Environment and aquatic vegetation of a riverside lagoon in Northern Sweden

Abstract

The environmental conditions and aquatic vegetation of a riverside lagoon (backwater, Sw. “ava”) in northern Sweden were studied during a 3-year period (1975–1977). Conditions were compared with those in the main river. During spring and summer, the lagoon is joined to the river by a back-eddy, up a channel. During the autumn and winter period, when the river flow is reduced and the water level sinks, the channel dries up and the lagoon becomes an isolated water-body. The coincidence of the time of lowest water level in the lagoon with the onset of ice formation in the autumn leads to freezing of the bottom sediments over wide areas, which induces rapid vegetational changes. The zonation of the littoral and sublittoral vegetation does not solely depend on hydrological factors; bottom freezing and tjaele formation (Sw. “tjäle”; frozen subsoil) are also important. The aquatic vegetation has been affected by human influences, such as timber floating and seine fishing. The aquatic vegetation in the lagoon is more species-rich than that in the main river, including, for instance, Stratiotes aloides L. This difference is due to the lentic, instead of lotic conditions and to the richer substrate in the lagoon. Four ecological groups of aquatic macrophytes were distinguished, one of which is entirely dependent on the environmental conditions prevailing in the lagoon.