Understanding the impact of farming on aquatic ecosystems
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Soil sedimentThe main mechanisms by which sediment is lost from farming activities are the erosion of river banks and surface run-off and sub-surface pathways from fields. Loss of soil from river banks is a natural process, but it can be accelerated where livestock have direct access to watercourses and loosen sediment by trampling and de-stabilising the river bank. Poaching of soils is also a problem especially during the winter months when soils are wet increasing their susceptibility to surface run-off. Compared to other sources, surface run-off from fields has the potential for a greater impact on aquatic ecosystems due to the associated pollutants, particularly phosphorus and pesticides, that can be lost with it. Soil is eroded by a variety of mechanisms and transported via numerous pathways. Susceptibility of soils to erosion has a high spatial variability as a result of varying climatic conditions and soil properties and, therefore, the intensity of the impact is highly variable, depending on the hydrodynamic and ecological characteristics of the receiving water body. Losses of soil from farmland will not necessarily be reflected in adjacent watercourses because sediment is transported and deposited downstream. This increases the spatial extent of the impact and may displace the impact from the immediate vicinity of the farmland. Furthermore, the sequential nature in which different sized fractions of sediment are deposited as the river loses its capacity to transport sediment also means that the intensity of the impact may vary spatially along the river, with the greatest impact being found in lowland streams. Erosion of soil sediment is estimated to result in losses ranging from 0.1 to 20 tonnes ha‑1 yr-1, depending upon soil characteristics, topography and land use.81 Losses are greatest from arable farmland, with the most vulnerable areas being the sandy soils in the southwest and southeast of England, East Anglia, the Midlands and the chalky soils of the South Downs. Areas of farmland kept for grazing and rearing of livestock situated adjacent to water courses have also proved problematic due to trampling of river banks and soils, with much of this type of farming activity being concentrated in north east and south west England. Highly susceptible land uses include late-sown winter cereal, potatoes, sugar beet, field vegetables and outdoor pigs.82 Preliminary source apportionment work shows that 75% of sediment input into rivers can be attributed to agriculture.83 Many studies on soil loss to water have concentrated on episodic and significant events (e.g. heavy rain) of soil loss rather than on the smaller more cumulative processes and tended to focus on where soil loss is present rather than where it is absent.84 The EA’s Article 5 risk assessment estimated that 21.0% of river water bodies (22.8% by length) were at risk from sediment delivery from agricultural activities. The risk was calculated from models that included only run-off from arable land but not field drains and eroding river banks.85 Point sources such as mines, quarries and gravel extraction sites were also omitted. No risk assessments were undertaken for sediment in lakes, transitional and coastal waters. The EA also reports that many chalk streams (approximately 101 out of 161) suffer from so-called ‘chalk stream malaise’. This is caused by a combination of silt smothering gravels and high phosphate inputs, and exacerbated by low river flows. It is also thought that local geology and hydrological conditions can worsen the impacts of any silt present in chalk streams by, for example, leading to river bed concretion.86 In addition, in 2004 half of the 62 principal salmon rivers in England and Wales with salmon action plans were at risk of missing their egg deposition targets, with siltation of spawning gravels a major factor.87 However, there appears to be no representative assessment of the geographic scale or the quantification of the potential problem of siltation in salmonid rivers. For example, it appears that there is no information on how representative the 62 principal salmon rivers are of the 34,500 km of rivers designated (salmonid or cyprinid) under the Freshwater Fish Directive and the 51,183 km of river assessed for Article 5 of the WFD. Not all of these rivers would naturally support salmonids. There is limited data on levels of sediment deposition/siltation in fresh water bodies: a report from the Life in UK Rivers project lists current data sources for rivers.88 The available data does not provide a representative assessment of siltation levels in rivers in England and Wales, and it has not been possible to establish simple links between siltation and suspended solids concentrations/loads, and siltation and land use. Suspended solids are monitored under the Freshwater Fish Directive which sets a guideline standard of 25 mg l-1 for designated salmonid and cyprinid fishery waters. This standard is exceeded in some rivers across the country, but particularly in central England, the Welsh borders and south east England.89 The Harmonised Monitoring Scheme also includes the measurement of suspended sediments at some of the 230 sites monitored, which are mainly located at the tidal limits of major rivers or at the points of confluence of significant tributaries.90 A study of the provenance of interstitial sediments in salmonid spawning gravels in 18 rivers in England and Wales found that the vast majority of sediment was derived from surface run-off, although there was large regional variation. For example, in the River Dee 98% of interstitial sediments were derived from surface run-off and just 2% from bank erosion.91 In southwest England, bank erosion is thought to be the main source of fine sediment in spawning gravels whereas in southern England surface soil erosion is more important. This difference reflects differences in agricultural activities in the two areas: livestock farming (with animals poaching river channel margins) in the former and mainly arable farming (with more stable river channels) in the latter. 