Drag coefficients of stream bryophytes: experimental determinations and ecological significance

1. Drag coefficients (C-D) of bryophyte-covered rocks were measured to see whether these differed between species. Replicate rocks, each supporting one of six bryophyte taxa, were attached to a base plate mounted on Teflon bearings in a how tank. The drag force exerted on racks with and without bryophyte material was measured by a strain gauge at different water velocities. 2. The difference in C-D between rocks with and without bryophyte material was calculated for each plant, and expressed as a percentage change (deltaC(D)). This varied significantly from 0 in three of the six taxa. The cushion-shaped moss Bryum blandum increased the C-D by around 10%. The moss Blindia lewinskyae and liverwort Cryptochila grandiflora decreased the C-D by around 40 and 30%, respectively, presumably reflecting their streamlined growth. 3. Drag characteristics of aquatic bryophytes may help explain differences in their resistance to fast flows. Furthermore, we suggest that some aquatic bryophytes can increase substrate stability by streamlining rocks, rendering them less prone to movement.