Controls on production of bryophytes in an arctic tundra stream

Abstract

SUMMARY1. Two bryophyte taxa (Hygrohypnum spp. and, to a lesser extent, Fontinalis neomexicana) were abundant in riffles within phosphorus‐fertilized reaches of the Kuparuk River (North Slope, Alaska), but were much less common in fertilized pools and virtually absent in unfertilized reaches of the river. We conducted field experiments using stems and clumps of both species and artificial bryophytes to test the hypotheses that bryophyte growth was strongly limited by low phosphorus concentrations in unfertilized reaches, and limited by epiphytes in fertilized pools.2. Stem tips of Hygrohypnum spp. did not elongate when grown in unfertilized pool and riffle environments. In fertilized reaches, Hygrohypnum elongated significantly, although there was no significant difference 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 significant difference in elongation of stem tips in control and fertilized riffles (2.1 ± 1.1 and 4.7 ± 0.1 cm, respectively) but not in tips 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 species followed similar patterns except in fertilized pools. Hygrohypnum spp. lost weight in control riffle environments and did not grow in pools, but accumulated 181 ± 44 and 335 ± 200% of initial biomass in fertilized riffles in 1992 (over 32 days) and 1993 (over 44 days), respectively. F. neomexicana accumulated 38 ± 39 and 98 ± 47% of initial biomass in 1992 in unfertilized and fertilized riffles, respectively. Total phosphorus concentrations of both bryophytes in 1992 were significantly greater when grown in fertilized riffles than control riffles.5. Artificial mosses (untwisted, natural fibre rope) and clumps of Hygrohypnum spp. were used to assess effects of flow regime on derrital and epiphyte accumulation in the fertilized zone. Epiphyte and detrital mass was 4–4.5 times greater on average on artificial mosses in slow‐flowing pool environments than in fast‐flowing riffle environments. 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 spp. and F. neomexicana could grow throughout the river, but are limited strongly by low phosphorus concentrations in unfertilized reaches and secondarily by detritus accumulation and interference competition with epiphytic algae in fertilized pools.