Effects of desiccation and temperature/irradiance on the metabolism of 2 arctic stream bryophyte taxa

Long-term (13 y) experimental P enrichment of a 4th-order reach of the Kupa ruk River (North Slope, Alaska) had little effect on the distribution, abun dance, or metabolism of a common, clump-forming bryophyte (Schistidium [Gri mmia] agassizii), but promoted rapid and extensive growth by mat-forming, s treamer bryophytes (Hygrohypnum alpestre and H. ochraceum) that formerly we re rare in the river. In previous research we showed that there were import ant physiological differences (i.e., photosynthetic efficiency) that helped explain why S. agassizii was able to persist in the river under cold, olig otrophic conditions, whereas H. alpestre and H. ochraceum were not. Here we hypothesize that S. agassizii should be more tolerant than Hygrohypnum spp . to desiccation stress, and that Hygrohypnum spp. would take greater advan tage (e.g, greater rates of net primary production) than S. agassizii of el evated temperatures. We did laboratory experiments using small-scale metabo lism chambers to test this hypothesis. Desiccation affected net photosynthe sis more in Hygrohypnum spp than in S. agassizii. Hygrohypnum spp, had a gr eater tolerance to temperatures >20 degrees C than S. agassizii. Further, n et photosynthetic rates (at light saturation) were significantly higher for Hygrohypnum spp. (1676-6342 mu g O-2 g(-1) dry mass [dm] h(-1)) than for S . agassizii (428-1163 mu g O-2 g(-1) dm h(-1)) at all temperatures measured . In fact, S. agassizii showed minimal response to increases in light avail ability and recovered rapidly from desiccation, but was inhibited by high t emperature. In contrast, Hygrohypnum spp. were susceptible to desiccation, and responded strongly to increased light and temperature. The physiologica l attributes we measured helped explain the distribution of our test specie s in arctic tundra streams.