Linking pattern and process: the effects of hydraulic conditions on cobble biofilm metabolism in an Australian upland stream

Mesoscale variability in hydraulic conditions plays an important role in stream ecosystem structure and function. While the structural effects of variability have been widely tested, its influence on ecosystem processes is less well understood. This study examines the influence of spatial hydraulic variability on cobble biofilm metabolism in a regulated Australian upland stream. Primary production and respiration on stream cobbles forming the substrate in areas subject to three different flow conditions were measured using benthic chambers. Overall the cobble biofilms were found to be strongly autotrophic (P/R >> 1). Rates of respiration and, to a lesser extent, production were greatest where flow velocities and turbulence were low. Concentrations of chlorophyll a, pheophytin a and organic material were also highest in areas of lowest flow velocities. Hydraulic conditions did not affect P/R ratios. The degree of autotrophy is likely to reflect the effects of a recent fire in reducing shading and increasing nutrient influx. The observed spatial patterns can be partly explained by the greater accumulation of biomass in low flow velocity areas, but qualitative differences in biofilms are also likely to be important.