Nuisance growth of Juncus bulbosus in lakes and rivers : experimental and observational studies

Nuisance growth of the aquatic macrophyte Juncus bulbosus has been observed since the 1980s in an increasing number of rivers and lakes in northern Europe. What causes such massive growth is not well understood, however, and our aim was to assess potential drivers behind the nuisance growth in both lakes and rivers. Our main hypothesis was based on the observation that there is a strong overlap between the areas receiving elevated nitrogen (N) deposition and those with the most severe growth of J. bulbosus. These areas generally have low levels of phosphorus (P) and we wanted to test whether the resulting elevated N:P ratios might promote J. bulbosus growth. To test this hypothesis, we collected data from 153 lakes and 28 river locations in Southern Norway (papers I, II and IV) and conducted a controlled growth experiment in which 100 J. bulbosus plants received different nutrient treatments (paper III). However, none of these approaches gave support for our hypothesis To search for other drivers for J. bulbosus nuisance growth, we collected a range of parameters from the lakes and rivers visited. These included catchment, lake water and sediment characteristics. Despite an extensive number of parameters tested, we were unable to detect any general drivers that could explain nuisance growth. Furthermore, a genetic screening (AFLP fingerprinting) showed no genetic differences between nuisance and nonnuisance plants. The upside of these “negative” conclusions however, is that we can now exclude several candidate parameters as the causes for nuisance growth. The most important being genetic differences and elevated N:P ratios. Our data did show that the most problematic nuisance growth occurred in the most oligotrophic lakes (paper I). Analyses of plant elemental composition (paper IV) showed that J. bulbosus plants had high C:N and C:P ratios compared to other freshwater macrophytes (and even higher in roots compared to shoots). This allows J. bulbosus to build large biomasses on small amounts of nutrients, rendering it highly competitive in nutrient poor habitats. In the growth experiment (paper III), we found an increasing uptake of sediment NH4 with increasing growth, pointing towards a special relevance of NH4 for J. bulbosus growth. Furthermore, analyses of plant elemental composition revealed higher N contents of river plants compared to lake plants (paper IV). Based on these results, relevant literature and indications from the lake and river surveys (papers I and II), we found indications that different triggers might be responsible for nuisance growth in rivers (NH4) compared to lakes (CO2). More detailed analyses are needed before we can draw definite conclusions.