COMPARISON OF EPILITHIC ALGAL AND BRYOPHYTE METABOLISM IN AN ARCTIC TUNDRA STREAM, ALASKA

Phosphorus has been added to a reach of the Kuparuk River Alaska, from late June to mid August every year since 1983. The P-fertilized reach of the river is now extensively colonized by 2 bryophytes (Schistidiu m agassizii and Hygrohypnum spp.), whereas only S. agassizii is common in unfertilized reaches of the river. We compared photosynthesis rate s of epilithic algae, S. agassizii, and Hygrohypnum spp. under referen ce and P-enriched conditions to extend our long-term records of bryoph yte dynamics within the fertilized reach and to describe the physiolog ical differences between major primary producers. Rates of primary pro duction were determined from changes in dissolved oxygen concentration during light and dark incubations of algal and bryophyte samples in c losed chambers. Net primary productivity per unit total chlorophyll a (CHL, a) was greater for epilithic algae (2.0-6.0 mg O-2 mg(-1) CHLt a h(-1)) than for bryophytes (0.2-1.7 mg O-2 mg(-1) CHLt a h(-1)). Howe ver, the greater biomass of the bryophyte community in the fertilized reach yielded area-specific productivity rates for Hygrohypnum spp. th at were 2 to 4 times greater than areal rates of epilithic algal produ ctivity. Bryophytes accounted for 80% of primary production in P-ferti lized reaches but only 9% in reference reaches. Thus, increased bryoph yte abundance in response to P increased total net primary production from 2.3 g C/h to 6.3 g C/h. Photosynthesis-irradiance (PI) parameters obtained by fitting data to a hyperbolic tangent model differed betwe en bryophyte species, between reference and fertilized stream reaches, and over the season. Differences in PI relationships between S. agass izii and Hygrohypnum spp. suggest that they use different life strateg ies (subsistence vs opportunism respectively) to exist in the arctic e nvironment.