CONTROLS ON PRODUCTION OF BRYOPHYTES IN AN ARCTIC TUNDRA STREAM

1. Two bryophyte taxa (Hygrohypnum spp. and, to a lesser extent, Fonti nalis neomexicana) were abundant in riffles within phosphorus-fertiliz ed reaches of the Kuparuk River (North Slope, Alaska), but were much l ess common in fertilized pools and virtually absent in unfertilized re aches of the river. We conducted field experiments using stems and clu mps of both species and artificial bryophytes to test the hypotheses t hat bryophyte growth was strongly limited by low phosphorus concentrat ions in unfertilized reaches, and limited by epiphytes in fertilized p ools. 2. Stem tips of Hygrohypnum spp. did not elongate when grown in unfertilized pool and riffle environments. In fertilized reaches, Hygr ohypnum elongated significantly, although there was no significant dif ference in elongation of stem tips placed in pools [2.5 +/- 0.9 cm (SD )] as opposed to riffles (2.8 +/- 0.9 cm) for 32 days. 3. Stem tips of F. neomexicana elongated significantly in all sites. There was a sign ificant difference in elongation of stem tips in control and fertilize d riffles (2.1 +/- 1.1 and 4.7 +/- 0.1 cm, respectively) but not in ti ps grown in control and fertilized pools (2.8 +/- 0.8 and 2.7 +/- 0.9 cm, respectively). 4. Biomass increments in clumps of these same speci es followed similar patterns except in fertilized pools. Hygrohypnum s pp. lost weight in control riffle environments and did not grow in poo ls, but accumulated 181 +/- 44 and 335 +/- 200% of initial biomass in fertilized riffles in 1992 (over 32 days) and 1993 (over 44 days), res pectively. F. neomexicana accumulated 38 +/- 39 and 98 +/- 47% of init ial biomass in 1999 in unfertilized and fertilized riffles, respective ly. Total phosphorus concentrations of both bryophytes in 1992 were si gnificantly greater when grown in fertilized riffles than control riff les. 5. Artificial mosses (untwisted, natural fibre rope) and clumps o f Hygrohypnum spp. were used to assess effects of flow regime on detri tal and epiphyte accumulation in the fertilized zone. Epiphyte and det rital mass was 4-4.5 times greater on average on artificial mosses in slow-flowing pool environments than in fast-flowing riffle environment s. Epiphyte chlorophyll a was 4 times greater on Hygrohypnum clumps in pools than in riffles. This difference was probably brought about by increased detrital deposition and reduced grazing by invertebrates in pools. It is likely that both Hygrohypnum spy. and F. neomexicana coul d grow throughout the river, but are limited strongly by low phosphoru s concentrations in unfertilized reaches and secondarily by detritus a ccumulation acid interference competition with epiphytic algae in fert ilized pools.