The transport of fine particulate organic matter in vegetated chalk streams

Abstract

Chalk streams are characterized by the presence of abundant stands of rooted macrophytes. There is typically a steep reduction in current velocity within stands and this promotes the trapping and accumulation of fine sediments. Fine sediments have also been shown to infill the gravel beds and shallow hyporheic zones of chalk streams in the process of colmation. We investigated how in-channel vegetation and colmation affect the reach-scale transport of fine particulate organic matter (FPOM, particles <1 mm in diameter) in a chalk stream. Corn pollen was used to model FPOM and a vegetated reach retained 62Ð5% of the corn pollen, while only 41Ð8% of the corn pollen was trapped in an unvegetated reach of similar length. In a second release, 51Ð2% of the corn pollen was retained in a vegetated reach, with 58Ð7% retained after the removal of the macrophytes to leave a bed coated with fine sediment. Two series of flume releases were used to examine the effect of colmation (infilling with fine sediment). In the first releases, retention of the corn pollen was 2% over the gravel-only bed, but infilling with sand increased retention to 34%. In the second releases, retention increased from 9% over a gravel-only bed up to 37% after infilling with sand. Our findings have implications for the residence times of sediment-bound nutrients and pesticides in chalk streams and for weed cutting, a common practice for flood risk management.

Instream and riparian implications of weed cutting in a chalk river

“…Existing accounts include impacts on invertebrate populations (Dawson et al, 1991), transport of fine particulate organic matter (Warren et al, 2009), fish habitat (Swales, 1982) and plant communities (Baattrup-Pedersen and Riis, 2004).…”

“…It is significant that even in this relatively clean river system, with low intensity urban and agricultural landuse, mobilised sediment elevated total concentrations by 50 -400% (Table 4) filtration by sand sized material in the river bed (Warren et al, 2009), production of dense faecal pellets by suspension feeders (e.g. blackfly larvae Diptera:Simuliidae), trapping of organic particles in biofilms and accumulation in the hyporheic zone in response to plant induced downwelling (Warren et al, 2009).…”

“…blackfly larvae Diptera:Simuliidae), trapping of organic particles in biofilms and accumulation in the hyporheic zone in response to plant induced downwelling (Warren et al, 2009). The resultant sediment deposits represent important stores of organic material, nutrients and other pollutants (e.g.…”

“…Wharton et al, 2006). Warren et al (2009) observed how sediment deposits may remain following a weed cut owing to the stabilising effect of biofilms. Thus, it is important that weed cover is sufficiently low in winter/spring to allow higher flows to clean fine sediment deposits from the river system.…”

Determining the sources of nutrient flux to water in headwater catchments: Examining the speciation balance to inform the targeting of mitigation measures

“…The NO 3 -N flux behaviour in the Wylye catchment was markedly different to that observed for PON, where the average GC was 0.82, indicating that the delivery of the particulate fraction was more episodic in nature and suggesting a different source and transport pathway, most likely driven by overland flow. It is also known that particulate nutrient fractions are often accumulated in the abundant plant beds characteristic of chalk streams (Prior and Johnes, 2002;Evans and Johnes, 2004) due to low water velocities and settling (Warren et al, 2009). Hence, there is an accessible source of nutrient-rich, fine particulate matter available for remobilisation and dispersal downstream during storm events, especially during autumn and winter when stream vegetation dies back (Jones et al, 2012).…”