THE SIGNIFICANCE OF CALCITE (TRAVERTINE) FORMATION BY ALGAE IN A MOSS-DOMINATED TRAVERTINE FROM MATLOCK BATH, ENGLAND

An investigation of epiphytic algae on the leaves of the bryophyte Cra toneuron commutatum var, commutatum from a thermal travertine-depositi ng spring revealed 18 species of diatoms and two cyanobacteria. Algae and calcite were detected on leaves immediately after their expansion, with diatoms Achnanthes minutissima aggr., Denticula tenuis and Navic ula carl var. cincta the most numerous. In terms of biomass (as biovol ume) and surface area, Rhopalodia gibba was the most important epiphyt e. The algal epiphytes attained maximum numbers 20-30 mm below the mos s cushion surface and thereafter declined due to lack of light. Traver tine (calcite) deposition increased in a Linear fashion with cushion d epth over the first 80 mm but with a lag in deposition at the stem ape x. About 1 mu g calcite was deposited per mm(2) leaf/day. A significan t association was found between the presence of the large diatom Rhopa lodia gibba and calcite deposition but no associations were found with the remaining algae. The results suggest that Rhapalodia exerts a pos itive effect on calcite deposition by presenting a favourable physico- chemical microenvironment through biochemical (including photosyntheti cally-driven) processes. Travertine deposition continued well below th e level where the moss leaves were photosynthetically active and below the level where the algae attained maximum abundance. Although diatom s have been shown to influence calcite precipitation, the deposition o f travertine was considered to be controlled mainly by inorganic proce sses.