Macrophyte vegetation in the river Otra upstream of Brokke, with a focus on the mass development of Juncus bulbosus

Summary

Mass development of bulbous rush (Juncus bulbosus) has been reported in the river Otra since 1964, after the establishment of the Brokke power plant. In this report, we bring together existing information on the distribution of aquatic macrophytes upstream of Brokke, report the results of macrophyte mapping in 2019, and discuss the results with respect to the question: why is there a mass development of Juncus bulbosus in the upper Otra? All available information indicates that Juncus bulbosus is a natural part of the macrophyte community in the upper Otra. It is likely that Juncus bulbosus and other macrophytes occurred in moderate abundances, in naturally slow flowing river reaches, prior to river regulation. These occurrences may already have been somewhat higher than in unimpacted reference conditions. However, massive abundances of macrophytes were only reported after the construction of Brokke power plant in 1964. ... Several factors enable increased growth and reduced dieback of Juncus bulbosus between Bykil and Brokke, and the sum of these factors may lead to “nuisance growth”. (1) Deep weir basins do not freeze deeply enough, meaning that more plants survive the winter and continue to build biomass over several years. (2) The water level is less variable and, on average, higher than before river regulation. Water plants can therefore grow over larger areas, leading to more visible biomass. (3) Reduced intensity of floods due to regulation causes reduced water plant losses, and higher minimum water levels leads to less freezing and drying of water plants. Consequently, water plants can build up a large biomass over several years. (4) Lower flow velocity and greater depth in weir basins leads to sedimentation of nutrient and carbon rich material. Degradation of this material leads to increased availability of CO2 and nutrients, which may be used by water plants for growth. (5) This effect is likely to be exacerbated by nutrients and organic carbon from wastewater treatment plants and other point and diffuse sources, even though measured water nutrient concentrations in the Otra are very low. An increased availability of CO2, which may be small but occurring over long periods of time, and often occurring together with an increased availability of NH4+ and phosphorus, may be a common factor which is generally related to Juncus bulbosus nuisance growth in different habitat types in Norway.