Water erosion and phosphorus problems in an agricultural catchment—Need for natural research for implementation of the EU Water Framework Directive
Introduction
Implementation of the European Water Framework Directive (WFD, CEC, 2000/60) will stress the need for measures to reduce the losses and transport of sediment and sediment-associated phosphorus (P) from catchments to receiving lakes. This is a consequence of P being the main nutrient in eutrophication (Vollenweider and Kerekes, 1982). A high nutrient load of P and a high internal load from the lake sediment can cause a low nitrogen/phosphorus (N/P) ratio during the summer, a factor shown to favour the Cyanobacteria (Smith, 1983). Such conditions will cause the water to have a ‘bad ecological status’ in contrast to the WFD goal of good status. The status of lakes is determined by the relationship between it and a reference condition and is classed as high, good, moderate, poor or bad according to the Directive. In order to reach the goal, assessments of diffuse and point source P losses in lake catchments are needed, as well as measures to reduce such losses. However, the links between the sources of P and sediment in the landscape and ecological impacts in waters are very complex. Another problem is that interactions between scientists in different natural disciplines are currently insufficient. Soil scientists tend to favour reductionist approaches at finer scales in the catchment, while limnologists favour empirical approaches for the lake. A coalition of natural disciplines within the land–water continuum is required in the future (Haygarth et al., 2005). Still another problem is how to determine the reference conditions for waters. Based on the fact that the ambition of WFD is to achieve ‘good ecological status’ already by 2015, decisions on measures for programmes have to be taken within a few years. Measures to combat erosion and phosphorus losses have to be evaluated on a currently weak base combined with conceptual understanding of P transport processes in order to suggest actions needed. In practice, only relative effects of measures to alleviate the total P load of the receiving water body can be estimated.
The main aim of this study was to outline measures to combat erosion and phosphorus losses in order to improve the water quality situation of a lake. This was implemented in the form of a case study. Another aim was to discuss further natural research needed to fulfil the targets set by the Directive to reduce and control P pollution through evidence-based policy.
Section snippets
The case study
Soil erosion and P losses are a general problem in silty and clay arable soils in central and northern Sweden (Ulén and Jakobsson, 2005). Lake Brunnsjön (Fig. 1) is situated in a silty soil area and suffers from heavy blooms of toxic blue-green Cyanobacteria and low diversity level indices. The lake is situated close to the town of Hedemora, (9000 inhabitants) and town residents consequently have an excellent view of its waters. The lake was previously a recipient of municipal sewage water and
Soil management
Since the area of non-cropped soil in the catchment is large during winter months, erosion and phosphorus lost with soil particles to surface waters was believed to be a substantial part of the problem. When the soil becomes wet there is a risk of the structure collapsing and mobilising both readily-dispersible clay and silt fractions. In addition, the soil concentration of organic matter is low, a factor that may reduce soil aggregate stability (Skøien, 1993). Direct relationships between the
Soil management
Results from the local experiment strongly indicated that tillage practice affects erosion and losses of PP via surface runoff (Table 2). For PP, the difference between plots with tillage/cultivation and plots with only occasional or no tillage was clearly significant (Table 3). The results also indicated the advantages of not tilling this particular soil during autumn and are in accordance with findings from southern Norway (Lundekvam and Skøien, 1998). However, P losses from spring-ploughed
Catchment status relative to the WFD reference conditions
A scientific problem that exists is how to relate the measured concentrations in surface runoff and drainage water to background levels. WFD requires classification of waters based on reference conditions that represent minimal anthropogenic impact. The concentrations of eroded material and of PP in surface and drainage water may depend on cultivation and tillage, but in a pristine state with constant vegetation cover, erosion may be limited to the water channel. Paleolimnological studies of
Effects of measures for the lake
A high background concentration highly reduces the possible ecological impact in the lake even for highly effective measures against P loss in the catchment source area. The combined effects of mitigated P losses from land, complex transformations in the landscape and final delivery to the lake have to be conceptually based at present and suggestions for the lake are given in Table 4. The finding that soil vegetation and other cover are quite unimportant for DRP losses is typical for catchments
Summary
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Soil management was found to be highly significant for erosion and phosphorus losses to surface water in local experiments.
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Spring cultivation was demonstrated to reduce erosion by 50%, yet deliver similar crop yields as after autumn ploughing.
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Total cessation of soil tillage (direct drilling) was associated with new problems.
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Catch crops of clover established better than ryegrass.
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Processes in the stream are possibly very important for the phosphorus concentration but the measures tested locally
Acknowledgements
Our sincere thanks to Marika Dahllöv, who took most of the local initiatives at Hedemora local authority, to Stefan Löfgren, Institute of Environmental Monitoring Assessment, Swedish University of Agricultural Sciences, and to Hans Olofsson at the County Administration Board of Dalecarlia for all their support. Malva Ahlkrona did much of the data compilation. The Swedish Board of Agriculture, the foundation ‘Stiftelsen lantbruksforskning’ the municipality of Hedemora and the water strategic
Barbro M. Ulén is a researcher and lecturer at the Agricultural University of Sweden. She is limnologist and during the past 28 years she has been studying water pollution from agricultural systems. Her main research interest is the environmental impact of agricultural activities on the quality of surface water and groundwater bodies. More specifically, she is studying processes and the impact of different transport pathways on the distribution of phosphorus in soil and water on different
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Barbro M. Ulén is a researcher and lecturer at the Agricultural University of Sweden. She is limnologist and during the past 28 years she has been studying water pollution from agricultural systems. Her main research interest is the environmental impact of agricultural activities on the quality of surface water and groundwater bodies. More specifically, she is studying processes and the impact of different transport pathways on the distribution of phosphorus in soil and water on different scales; lysimeters, experimental plots, experimental fields, agriculture catchments, agricultural streams and lakes. She has also been working with water monitoring activities and modelling in the Baltic States and in Russia.
Teresa Kalisky is an agronomist who was project leader for the River Mässingsboån and Lake Brunnsjön project at Hedemora local authority, a position founded by the Ministry of the Environment during 2001–2002. Since 2003, she has been working on water planning and other environmental issues at the office of Regional Planning and Urban Transportation of Stockholm County Council. Since 2004 she has also been working on synthesising knowledge about water related problems as project secretary of the ‘water dialogue’, an internet based forum for researchers, politicians and practitioners.