HYDRAULIC INFLUENCES ON PERIPHYTON AND BENTHIC MACROINVERTEBRATES - SIMULATING THE EFFECTS OF UPSTREAM BED ROUGHNESS

1. Hydraulic conditions, periphyton biomass and invertebrate communiti es were compared on artificial substrates exposed to a range of upstre am roughness conditions across an area of uniform current velocity and depth in a gravel-bedded river. The effect of river bed roughness was simulated by installing roughness elements upstream of artificial sub strates. 2. Increasing upstream roughness reduced the average near-bed velocity above the substrates and increased short-term variability in velocity (i.e. turbulence). 3. Periphyton chlorophyll n density showe d a general decline with near-bed velocity and was significantly lower on the substrates exposed to the river bed reference and 0 mm roughne ss treatments than the 110 mm roughness elements. Chlorophyll a was al so negatively correlated with the abundance of larger collector-browsi ng invertebrates. This indicates that effects of the changes in hydrau lic conditions on invertebrates may have contributed to the observed t reatment effects on periphyton. 4. Invertebrate abundance and diversit y declined with increasing upstream roughness. Filter-feeders, collect or-browsers and predatory invertebrates all declined in abundance with increasing upstream roughness, but the effect was strongest for filte r-feeders. Eight of the nine most common taxa showed significant treat ment effects. The orthoclad chironomid, Eukiefferiella sp., was not in fluenced strongly by upstream roughness, but its abundance was correla ted significantly with periphyton biomass.