Lars M. Svendsen

Lake and catchment management in Denmark

The majority of Danish lakes are highly eutrophic due to high nutrient input from domestic sources and agricultural activities. Reduced nutrient retention, and more rapid removal, in catchments as a result of agricultural drainage of wetlands and lakes and channelisation or culverting of streams also play a role. Attempts have recently been made to reduce nutrient loading on lakes by intervening at the source level and by improving the retention capacity of catchment areas. The former measures include phosphorus stripping and nitrogen removal at sewage works, increased use of phosphate-free detergents, and regulations concerning animal fertiliser storage capacity, fertiliser application practices, fertilisation plans and green cover in winter. In order to improve nutrient retention capacity of catchments, wetlands and lakes have been re-established and channelised streams have been remeandered. In addition, cultivation-free buffer strips have been established alongside natural streams and there has been a switch to manual weed control. These measures have resulted in a 73% reduction of the mean total phosphorus concentration of point-source polluted streams since 1978; in contrast, there has been no significant change in the total nitrogen concentration. Despite the major reduction in stream phosphorus concentrations, lake water quality has often not improved. This may reflect a too high external or internal phosphorus loading or biological resistance. Various physico-chemical restoration measures have been used, including dredging and oxidation of the hypolimnion with nitrate and oxygen. Biological restoration measures have been employed in 17 Danish lakes. The methods include reducing the abundance of cyprinids, stocking with 0+ pike ( Esox lucius) to control 0+ cyprinids, and promoting macrophyte recolonization by protecting germinal submerged macrophyte beds against grazing waterfowl and transplanting out macrophyte shoots. In several lakes, marked and long-lasting improvements have been obtained. The findings to date indicate that fish manipulation has a long-term effect in shallow lakes, providing nutrient loading is reduced to a level so low as to ensure an equilibrium lake water phosphorus concentration of less than 0.05–0.1 mg phosphorus l−1. If nitrogen loading is very low, however, positive results may be obtained at higher phosphorus concentrations. Macrophyte refuges and transplantation seem to be the most successful as restoration measures in the same nutrient-phosphorus regime as fish manipulation.

Retention of nitrogen and phosphorus in a Danish lowland river system: implications for the export from the watershed

In a Danish lowland river system intensive measurements were made, in four 80 m reaches, of the nitrogen (N) and phosphorus (P) stored in the stream sediment. The results were used for calculation of the total retention in the river system during two summers (June to August). In addition, the mobilization of nutrients from the stream bottom in autumn 1987 was compared with the export from the watershed.During the study period (June 1987 to September 1988) the amounts of N and P stored in stream reaches were determined fortnightly using a core-sample technique. In reaches dominated by submersed macrophytes, 25–40 g N m−2 and 20–30 g P m−2 were stored during two summers, against only 10–15g N and P m−2 for sandy and gravely reaches. In riparian zones with emergent macrophytes the retention was even higher than in the submersed macrophytes. Gross retention exceeded net retention by a factor of two to three.Net retention of P in the river system during the summer of 1987 was equal to the summer export from the watershed. On an annual basis, retention in the summer constituted 20% of the P export. In contrast, retention in the summer of 1988 amounted to 60% of the total P export during the same period (38% reduction) and 22% in comparison with the annual export. The corresponding figures for N were lower, showing reductions of 16% and 12% of the export of total N in the two summer periods, and about 1% of the annual exports.In September 1987 6.4% of the total N export and 65% of the total P export from the watershed consisted of resuspended material. In 1987 the N and P retained during the summer was almost completely resuspended during storm events during September to November.

