Response of submerged macrophytes in Danish lakes to nutrient loading reductions and biomanipulation

During the last two decades the nutrient loading to Danish lakes has been reduced with the aim to improve water quality. However, because of internal P-loading and biological resistance, the expected improvement has been delayed. Therefore, to reduce the duration of the recovery period and to accelerate recolonisation of submerged macrophytes, several lakes have been biomanipulated with the purpose of improving the top-down control by zooplankton. To elucidate the effects of these measures, we undertook an analysis of data on submerged macrophytes monitored annually in 17 lakes for 8 years. The results obtained show that the macrophyte coverage in non-biomanipulated lakes remained relatively stable following the external nutrient reduction. However, a small increase in macrophyte coverage occurred in a few lakes. In two of the four biomanipulated lakes, in contrast, macrophyte coverage increased from 0 up to 80% within 2–4 years following manipulation. In the other two lakes macrophyte colonisation failed. However, in lakes with a successful recolonisation, large inter-annual variations in macrophyte coverage, varying between 2 and 80% among growth seasons, occurred. We conclude that the potential of macrophyte recolonisation after nutrient loading reduction on the short term is higher in biomanipulated lakes than in lakes subjected to loading reduction only, although biomanipulation does not provide a guarantee for macrophyte recolonisation or a stable macrophyte community when colonisation occurs.