Succession and accrual of benthic algae on cobbles of an upland river following scouring

Abstract

Flow releases from dams can be used to scour benthic algae, simulating the effects of natural spates and maintaining benthic algae in an early successional stage for increased grazer palatability. The timing of releases needs to consider the natural periodicity of flow events and the speed of regrowth and community succession changes. We studied benthic algal regrowth and succession using manipulative field experiments during summer and winter in the upland regulated Severn River, New South Wales, Australia. Benthic algal biomass accrual as chlorophyll a and community changes were determined after artificially scoured cobbles were returned to the river. In summer, algal biomass and diversity on scoured cobbles took 2 weeks to return to levels similar to reference cobbles and 5 weeks in winter. Chlorophyll a during summer was initially 0.24 ± 0.06 mg m−2 on scoured cobbles, but by day 16 had increased to 9.74 ± 1.97 mg m−2 and was no longer significantly different from reference cobbles. In winter, chlorophyll a was initially 0.47 ± 0.13 mg m−2 on scoured cobbles, but by day 37 had increased to 44.7 ± 10.9 mg m−2 and was no longer significantly different from reference cobbles. Peak chlorophyll a accrual during summer and winter was 1.64 and 2.63 mg m−2 d−1, respectively. Early succession in both experiments was dominated by diatoms such as Cocconeis, Synedra, and Fragilaria. A proliferation of the filamentous green alga Stigeoclonium was indicative of a late succession community. The implications for flow management based on resetting of benthic algae by scouring in riffle reaches of rivers are discussed.