REMEDIAL MEASURES CONNECTED WITH AQUATIC MACROPHYTES IN NORWEGIAN REGULATED RIVERS AND RESERVOIRS

Abstract

Enhanced growth of aquatic macrophytes have been recorded from some Norwegian rivers regulated for generating hydropower, and reducing the aquatic vegetation to manageable levels is desirable. Weir building is a common practice on Norwegian low flow reaches, but this in turn can result in silting and nuisance growth of macrophytes. Using herbicides to treat aquatic systems is not allowed by the Norwegian environmental authorities and alternative methods of macrophyte control have to be employed. In contrast, many regulated rivers and reservoirs are devoid of macrophytes and the management aim then is to restore native aquatic vegetation. Mechanical methods using harvesters and cutting devices are often difficult to implement in running water. Better results have been achieved utilizing flushing flows to scour sediments and associated macrophytes. Using a sharply peaking flow gives a more efficient control of excessive macrophyte growth because only the initial surge actively scours the river bed. Moss quantities flushed out in a number of controlled flows exhibited a strong log–linear relationship to significant flow, Q, in River Suldalslagen, south-western Norway. Temporarily halting hydroelectric facilities during cold spells to drain the river and thereby expose submerged macrophytes to frost, has been applied to the River Otra in southern Norway. An attendant ice run resulted in heavy loss of macrophytes in this river. The benefit of winter draw-downs combined with flushing flow is partly offset by downstream accumulation of the removed plants. The potential to revegetate shores of reservoirs and impoundments has been studied in Meltingen, a regulated lake in mid-Norway. This field study indicates that amending the shore substrate by light dressing with fertilizer results in enhanced survival of amphibious species. This study also demonstrates the need for sediment seed banks or nearby sanctuaries from which aquatic species can extend into the reservoir during years with a favourable high water level. Reducing the designated fluctuation range of reservoir elevation from 7 to 1.6 m resulted in recolonization by submerged macrophytes in Hartevatn, a reservoir on the upper River Otra.