MECHANISMS OF STREAM PHOSPHORUS RETENTION - AN EXPERIMENTAL-STUDY

Using the spiralling concept, this study assessed the relative importance of temperature, current velocity, leaf biomass, fine particulate organic matter, and leaf type in determining phosphorus retention in woodland streams. To accomplish this objective, streamside artificial streams and two contrasting leaf types were used. A labile leaf type (dogwood), typical of successional forest vegetation, and a more refractory leaf type (oak) characteristic of mature forest species were chosen. Leaves were picked just before abscission and air dried. Dogwood leaves were added to three streams and oak leaves to three other streams. Phosphorus uptake experiments were conducted in the streams during November 1987-June 1988 and November 1988-June 1989. All streams were least retentive of phosphorus in December, the coldest month of the year, and more retentive in the warmer spring and summer months. Phosphorus uptake was not correlated with microbial biomass or activity on leaves or with leaf biomass. However, streams with oak leaves were more retentive than streams with dogwood leaves. Measurements of penetrance (i.e., weight required to push a metal rod through the leaf) revealed that dogwood leaves were softer than oak leaves. The soft dogwood leaves were less effective at retarding water flow and therefore velocity was typically faster in dogwood streams (p < 0.05; Pearson's correlation). This higher velocity apparently contributed to lower retention. These results suggest that phosphorus retention under conditions of this experiment was governed primarily by temperature and water velocity. Coarse and fine particulate organic matter biomass and composition did influence retention, but their impact was secondary to the effects of temperature and velocity.