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 62.5% of the corn pollen, while only 41.8% of the corn pollen was trapped in an unvegetated reach of similar length. In a second release, 51.2% of the corn pollen was retained in a vegetated reach, with 58.7% 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. Copyright © 2009 John Wiley & Sons, Ltd.