Sediment‐associated organic matter sources and sediment oxygen demand in a Special Area of Conservation (SAC): A case study of the River Axe, UK

Abstract

Oxygen demand in river substrates providing important habitats for the early life stages of aquatic ecology, including lithophilous fish, can arise due to the oxidation of sediment‐associated organic matter. Oxygen depletion associated with this component of river biogeochemical cycling, will, in part, depend on the sources of such material. A reconnaissance survey was therefore undertaken to assess the relative contributions from bed sediment‐associated organic matter sources potentially impacting on the River Axe Special Area of Conservation (SAC), in SW England. Source fingerprinting, including Monte Carlo uncertainty analysis, suggested that the relative frequency‐weighted average median source contributions ranged between 19% (uncertainty range 0–82%) and 64% (uncertainty range 0–99%) for farmyard manures or slurries, 4% (uncertainty range 0–49%) and 35% (uncertainty range 0–100%) for damaged road verges, 2% (uncertainty range 0–100%) and 68% (uncertainty range 0–100%) for decaying instream vegetation, and 2% (full uncertainty range 0–15%) and 6% (uncertainty range 0–48%) for human septic waste. A reconnaissance survey of sediment oxygen demand (SOD) along the channel designated as a SAC yielded a mean SOD5 of 4 mg O2 g−1 dry sediment and a corresponding SOD20 of 7 mg O2 g−1 dry sediment, compared with respective ranges of 1–15 and 2–30 mg O2 g−1 dry sediment, measured by the authors for a range of river types across the UK. The findings of the reconnaissance survey were used in an agency (SW region) catchment appraisal exercise for informing targeted management to help protect the SAC.

The magnitude and significance of sediment oxygen demand in gravel spawning beds for the incubation of salmonid embryos

“…The sediment samples collected for SOD measurements were processed following the procedures detailed in Collins et al (, this volume). Samples were wet sieved using river water, and the <63‐μm fraction retained, as previous experience has shown that the highest SOD corresponds to this grain size fraction (Bateman, ).…”

Land use affects lowland stream ecosystems through dissolved oxygen regimes

“…One of the most important stressors originating from the adjacent land is the input of fine sediments into streams 5 . These fine sediments accumulate in deposition zones, like stream bed depressions and pools 6 , especially in streams with low current velocities such as lowland streams 7 . Fine sediments increase the turbidity 8 , decrease the underwater light availability for primary producers 9 and reduce the availability of suitable streambed habitats for aquatic invertebrates 10 .…”

The fine sediment conundrum; quantifying, mitigating and managing the issues

“…Available methods for investigating sediment sources can be divided into indirect and direct approaches (Collins & Walling, ). The most commonly applied direct method of identifying catchment sediment sources is the fingerprinting approach that quantifies the relative contributions of individual sediment sources to target sediment samples, including those collected in gravel beds or from the suspended load (Collins, Foster, et al, ; Owens et al, ). Potential sources of sediment and associated organic matter are identified and sampled, such as agricultural top soils, channel banks, damaged road verges, septic tanks, farmyard manures, and decaying instream vegetation.…”

“…Collins, Zhang, et al () examined the provenance of fine sediment‐associated organic matter and complimented this with sediment oxygen demand measurements. By utilizing the two methods simultaneously, it was possible to account for the key sources of sediment‐associated organic matter that contributed to oxygen demand and therefore habitat and ecological degradation.…”