THE ECOLOGICAL ROLE OF AQUATIC BRYOPHYTES IN A WOODLAND STREAM

Seasonal phenology, abundance, distribution patterns, photosynthetic rates, and phosphorus uptake rates of aquatic bryophytes in Walker Branch, Tennessee over a 13-mo period are reported in this study. The most abundant bryophyte in Walker Branch during 1990-1991 was the leafy liverwort Porella pinnata, followed by the mosses Brachythecium cf. campestra and Amblystegium sp. Bryophyte abundance peaked in late summer and was reduced by a severe winter storm. Abundance was positively associated with stable substrate types (bedrock, boulders) and channel units characterized by rapid water velocity (bedrock steps, riffles). Where present, Porella exhibited significantly greater area-specific rates of photosynthesis and phosphorus uptake than periphyton, irrespective of site or season. Biomass-specific rates also were greater for Porella in fall; in winter and spring, however, periphyton on cobbles (but not on introduced cylinders) had significantly greater biomass-specific photosynthetic and phosphorus uptake rates. When these data are extrapolated to the entire 925-m2 sampling reach, taking into account the patchy distribution of Porella, photosynthesis and phosphorus uptake rates are similar between Porella and periphyton in late summer and fall, but are 3-5x greater by periphyton in spring and early summer.