Diversity and distribution of stream bryophytes: does pH matter?

Bryophytes can strongly influence biodiversity and ecosystem function in low-order streams. Mosses and liverworts have substantial biodiversity and biomass in streams, yet few investigators have examined which factors influence bryophyte species distributions, and fewer have examined assemblages across a wide pH gradient. We examined bryophyte assemblages across a pH gradient in 26 southeastern New York (USA) streams. We recorded bryophyte species richness, diversity, and abundance, in conjunction with pH, width, depth, canopy cover, substratum size, temperature, dissolved O-2, turbidity, conductivity, current velocity, dissolved organic C, dissolved Ca, Mg, Fe, NH4+, NO3-, soluble reactive P (SRP), and abundance of other autotrophs. pH ranged from similar to 4 to 7 and corresponded to the type of underlying bedrock. Nearly all streams had low or undetectable concentrations of SRP. Several species, e.g., Hygrohypnum eugyrium and Codriophorus aduncoides, were apparent pH generalists, whereas others were pH specialists. For example, Hygrohypnum ochraceum occurred only in neutral pH, whereas Andreaea rothii was restricted to low-pH sites. Vector fitting on a nonmetric multidimensional scaling ordination identified stream pH and amount of bedrock as key factors affecting species distributions. Latitude, longitude, altitude, canopy cover, and aqueous Ca, Mg, SRP, and turbidity also were correlated with species distributions, but many factors were colinear. Nearly all sites had low SRP, and species distributions were strongly related to pH, so we assayed several species and populations for phosphomonoesterase (PMEase) activity under different pH conditions. Species and populations from low-pH streams tended to have stronger PMEase activity at lower pH than those from higher pH streams, suggesting that species and populations may be adapted to specific pH conditions. Differential PMEase activity may be one mechanism by which aquatic bryophytes persist in low-pH environments.