Phosphorus Dynamics in a Woodland Stream Ecosystem: A Study of Nutrient Spiralling

Abstract

The term spiralling refers to the interdependent processes of cycling and downstream transport of nutrients in a stream ecosystem. To describe spiralling in Walker Branch, a first—order woodland stream in Tennessee, we released 32 PO 4 to the stream water and measured its uptake from the water and then followed its dynamics in coarse particulate organic matter (CPOM), fine particulate organic matter (FPOM), aufwuchs, grazers, shredders, collectors, net—spinning filter feeders, and predators over a 6—wk period. Rates of transfer among compartments and rates of downstream transport were estimated by fitting a partial differential equation model of the ecosystem to the data. With the resulting coefficients, the model was run to steady state to estimate standing stocks and fluxes of exchangeable phosphorus. Phosphorus moved downstream at an average velocity of 10.4 m/d, cycling once every 18.4 d. The average downstream distance associated with one cycle, defined as the spiralling length, was therefore 190 m (10.4 m/d ° d). Spiralling length, at steady state, is approximately the ratio of the total downstream flux of phosphorus per unit width of stream (720 mg°d—1°m—1) to the rate of P uptake from the water (3.90 mg°m—2.d—1). CPOM accounted for 60% of the uptake, FPOM for 35%, and aufwuchs for 5%. Turnover times of P in particulates ranged from 5.6 to 6.7, except for FPOM, which showed a second, slower turnover time of 99 d. Of the P uptake from water by particulates, 2.8% was transferred to consumers, while the remainder returned directly to the water. About 30% of the consumer uptake, in turn, was transferred to predators. The spiralling length was partitioned into: (1) an uptake length associated with transport in the water column (165 m), (2) a particulate turnover length associated with transport in FPOM and CPOM (25 m), and (3) a consumer turnover length associated with animal drift (0.05 m). FPOM transport accounted for 99% of the particulate turnover length. The small consumer turnover length reflected low consumer uptake of P from particulates and slow downstream drift velocity (0.013 m/d). In spite of the low rate of phosphorus uptake, the combined consumer—and—predator community accounted for 25% of the standing stock of exchangeable P in the stream. The retentiveness of this community is attributable both to the low drift rate and to a long turnover time (152 d) for P within the community.