Phosphorus Spiralling in a Woodland Stream: Seasonal Variations

Four radiotracer releases were performed over an annual period in 1981—1982 to determine seasonal variation in indices and pathways of phosphorus spiralling in Walker Branch, a small woodland stream in eastern Tennessee, USA. Each release consisted of an addition of °370 MBq each of carrier—free ³ ²PO₄ and ³H₂O over a 1—h period during baseflow. Concentrations of ³ ²P and ³H dissolved in stream water were measured intensitively at several stations downstream for the radiotracer input during and immediately following each release. Activity of ³ ²P in coarse particulate organic matter (CPOM), fine particulate organic matter (FPOM), and aufwuchs was measured 2—3 h after each release and at various intervals for 7 wk. Total biomass of CPOM, FPOm, and aufwuchs at the time of each release was also measured. Uptake of ³ ²PO₄ from the water was greatest in November and lowest in August, Uptake length (Sw) of phosphorus, defined as the average distance travelled by a PO₄ ion dissolved in water, varied from 22 m in November to 97 m in August. Uptake of ³ ²PO₄ by CPOM was generally greatest, with °50% of total uptake, while that by aufwuchs was lowest, with <15% of the total. CPOM abundance was the major determinant of whole—steam PO₄ uptake rate and Sw. Turnover length (Sₚ) of phosphorus, defined as the average distance traveled by an atom of P taken up by particulate material, was short compared to Sw, varying from 1 m in November to 3 m in January. Consequently total spiralling length (S) of P varied from 23 in November, just after peak autumn leaf fall, to 99 m in August, and reflected primarily the travel of P in the dissolved form. Our results indicate that the greatest increase in Sw, (and consequently in S) in Walker Branch occurs in late autumn or winter after storms reduce the abundance of CPOM in the lower portions of the stream bed. Although we calculate that Sₚ may increase by one of two orders of magnitude for short periods during storms, the greatest effect of storms on P spiralling over the long term is their impact on the quality of CPOM and FPOM in the stream bed after the return to baseflow. For most of the year, detrital organic carbon probably influences phosphorus spiralling more than phosphorus spiralling influences the processing of organic carbon in Walker Branch. Only during the fall and early winter periods, when CPOM abundance is high and Sw is short, does phosphorus spiralling exert strong control over biotic processes downstream.