Hydraulic influences on periphyton and benthic macroinvertebrates: simulating the effects of upstream bed roughness

1. Hydraulic conditions, periphyton biomass and invertebrate communities were compared on artificial substrates exposed to a range of upstream 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 substrates.
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 a density showed a general decline with near-bed velocity and was significantly lower on the substrates exposed to the river bed reference and 0 mm roughness treatments than the 110 mm roughness elements. Chlorophyll a was also negatively correlated with the abundance of larger collector-browsing invertebrates. This indicates that effects of the changes in hydraulic conditions on invertebrates may have contributed to the observed treatment effects on periphyton.
4. Invertebrate abundance and diversity declined with increasing upstream roughness. Filter-feeders, collector-browsers and predatory invertebrates all declined in abundance with increasing upstream roughness, but the effect was strongest for filter-feeders. Eight of the nine most common taxa showed significant treatment effects. The orthoclad chironomid, Eukiefferiella sp., was not influenced strongly by upstream roughness, but its abundance was correlated significantly with periphyton biomass.