Phytoplankton, zooplankton, submerged vegetation and main nutrients have been monitored in 48 eutrophic ponds from the Brussels Capital Region (Belgium) between 2005 and 2008. Nine ponds have been biomanipulated in order to improve their ecological quality and prevent the occurrence of noxious cyanobacterial blooms. The 4-year study of a large number of ponds allowed identification of the factors having the strongest influence on phytoplankton growth. Continuous monitoring of the biomanipulated ponds allowed the significance of changes caused by biomanipulation to be tested as well as the main reasons of biomanipulation successes and failures to be elucidated. The main factors controlling phytoplankton in the ponds studied appeared to be grazing by large cladocerans and inhibition of phytoplankton growth by submerged vegetation. Biomanipulation resulted in a significant decrease in phytoplankton biomass in general and biomass of bloom-forming cyanobacteria in particular that were associated with a significant increase in large Cladocera density and size. In six out of nine ponds biomanipulation resulted in the restoration of submerged vegetation. The maintenance of the restored clearwater state in the biomanipulated ponds was strongly dependent on fish recolonisation and nutrient level. In the absence of fish, the clearwater state could be maintained by submerged vegetation or large zooplankton grazing alone. In case of fish recolonisation, restoration of extensive submerged vegetation could buffer, to a considerable degree, the effect of fish except for ponds with high nutrient levels.

The influence of different macrophyte taxa or growth forms on biological and environmental variables is often analysed in one-lake studies. However, the unique combination of non-vegetational characteristics of a waterbody, i.e. its site identity, can be an influential factor in itself, shaping the measured parameters irrespective of the presence or absence of certain macrophyte species. In this situation, the relative strengths of all factors can be determined best in a study that explicitly accounts for differences in the identity of the waterbodies. Several functional macrophyte groups are known to provide a potent microinvertebrate refuge or permanent habitat. The objective of this study was to detect patterns in the zooplankton assemblages associated with different extensive habitats of macrophyte species and to relate these patterns to three major factors: the microhabitat, the pond identity and the seasonality in the warmer months of the year. Five ponds located in the Woluwe catchment of the Brussels-Capital Region (Belgium) were studied monthly for macrophyte and zooplankton characteristics from July until October 2005. The vegetation in the clear ponds was characterized by extensive monospecific stands (Ceratophyllum, Chara, Nitella, Potamogeton, Nuphar and filamentous algae). Zooplankton could be analysed in seven different vegetation types and in the open water zones and contained a total of 17 cladoceran and 27 rotifer genera. Principal components analysis (PCA) ordination of zooplankton communities showed a seasonal gradient and a tendency to group within-pond habitats, although they differed in macrophyte species and habitat structure. Despite the absence of clustering of similar microhabitats across ponds, percent volume infested (PVI), vegetation biomass density and Daphnia length (used as a proxy for fish predation pressure) contributed significantly positive to the Shannon zooplankton biodiversity indices. Moreover, densities of most zooplankton subgroups and of total zooplankton were significantly and positively related to PVI. It is assumed that in eutrophic ponds, extensive, often monospecific macrophyte vegetations provide an ecological environment suitable for both macrophyte-associated species and migrating pelagic zooplankton, thereby maintaining a high microinvertebrate biodiversity.

-Blooms of toxic cyanobacteria became a common feature of temperate lakes and ponds owing to human induced eutrophication. Occurrence of cyanobacterial blooms in an urban context may pose serious health concerns. This necessitates the development of tools for assessment of the risk of noxious bloom occurrence. A five year study of 42 Brussels ponds showed that cyanobacteria have threshold rather than linear relationships with environmental variables controlling them. Hence, linear relationships have limited predictive capacity for cyanobacterial blooms. A probabilistic approach to prediction of bloom occurrence using environmental thresholds as conditions in conditional probability calculation proved to be more useful. It permitted the risk of cyanobacterial bloom occurrence to be quantified and thus the conditions and thence the ponds the most prone to cyanobacterial bloom development to be singled out. This approach can be applied for the assessment of the risk of cyanobacterial bloom occurrence in urban ponds and thus can facilitate monitoring planning, remediation efforts and setting restoration priorities.

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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.