Retention of nutrients in river basins

In Denmark, as in many other European countries, the diffuse losses of nitrogen (N) and phosphorus (P) from the rural landscape are the major causes of surface water eutrophication and groundwater pollution. The export of total N and total P from the Gjern river basin amounted to 18.2 kg ha−1 and 0.63 kg P ha−1 during June 1994 to May 1995. Diffuse losses of N and P from agricultural areas were the main nutrient source in the river basin contributing 76% and 51%, respectively, of the total export.Investigations of nutrient cycling in the Gjern river basin have revealed the importance of permanent nutrient sinks (denitrification and overbank sedimentation) and temporary nutrient storage in watercourses. Temporary retention of N and P in the watercourses thus amounted to 7.2–16.1 g N m−2 yr−1 and 3.7–8.3 g P m−2 yr−1 during low-flow periods. Deposition of P on temporarily flooded riparian areas amounted from 0.16 to 6.50 g P m−2 during single irrigation and overbank flood events, whereas denitrification of nitrate amounted on average to 7.96 kg N yr−1 per running metre watercourse in a minerotrophic fen and 1.53 kg N yr−1 per linear metre watercourse in a wet meadow. On average, annual retention of N and P in 18 Danish shallow lakes amounted to 32.5 g N m−2 yr−1 and 0.30 g P m−2 yr−1, respectively, during the period 1989–1995.The results indicate that permanent nutrient sinks and temporary nutrient storage in river systems represent an important component of river basin nutrient budgets. Model estimates of the natural retention potential of the Gjern river basin revealed an increase from 38.8 to 81.4 tonnes yr−1 and that P-retention increased from −0.80 to 0.90 tonnes yr−1 following restoration of the water courses, riparian areas and a shallow lake. Catchment management measures such as nature restoration at the river basin scale can thus help to combat diffuse nutrient pollution.

Long-Term, Habitat Specific Response of a Macroinvertebrate Community to River Restoration

1. In 1989 a 1.3 km channelized reach of the River Gelsa was restored to a new 1.9 km meandering course. The restored reach was subsequently (1989–95) monitored for changes in physical and biological features compared with an upstream channelized reach. Weed clearance and other types of maintenance practices have not been undertaken in either reach since 1990. 2. By 1993 the restored reach had already stabilized, both physically and with respect to diversity and density of the macroinvertebrate community. 3. The upstream reach gradually improved physically during the study period, but remained less heterogeneous than the restored reach, with only a very limited amount of stony substrate. Stone-dwelling macroinvertebrate species were consequently still scarce in 1995, while overall diversity and density of the macroinvertebrate community was similar to that in the restored reach. 4. The results indicate that natural rehabilitation of physical features is a rather fast process, but in some ways cannot match the almost instantaneous heterogeneity obtained by active restoration measures.

Restoration of the rivers Brede, Cole and Skerne: a joint Danish and British EU‐LIFE demonstration project, III—channel morphology, hydrodynamics and transport of sediment and nutrients

1. A comprehensive monitoring programme was initiated for the Brede, Cole and Skerne river restoration projects in order to elucidate the impact of re-meandering on flood levels, floodplain inundation, adjustment of river morphology, sediment transport and overbank sediment deposition. 2. Reducing the bankfull capacity, raising the bed level and lowering the bank level allowed an increase in flooding frequency and in the amount of water passing onto the floodplain in all three rivers. In the river Brede, restoration of the natural hydrological contact between the river and its floodplain resulted in high deposition of sediment (189 t year−1) and sediment-associated phosphorus (770 kg P year−1). 3. Construction work caused excessive downstream loss of sediment and phosphorus as documented from sediment mass balances for the River Brede and River Cole. Short-term adjustments in river morphology were recorded in the River Cole based on the fluvial auditing procedure. Post-restoration morphology changed compared with that before restoration in terms of both total diversity and the type of features recorded.

Scenario analysis of nutrient management at the river basin scale

A new river basin model (TRANS) for studying the transport, removal and accumulation of nutrients in rivers, lakes and riparian areas has been developed and tested on data from a 115 km2 river basin in Denmark (river Gjern). The model combines catchment information on soil type and land use with a physical hydrodynamic modelling system and several semi-dynamic empirical models on diffuse nutrient loading and nutrient retention in rivers, lakes and riparian areas. Three main river basin management scenarios were defined for this basin and the environmental effects analysed using the model: 1. Improved sewage treatment at sewage works; 2. 20% set-aside of arable land; 3. river system rehabilitation. Combining the three scenarios in an overall river basin management strategy reduced annual total nitrogen export from the river basin by 93 tonnes, corresponding to a 53% reduction in nitrogen export during the hydrological year 1994–95. Similarly, annual total phosphorus export was reduced by 2.6 tonnes corresponding to a 46% reduction in phosphorus export.

