Changes in macroalgae and bottom fauna in the winter period in the regulated Alta River in Northern Norway

Abstract

AbstractThe study was carried out in the upper part of the Alta River (69°58N; 23°24E), on a 3 km stretch that is not covered by ice due to increased winter temperatures after hydroelectric power development in 1987. The aim was to detect potential effects on the macroalgae and bottom fauna from the altered physical conditions. The presented data, based on Surber samples, describe the late winter situation (March to May) for the macroalgae (1996–2004) and the bottom fauna (1993–2004). The maximum biomasses of algae showed a highly significant decline from close to 200 g dry weight (ash free) m−2 in 1996 to less than 50 g dry weight m−2 in the last years of the investigation. Microspora amoena was the overall dominating species in the first phase of the study and Didymosphenia geminata in the last 5 years. Ulothrix zonata and Hydrurus foetidus were common throughout the entire study. A highly significant linear decline (r2 = 0.752, p = 0.001) was found in the total bottom fauna densities, showing means from 9500 to 16 000 ind. m−2 in 1993–1996 and from 2200 to 2700 ind. m−2 in 2001–2004. The reduction primarily concerned Chironomidae larvae. The mean biomass of the bottom fauna did not change to the same extent due to low individual weight of the Chironomidae larvae. A strong correlation between algal biomass and bottom fauna density was found (r2 = 0.796, p = 0.0029). The study demonstrates a eutrophication effect in the ice‐free river section following the regulation. The effect decreased through the investigated period and terminated 13–15 years after the regulation took place. Copyright © 2008 John Wiley & Sons, Ltd.