Stabilizing the clear-water state in eutrophic ponds after biomanipulation: submerged vegetation versus fish recolonization

Abstract

Biomanipulation through fish removal is a tool commonly used to restore a clear-water state in lakes. Biomanipulation of ponds is, however, less well documented, although their importance for biodiversity conservation and public amenities is undisputed. In ponds, a more complete fish removal can be carried out as compared to lakes and therefore a stronger response is expected. Fish recolonization can, however, potentially compromise the longer term success of biomanipulation. Therefore, we investigated the impact of fish recolonization on zooplankton, phytoplankton, and nutrients for several years after complete drawdown and fish removal in function of submerged vegetation cover in 12 peri-urban eutrophic ponds situated in Brussels (Belgium). Fish recolonization after biomanipulation had a considerable impact on zooplankton grazers, reducing their size and density substantially, independent of the extent of submerged vegetation cover. Only ponds with \30% cover of submerged vegetation shifted back to a turbid state after fish recolonization, coinciding with an increase in density of small cladocerans, rotifers, and cyclopoid copepods. In ponds with [30% submerged vegetation cover, macrophytes prevented an increase in phytoplankton growth despite the disappearance of large zooplankton grazers. Our results suggest that macrophytes, rather than by providing a refuge for zooplankton grazers, control phytoplankton through other associated mechanisms and confirm that the recovery of submerged macrophytes is essential for biomanipulation success. Although the longer term effect of biomanipulation is disputable, increased ecological quality could be maintained for several years, which is particularly interesting in an urban area where nutrient loading reduction is often not feasible.

Stability in Planktonic Rotifer Assemblages Found in Four Old Diversified Forest Ponds of Eastern Poland

“…A similarly low density of planktonic rotifers was recorded in other reservoirs of this type [14][15][16]. In the majority of other ponds, however, higher or much higher rotifer density was observed by the authors [12,[17][18]. Differences in rotifer density observed in ponds in different years of the study are statistically signifi cant, except for the differences in density noted in The biomass of planktonic rotifers in the studied ponds was in most cases insignifi cant and its value was similar at a comparable level of stability, signifi cance, and abundance (Fig.…”

Effects of Pond Draining on Biodiversity and Water Quality of Farm Ponds

“…Submerged macrophytes can inhibit phytoplankton growth through several mechanisms, such as reducing nutrient availability, allelopathy, providing refuge for zooplankton grazers, and shading (Jeppesen ). Although the role of submerged macrophytes as a refuge for zooplankton is unclear in ponds, nutrient limitation of phytoplankton by submerged macrophytes and other associated mechanisms are hypothesized to be possible mechanisms for the regime shifts following biomanipulation (Jeppesen ; De Backer et al ). Where biomanipulation is plausible, we suggest restoration of submerged macrophytes be considered concurrently in the management plan.…”

Sustainable Management of Eutrophic Lakes and Reservoirs

“…Macrophyte restoration experiments have been attempted in a number of places [23,80]. When populations of planktivorous fish are reduced substantially to allow zooplankton to control phytoplankton populations and when macrophyte coverage exceeds 30%, water clarity is good and eutrophic conditions are under control [6].…”