Guidance Freshwater quality

Index section Freshwater quality

Introduction

The lake IJssel (IJsselmeer), the lake Marker (Markermeer) and its marginal lakes are together referred to as "the wet heart of the Netherlands". In the 1960s and the 1970s the water quality here was poor, because of eutrophication. Since then, the water quality in many of the marginal lakes has improved, thanks to a reduction in the amounts of nutrients entering the water and to the fishing of bream. The result has been an increase in duckweed and fish species (not bream) and water birds in the Veluwemeer.

Zebra mussels play an important role in the water bodies in the "wet heart". This mussel filters the water, so if the mussel colonies are sufficiently large, the water becomes clearer. The mussels are also an important source of food for waterfowl, particularly for the tufted duck and pochard. The decline in zebra mussels in Lake Mark has negatively affected the clarity of the water and the number of overwintering tufted ducks

Since 1950 the water quality of pools, streams, ditches and fens has declined and therefore many plant and animal species have declined or disappeared from these small bodies of water. The species that have suffered include the dragonflies and fish found in brooks. Many of the stoneflies associated with flowing water have disappeared.