Longitudinal variability in the oxygen demand of channel bed matrix sediment in a UK agricultural catchment: implications for managing the sediment problem

Dissolved oxygen has been shown to be an important control on the ecological status of rivers. The presence of excess fine-grained matrix sediment in channel beds can exert an oxygen demand in this critical habitat for fish spawning and invertebrates. Therefore, reducing the oxygen demand of channel bed sediment through targeted intervention represents a promising way of delivering better cost-benefit from environmental management investments. At present, limited research has been conducted into the potential for channel
bed sediment oxygen demand to be reduced through targeted management. Therefore, sediment oxygen demand (SOD) was measured in pools, riffles, bars and runs at nine sites
along the upper River Taw in the southwest of the UK. This river flows from upland semi-natural grassland to lowland agriculture with sewage treatment work discharges representing a range of land uses and sources. SOD was measured over a five-day period on the <25 µm fraction of the bed matrix sediment using a laboratory based polarographic dissolved oxygen probe. Samples of potential sediment sources within the study catchment were also analysed,and a colour-based sediment source tracing method was used to determine the provenance of the channel bed sediment. SOD did not vary significantly longitudinally or by riverine feature
and was much higher in the river bed matrix sediment than its sources. Using settling to isolate the ultra-fine fraction of the sediment showed the highest SOD was concentrated here.
It is likely that the entrapment of autochthonous algal material in this fraction is the cause of high SOD in the channel bed matrix sediment. Since reducing within-stream productivity is likely to be challenging, a combined approach targeting sediment source protection to water erosion and use of in-channel measures to increase matrix sediment exfiltration is warranted. This dual approach would reduce the reduction in pore spaces in the bed matrix by fine sediment ingress, thereby limiting the entrapment of the ultra-fine material controlling SOD5.