Lake enrichment by submersed macrophytes: A Norwegian whole-lake experience with Elodea canadensis

The submerged macrophyte Elodea canadensis Michx. heaviliy infested the mesotrophic lake Steinsfjord (SE Norway, area 13.9 km 2) in the late 1970's. This study reports an assessment of the impact of Elodea, primarily focussing on the lake's response at the ecosystem level. The species colonized 79% of the lake floor within the 0–6-m depth range. Averaged over the infested area, Elodea attained a peak biomass up to 310 g DW m −2. The combined production of phytoplankton and macrophytes increased significantly, but we could not demonstrate major nutrient enrichment of the lake waters, and in general pelagial primary production declined. However, some short-term dieback incidents, mainly in the summer, clearly enriched the lake waters. The impacted ecosystem rapidly stabilized after the initial perturbation. Sufficient depletion of sediment nutrients, coupled with slow in situ regeneration of nutrients allows for effective negative feedback. Ecosystem complexity (species richness) was largely unaffected, although one aspect of complexity, i.e. evenness, in fact decreased. The observed production enhancement is due to the invader's superior utilization of space and sediment nutrient resources.