Vegetation Changes in the Nutrient-Rich Shallow Lake Hjälstaviken

Abstract

Hjälstaviken used to be a bay of Lake Mälaren. The bay is now separated from the large lake and forms a shallow lake with abundant waterfowl (Lindroth, 1931, Öhrn, 1940). The area of lake Hjälstaviken is about 1.4 km². The reed belts around the lake occupy 1.1 km² and only 30 ha. of the total lake area is open water. Maximum depth is 1.5 m and mean depth is 0.5 m.

Life in the lake has changed during the last decades. Aquatic plants were to be found in the whole lake (Hörtadius, 1923; Almqvist, 1929). Today there is only a broad reed belt. This in turn affects the bottom fauna and the bird life.

There are many reasons for the altered situation in Hjälstaviken. One reason is the supply of nutrients to the lake by the drainage from the surrounding farm land.

The purpose of this investigation was to clarify why the biota in Hjälstaviken have changed, how much nutrients are supplied to the lake, and which are the chemical conditions in the lake water and bottom sediments. The drainage basin of the lake is 60 km² and 75 per cent of this is farm land. The region around the lake consists of nutrient rich post-glacial silt, which is easily eroded by the spring- and autumn-floods and deposited in the lake. The lake has only one large inflow, Trögbodiket, which drains 43 km² of the whole drainage area. Prior to the influent point, the Trögbodiket is overgrown. Most of the eroded material is deposited in this zone. The same estuary situation occurs in the first three-hundred meters of the outflow, Utloppsbäcken.