—1 mm) exceeded inputs. (Thus there was a net conversion of other forms of P to the FPP fraction, which was the predominant form (62% of the total) exported downstream.