Shallow lakes in lowland river systems: Role in transport and transformations of nutrients and in biological diversity

Abstract

Shallow, small-sized, macrophyte-dominated lakes are frequently inserted into the upstream, initial sections of low-grade river systems in post-glacial lowland landscapes. In terms of the River Continuum Concept they function as ‘disturbing inserts’ which are: 1. sinks for transported nitrate nitrogen by providing more sites for effective denitrification (anoxic, organic sediments, vegetation beds), 2. usually sink also for total phosphorus fluvial load except in summer periods when the net export of this nutrient takes place due to internal loading, being the source of it for downstream river section, 3. transform the coarse, low-organic fluvial suspended matter into the subtle, rich-organic suspension in which living plankton organisms are present. One can say that these lakes being a stagnant section of a system, are shortening the length of nutrient spiralling within the river ecosystem. The fluvial lakes increase the patchiness of a river system providing the sites for aquatic vegetation patches and sediment patches more stable in relation to typical river conditions; in this way, they enlarge the area of refuges in a river system. The greater the refugial area in a system the more stabilised are the predator (fish) — prey (invertebrates) relations in the river food web. The inflows (deltas) as well as outflows from the lake are important sites for the transformation of fluvial matter and nutrients, as well as for river habitat patchiness. The lakes inserted into the fluvial system thus provide the examples of the Serial Discontinuity analogical to the man-made impoundments on a large river system.