Lake and catchment management in Denmark

The majority of Danish lakes are highly eutrophic due to high nutrient inpu t from domestic sources and agricultural activities. Reduced nutrient reten tion, and more rapid removal, in catchments as a result of agricultural dra inage of wetlands and lakes and channelisation or culverting of streams als o 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 strippi ng and nitrogen removal at sewage works, increased use of phosphate-free de tergents, and regulations concerning animal fertiliser storage capacity, fe rtiliser application practices, fertilisation plans and green cover in wint er. In order to improve nutrient retention capacity of catchments, wetlands and lakes have been re-established and channelised streams have been remea ndered. In addition, cultivation-free buffer strips have been established a longside natural streams and there has been a switch to manual weed control . These measures have resulted in a 73% reduction of the mean total phospho rus 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 in ternal phosphorus loading or biological resistance. Various physico-chemica l restoration measures have been used, including dredging and oxidation of the hypolimnion with nitrate and oxygen. Biological restoration measures ha ve been employed in 17 Danish lakes. The methods include reducing the abund ance of cyprinids, stocking with 0(+) pike ( Esox lucius) to control 0(+) c yprinids, and promoting macrophyte recolonization by protecting germinal su bmerged macrophyte beds against grazing waterfowl and transplanting out mac rophyte 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 concentra tion 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 concen trations. Macrophyte refuges and transplantation seem to be the most succes sful as restoration measures in the same nutrient-phosphorus regime as fish manipulation.