Thresholds Database > Lake eutrophication, Denmark

Background

Danish lakes are typically small and shallow, with a high point source loading of nutrients from arable land.

Alternate Regimes

1. Clear water, submerged vegetation, preferred fish species

2. Eutrophic, turbid, few fish

Mechanism

Phosphorus and nitrogen input from agricultural land, has decreased oxygen levels in the water, increasing turbidity, with an associated reduction in submerged vegetation and a switch in favour of anoxic fish species.

Management Decisions in Each Regime

State 1: Since the mid 1800's, many of the wetlands, lakes or streams have disappeared due to land reclamation. Most of the remaining streams were culverted or channelised, stream beds were lowered and widened to diminish the risk of flooding. This has meant that the removal of nutrients by wetlands has decreased.



State 2: The Danish government has implemented action plans, the objectives being, 1) to reduce external nutrient loading and, 2) to improve nutrient retention and removal by wetlands. The national Action Plans aim to reduce the land-based nitrogen and phosphorus loading of the aquatic environment by 50% and 80% respectively. A number of strategies have been implemented to reduce nutrient loadings at their source, including the promotion of phosphorus-free detergents, regulation of fertilizer use, increasing catchment retention, remeandering channelled streams, dredging and oxidating the hypolimnion, reducing the abundance cyprinids and stocking with pike, protecting macrophyte beds from waterfowl and transplanting macrophyte shoots. For a detailed description of these and other strategies, refer to Jeppesen et al. (1999). Examples are given of fish manipulation in Lake Lyng and Lake Vaeng.