Mass development of monospecific submerged macrophyte vegetation after the restoration of shallow lakes: Roles of light, sediment nutrient levels, and propagule density

After restoration, eutrophicated shallow freshwaters may show mass development of only one or two submerged macrophyte species, lowering biodiversity and hampering recreation. It is unclear which environmental factors govern this high percentage of the volume inhabited (PVI2) by submerged macrophytes, and whether the development of a more diverse, low canopy vegetation is likely to occur if dominant species decline in abundance. We hypothesized that (1) adequate light and high sediment nutrient availability leads to massive development of submerged macrophytes, and (2) that macrophyte species richness is low at high PVI, but that this is not caused by a lack of viable propagules of non-dominant species (especially charophytes). To test these hypotheses, fifteen shallow waters in the Netherlands were studied with respect to submerged vegetation (including propagules), water, and sediment characteristics. The probability of high submerged macrophyte PVI is highest in shallow waters where light availability in the water layer and phosphorus availability in the sediment are abundant. These conditions typically occur upon restoration of eutrophic waterbodies by reducing water nutrient loading or applying biomanipulation. Other factors, as top-down control, can additionally influence realised PVI. Viable propagules of species other than the dominant ones, including charophytes, were found in most of the sediments, indicating that once the dominant species declines, there is local potential for a diverse submerged vegetation to develop. Results can be used to predict when mass development occurs and to tackle the factors causing mass development.