Longitudinal Variability in the Oxygen Demand of Channel Bed Matrix Sediment in a <scp>UK</scp> Agricultural Catchment: Implications for Managing the Sediment Problem

ABSTRACT 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 may deliver better cost–benefit from catchment management. To assess the potential for targeted interventions to deliver benefits, sediment oxygen demand (SOD) was measured in pools, riffles, bars, and runs at nine sites along the River Taw in the southwest of the UK. This river flows from upland semi‐natural grassland, to lowland agriculture with sewage treatment work discharges. SOD was measured for 5 days on the &lt; 25 μm fraction of the bed matrix sediment using a laboratory‐based dissolved oxygen probe. Samples of potential sediment sources were also analysed, and a colour‐based method was used to determine the provenance of the channel bed sediment. SOD did not vary significantly longitudinally or by riverine feature and was 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. The entrapment of autochthonous algal material in this fraction is the probable source of this high SOD. Since reducing within‐stream productivity is likely to be challenging, a combined approach targeting sediment source protection to water erosion and in‐channel measures to increase matrix sediment exfiltration is warranted. This 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 SOD.