Effects of nuisance submerged vegetation on the fauna in Norwegian rivers

The abundance of aquatic vegetation is increasing in rivers and lakes worldwide. The aim of this study was to find how the macrophyte Juncus bulbosus Linnaeus affects salmonids and benthic macroinvertebrates in Norwegian rivers. The proliferation of J. bulbosus in the study rivers commenced after the development of hydropower in the 1960s and 1970s. J. bulbosus is now considered a nuisance for humans in many areas of the rivers. We found a higher density of juvenile fish and higher density, weight and species richness of invertebrates in areas with J. bulbosus than in areas with gravel, suggesting that the vegetation is not limiting fish and invertebrates. This may be because macrophytes increase the surface area and provide shelter, food and a variety of ecological niches. Adult salmonid fish can be negatively affected when their spawning grounds are covered by vegetation. However, overgrowth is not common and may take years since fish clear the river bed of macrophytes during redd digging, indicating competition between macrophytes and fish for riverbed habitat. Our results suggest that one should not assume that outgrowths of macrophytes have negative impacts on the ecosystem. It is important to map all impacts and distinguish nuisance to humans from effects on the ecosystem.