Bank erosion in a Danish lowland stream system

This paper deals with quantification of bank erosion in an entire lowland stream system and its contribution to suspended sediment export from the stream basin. Bank erosion was measured at 33 stream reaches over a one-year period and suspended sediment transport concomitantly determined near the outlet of the stream system. There was considerable spatial variation in bank erosion within the stream system, erosion being most severe on the unprotected lower parts of the banks. No relationship could be established between stream bank erosion and land use, although erosion was lower in forest streams than in streams on land used for grazing cattle. For the stream system as a whole the erosion rate was 11 mm y-1 for each bank, or 0.020 m3 per m stream reach. This corresponds to a total of 2000 tonnes bank material, of which the <63 μm fraction accounted for 20–30% (no sample pre-treatment). In comparison, total annual export of suspended sediment from the stream system amounted to 675 tonnes.

Rivers of the Central European Highlands and Plains

The ecoregion of the central European highlands and plains is drained by some of the main rivers that flow into the Baltic and North Seas, including the Weser, Elbe, and Oder Rivers. In addition to these rivers, this chapter describes some smaller but peculiar rivers, such as the Em (Sweden), Skjern (Denmark), Spree (Germany) and Drawa (Poland) rivers. The Weser River exhibits a balanced longitudinal sequence of geomorphologically distinctive river sections typical of the Central European Highlands and Plains. The Weser and its tributaries provide important ecological services to society, including drinking water, sewage removal, water for irrigation, cooling water for power plants and industrial facilities, hydropower, habitat for organisms, and recreation and tourism. With a length of 1094 km, the 8th order River Elbe (Czech: Labe) is the third longest river in central Europe (after the Danube and Rhine). The Elbe is often seen as a river still possessing a natural river bed with active flood-plains. The Oder (Polish and Czech: Odra) is the sixth largest river flowing into to the Baltic Sea, with an annual discharge volume of 17.3 km. Being 854 km long, the Oder is the second longest river in Poland (after the Vistula). It has been used early for navigation both in north-south and east-west directions, as it has been connected early with the Elbe catchment via two canals.

Deposition of sediment and phosphorus during overbank flooding

During the last decade, more emphasis has been placed on the fluxes of suspended sediment and associated nutrients, pesticides and heavy metals through freshwater drainage systems (e.g. MEAoE 1995). This has resulted not only because of eutrophlc and toxic effects within streams and lakes themselves, but also because of the environmental impact in coastal marine waters (e.g. KR.oNVANG et al. 1993). In recent years, phosphorus (P) has received special anention because little is known about diffuse sources and pathways (HAYGARTH et al. 1998). In particular, the ability o f P to be boun d to particulate matter means that the flux can be highly discominuous, with periods of suspension and transport alternating with periods of sedimentation and rest (SVENDSEN & KR.ONVANG 1995, KR.ONVANG et al. 1997). Overbank storage is an important natural sink for suspended sediment and associated substances in river systems (WALLING & HE 1994). In many river systems overbank flooding is, however, prevented due to the regulation of water courses for the purposes of drainage of agricultural areas, ship traffic, dams and reservoirs, ete. (EuROPEAN ENVIRONMENT AGENCY 1999). This paper summarises the results of a 2-year insitu study on sedimentation rates of sediment and phosphorus in a Danish lowland river valley of 0.5 ha. The sedimentation rates during single overbank flooding and irrigation events o f the river valley were measured.

Danish Buffer Strips as Sediment and Phosphorus Traps

Excessive soi! erosion and sediment delivery to streams results in increased sediment-bound pollutant transport, decreased water storage capaciry, flooding, and destruction of aquatic habitats (DILLAHA & lNAMDAR 1997). Buffer strips are widely accepted as being beneficial for sediment and pollutant retention and habitat qualiry, and hence are an essential watershed protection practice. Grass or dense herbaceous vegetation is most effective at trapping partides from overland storm flows (ÜSBORNE & KoVACIC 1993). However, considerable uncertainty remains regarding the site-specific effectiveness of buffer strips for sediment and pollutant remova[ (DILLAHA & lNAMDAR 1997). The present study examines the characteristics of buffer strips along the edges of agricultural fields in Denmark, focusing on their ability to trap sediment and phosphorus (P). Detailed measurements of rill volume and sediment deposition enabled us to determine sediment and P budgets for four slope units and to highlight the importance of buffer strips as sediment and P traps.