92 93 A further survey looked at siltation within artificial redds (spawning gravels) at 43 sites across 19 catchments in England and Wales.94 It was thought that the silt (<0.85 mm particle size) accumulating at two of the sites could adversely affect salmonid reproduction. It was hoped that the results from this survey could be extrapolated to other rivers across England and Wales to obtain a more representative assessment. The UKTAG has not proposed any standards for suspended sediment that would lead a water body to be classified as less than good ecological status partly because the current Freshwater Fish Directive annual average suspended standard of 25 mg/l was considered to be inappropriate and because of the lack of suitable data to develop an alternative standard95. Sediment is constantly introduced into waterbodies through the natural processes of erosion and runoff. Aquatic communities are adapted to cope with these natural levels of input and sediment provides a habitat for many organisms. In the context of the WFD this would equate to ‘reference conditions’ where natural levels of settled and suspended sediment would lead to typical plant and animal communities that would be different for different types of water body. The detrimental effects of increased loadings of sediment to water bodies resulting from human activities can affect all trophic levels of the ecosystem. In suspension, sediment can: Reduce primary production by increasing water turbidity. In marine waters there may be a resultant reduction in growth rates, aerial coverage and depth of colonisation of macrophytes and macroalgae. Macrophytes may be replaced by floating algae with resultant changes in fish and invertebrate communities.96 Alter water pH. Increase water temperatures by increasing heat absorption. Fish such as salmon and trout require temperatures of between 5 and 15ºC for normal growth and maybe vulnerable to increases in temperature.97 Clog the gills of fish and crayfish, which impairs respiratory function and damages internal organs, 98 and overload the feeding appendages of filter-feeding organisms such as mussels, thereby interfering with their digestive and respiratory systems.99 Destroy the protective mucous covering the eyes and scales of fish, which makes them more susceptible to disease and infection.100 Be vectors of pollutants (organic matter, nutrients, chemical and microbial) that can, for example, secondarily affect filter feeding animals.101 As a result, high suspended sediment can cause a shift from filter-feeding organisms to deposit-feeding organisms. When deposited, sediment can: Suppress growth of plants by reducing light penetration to, and oxygen uptake by, submerged aquatic plants and causing instability for rooted plants such as brook water crowfoot.102 Blanket or smother benthic invertebrate habitats. Overall, sedimentation and turbidity has been found to be inversely related to invertebrate densities and biomass 103 with, for example, caddisfly104, mayfly and stonefly105 larvae being adversely affected by deposition of silt and smothering and burial by fine sediments. Marine species which reside in burrows are particularly vulnerable to smothering. Sediment boring species (e.g. the bivalve Pholas dactylus or burrowing species such as the polychaete worm Polydora ciliata) are unlikely to recover from smothering due to clogging of their burrows but may be able to tolerate high concentrations of sediment in suspension.106 Reduce the suitability of hard substrata as a habitat for colonising organisms, such as mussels and oysters and other biofouling organisms.107 Species of macroalgae such as Laminaria digitata, which possess a holdfast for attachment to rocks, may be unable to establish themselves where there is excessive deposition of fine cohesive sediments. Blanket spawning gravels of fish species such as Atlantic salmon108 109, sea trout, allis and twaite shad and reduce egg and larvae survival rates.110 Modify the morphology and hydrodynamic conditions of (river) waterbodies by reducing the speed and turbulence of flow, thereby further reducing its ability to assimilate other pollutants. Have secondary effects on benthic organisms and on organisms with benthic stages of their life cycles via pollutants associated with sediment. Research is currently underway to link agricultural land use to the biodiversity of benthic invertebrates. Preliminary results from the EU-Life funded SOWAP (Soil and Water Protection) project, a study looking at the impact of soil loss on aquatic ecology, show that catchments with intensively cultivated land have a lower diversity of benthic organisms than those catchments with woodland or natural grassland.111 Download 432.61 Kb. Share with your friends:
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