James L. Pretty

Understanding the controls on deposited fine sediment in the streams of agricultural catchments.

Excessive sediment pressure on aquatic habitats is of global concern. A unique dataset, comprising instantaneous measurements of deposited fine sediment in 230 agricultural streams across England and Wales, was analysed in relation to 20 potential explanatory catchment and channel variables. The most effective explanatory variable for the amount of deposited sediment was found to be stream power, calculated for bankfull flow and used to index the capacity of the stream to transport sediment. Both stream power and velocity category were highly significant (p ≪ 0.001), explaining some 57% variation in total fine sediment mass. Modelled sediment pressure, predominantly from agriculture, was marginally significant (p<0.05) and explained a further 1% variation. The relationship was slightly stronger for erosional zones, providing 62% explanation overall. In the case of the deposited surface drape, stream power was again found to be the most effective explanatory variable (p<0.001) but velocity category, baseflow index and modelled sediment pressure were all significant (p<0.01); each provided an additional 2% explanation to an overall 50%. It is suggested that, in general, the study sites were transport-limited and the majority of stream beds were saturated by fine sediment. For sites below saturation, the upper envelope of measured fine sediment mass increased with modelled sediment pressure. The practical implications of these findings are that (i) targets for fine sediment loads need to take into account the ability of streams to transport/retain fine sediment, and (ii) where agricultural mitigation measures are implemented to reduce delivery of sediment, river management to mobilise/remove fines may also be needed in order to effect an improvement in ecological status in cases where streams are already saturated with fines and unlikely to self-cleanse.

Ecology of Industrial Pollution: Ecological monitoring and assessment of pollution in rivers

Introduction Many organisms respond to pollution in a predictable way, and it has long been realised that the biota can be used to determine the extent of pollution at a site, a technique termed biomonitoring. Much of the science of biomonitoring developed in aquatic systems, driven by concerns about the impact of industrial and domestic pollution on potable water resources. Over the past century, aquatic biomonitoring has travelled a long way from the early methodologies, and much about the pitfalls and benefits of using biota to assess pollution or other stressors has been discovered. Here we describe the history of biomonitoring and how our understanding has developed, with particular focus on RIVPACS (River InVertebrate Prediction And Classification System). This system marked a major advance in biomonitoring techniques, introducing the reference condition approach, where the physical and geographical characteristics of the river were taken into account when determining what taxa would be expected to be present if the site were not polluted. Assessment of a site was then based on a comparison of the observed community and derived scores, to that expected if the site were not polluted. RIVPACS was also the first biomonitoring tool to incorporate a measure of uncertainty; any assessment is based on spatially and temporally variable samples and it is necessary to calculate the confidence associated with the quality class derived using these samples.

Do agri-environment schemes result in improved water quality?

1. Improved water quality, through a reduction in diffuse pollution from agricultural sources, is an expected benefit of agri-environment schemes, but this has yet to be demonstrated in practice. Here, we evaluate the impact of Welsh agri-environment schemes on water quality and freshwater ecosystem condition through a combined monitoring and modelling framework. 2. To determine the influence of the agri-environment schemes on ecosystem condition, spatially independent catchments dominated by a single scheme (>40% of catchment) were compared to control catchments dominated (>70%) by agricultural land that was not part of any scheme. Biological indicators of water quality were monitored at the outfall of each catchment and a spatially explicit modelling framework of diffuse pollutant emissions applied to each of the 80 catchments. 3. Direct comparison (scheme/non-scheme) was unable to identify any significant effect of agrienvironment scheme participation. However, derived biological indicators that reflected organic pollution, eutrophication and pesticide run-off were strongly correlated with modelled concentrations of corresponding diffuse pollutants, thus providing a ground-truth for the models. Scenarios that assessed the correct counterfactuals (i.e. the influence of scheme entry on pollutant output) were developed for the whole of Wales. The models indicated an important effect of scheme entry on water quality, but this effect was not evenly distributed across the landscape. 4. Synthesis and applications. Our results indicate that agri-environment schemes can deliver improvements in water quality, through a reduction in diffuse pollution from agricultural sources. However, it is not easy to demonstrate scheme effectiveness; the combination of field survey and modelling used here provides a framework for addressing these difficulties. A spatially targeted approach for agri-environment scheme options to protect water resources from diffuse pollution is likely to be most effective at delivering water quality improvements.

Diatoms As Indicators Of Fine Sediment Stress

Excessive delivery of fine sediments to water bodies has a detrimental impact on the biotic elements used for water body status classification. Although diatoms are typically used to assess stress from eutrophication, as fine sediment has the potential to impact diatoms in many ways, it is not surprising that an index based on benthic diatom assemblages has been proposed: the relative abundance of motile species. This measure is based on the fact that many raphid diatom species are capable of migrating through deposited sediment to avoid negative impacts. However, the use of such an index has yet to be fully tested. Various data analysis techniques were used to explore how indices based on diatom assemblages (related to eutrophication and siltation), diatom species, the traits motility, and nutrient affinity responded to a gradient of percentage cover of fine sediment. Although diatom species showed marked variation in their affinity for percentage cover of fine sediment, the relationship between motility (both percent motile and the trait motility) and deposited fine sediment is not sufficiently strong to be used as a reliable indicator of fine sediment stress. We present an approach, which could potentially be used to develop a new index (diatom indicator of sediment conditions) on the basis of the response of diatoms to fine sediment, but caution that this index requires further development before use. Despite the hydromorphology having considerable potential to affect benthic diatoms, existing indices designed to assess eutrophication were robust to hydromorphological modification, reducing the possibility of false diagnosis of impacts.

A possible strategy for enhancing the stream biota in areas of intensive commercial forestry

The intimate relationship berween river channels and riparian trees in north-temperate regions is well established. eoarse woody debris (eWD) determines channel morphology to a large exrent (GuRNELL er al. 1995) and rerains plant derritus (BILBY & LIKENS 1980). Invertebrate derririvores depend upon effective retention and respond positively to elevated derrital standing stocks, particularly in streams with low natural retention (DossoN & HILDREW 1992). Research in catchments containing broad-leaved trees suggests that invertebrate producriviry is ultimately governed by seasonal low points in food availabiliry (RICHARDSON 1991). Therefore, management of streams in foresred areas requires some maintenance of effective retention. eonifer plantations in the British uplands are normally harvested before rrees are mature, and the srreams running through rhem have low derrital standing stocks, despite rhe large quantities of derrirus in rransit (eARISS & DossoN 1997). The low number of inverrebrates commonly encounrered in forested streams is generally attributed to acidification (HARRIMAN & MORRISON 1982, ÜRMEROD et al. 1993), but may, equally, result from low standing stocks of detrirus. This study is part of an invesrigation into the introduction of eWD as a means of increasing detrirus retention in forested srreams. Ir aimed to determine the effect of eWD on the detrital sranding stock and the invertebrate communiry o n the srream bed and hence to evaluare rhe viabiliry of using ewD as a management option in foresred srreams with low derrital retention.