Fingerprinting source contributions to bed sediment‐associated organic matter in the headwater subcatchments of the River Itchen SAC, Hampshire, UK

Abstract

Excessive accumulation of sediment-associated organic matter in river beds has detrimental impacts on aquatic ecology, including fish. Sediment and source samples were collected from 3 headwater tributaries of the River Itchen, in southern England. Near infrared spectra and bulk stable carbon and nitrogen isotopes were employed as fingerprint properties to quantify the relative contributions from catchment-based (farmyard manures/slurries, damaged road verges, and septic tanks) and channel-based (decaying instream vegetation, watercress farms, and fish farms) sources. The findings suggested that over the duration of the sampling period, the final estimated median relative contributions in the Candover Stream subcatchment were in the order: farmyard measures/slurries 74% > decaying instream vegetation 15% > watercress farms 6% > septic tanks 3% > damaged road verges 2%. For the River Arle subcatchment, corresponding estimated median relative contributions were in the order: watercress farms 40% > farmyard manures/slurries 28% > decaying instream vegetation 21% > fish farms 7% > damaged road verges and septic tanks 2% each. Similar contributions were estimated for the Tichborne Stream subcatchment: watercress farms 36% > farmyard manures/slurries 26% > decaying instream vegetation 26%and damaged road verges and septic tanks at 6% each. These source apportionment estimates will contribute to stakeholder engagement and targeted management interventions.

Linking the field to the river 30 years on: Policy, practice and professional geomorphology

“…• Identify sources of harmful organic matter in the bed sediments of the River Itchen (Zhang et al, 2017) • Understand the importance of roadside verges as a sediment source in the Hampshire Avon (Collins et al, 2010) • Support WFD compliance work in the River Beane (Environment Agency, 2019b), and the River Taw (West Country Rivers Trust, 2015)…”

“…Walling & Amos (1993) provide an early example of sediment fingerprinting for applied understanding of sediment sources in the River Piddle, UK. More recently, the Environment Agency has either commissioned or benefited from a diverse range of catchment‐based sediment fingerprinting projects, including studies to: Reduce impacts of fine sediment on River Torridge freshwater pearl mussels (Pulley et al, 2019) Target Catchment Sensitive Farming on the River Parrett (Collins et al, 2010), and the River Win (Environment Agency, 2019a) Identify sources of harmful organic matter in the bed sediments of the River Itchen (Zhang et al, 2017) Understand the importance of roadside verges as a sediment source in the Hampshire Avon (Collins et al, 2010) Support WFD compliance work in the River Beane (Environment Agency, 2019b), and the River Taw (West Country Rivers Trust, 2015) Investigate runoff management measures for flood risk management (Blake et al, 2012) Looking ahead, application of fingerprinting to future catchment management will need to consider costs of data analysis increasingly carefully. Recent work by Pulley and Collins (2018) offers an opportunity to reduce costs through development of open‐source software that simplifies the complex statistical analysis.…”

The scope for a system-based approach to determine fine sediment targets for chalk streams

“…The impacts of organic matter content of infiltrating fine sediment could be extensive within a chalk stream as these streams regularly record high proportions of organic matter within accumulated fine sediment compared with other UK fluvial systems (Sear et al, 1999;Greig et al, 2005a;Heywood and Walling, 2007). The presence of abundant aquatic macrophytes throughout chalk streams contributes in this respect (Bateman, 2012;Collins et al, 2017;Zhang et al, 2017). Anthropogenic activities also contribute to the relativity high organic matter content.…”

“…Inclusion of such geographically varied factors in sediment targets, however, necessarily adds another level of complexity for the management of fine sediment. For example, the inclusion of targets for sediment-associated organic matter introduces the need to consider a far greater range of catchment sources than currently scoped within conventional sediment management strategies, including in-channel biota, riparian vegetation litter, damaged road verges and septic waste (Collins et al, 2010(Collins et al, , 2014(Collins et al, , 2017Sear et al, 2017;Zhang et al, 2017). A possible means to address the problems associated with overly-simplistic strategic sediment management targets is to determine system-based targets, centred on the understanding of the system sediment budget and ecological responses to elevated fine sediment pressure and subsequent impacts.…”

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

“…Four papers within this special issue illustrate and reflect on how sediment fingerprinting can be implemented in the management of sediment and associated organic matter using diverse case study examples. Zhang et al () present the findings of a study conducted in three tributaries of the River Itchen, in southern England, which successfully identifies the main sources of sediment‐associated organic matter inputs. In all three subcatchments, the top three sources were found to be watercress farms, farmyard manures/slurries, and decaying instream vegetation, although the relative contributions and importance varied.…”