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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.

The aquatic vegetation in nutrient-rich shallow lakes and ponds is structured by the interplay of multiple biotic and abiotic drivers. We tested the contribution of the macrophyte propagule bank and the delayed as well as direct impact of waterbirds on submerged aquatic vegetation in a peri-urban pond system. To clarify the functional hierarchy of predictor variables, effects of herbivorous waterfowl and propagule bank potential were ranked relative to environmental, phytoplankton, and zooplankton indicators. Two aspects of the aquatic vegetation – community composition and total pond-scale cover – were discriminated. Within vegetation communities, phytoplankton biovolume and waterfowl herbivory during summer were linked to low macrophyte abundance, whereas propagule density of angiosperms was positively associated with specific assemblages of submerged macrophytes. High algal biovolume and summer waterfowl grazing seemed to affect maximal pond-scale cover of submerged aquatic vegetation. The presence of waterfowl in cold and spring periods was unrelated to vegetation structure in the consecutive main growth season. In addition, availability of propagules in the sediment did not automatically prompt pond-wide vegetation cover (especially when overruled by high waterfowl densities), nor did it guarantee a position in the submerged macrophyte community. Nonetheless, propagule bank potential was related to the waterbody’s general ecological status, since turbid ponds exhibited impoverished propagule reserves compared to ponds residing in a clear, macrophyte-dominated state. Inadequate recruitment therefore represents a plausible bottleneck for macrophyte establishment. We conclude that phytoplankton-caused turbidity and high waterfowl biomass densities greatly restrict submerged macrophyte abundance. Propagule banks also participate in structuring submerged aquatic vegetation, though a stronger role is reserved for herbivorous waterfowl.

No abstract

Despite the presence of high nutrient concentrations, most ponds located around Brussels (Belgium) show a considerable variation in turbidity. The importance of submerged macrophytes in maintaining the clear-water state requires identification of the main factors determining macrophyte abundance and diversity in ponds and small lakes. In this study, the inter-relationships between submerged macrophyte cover, fish abundance and turbidity were investigated in 13 eutrophic peri-urban ponds. Along a turbidity gradient, vegetation switched from dominance by Stoneworts (Chara and Nitella spp.) in the clearest ponds, to dominance by Potamogeton pectinatus in ponds with a slightly lower water transparency. Despite the presence of both P. pectinatus and Stoneworts in each of the vegetated ponds, only one became dominant. Only a very low abundance (around 20%) of submerged vegetation was found in ponds of intermediate turbidity, while macrophytes were absent in turbid ponds. Multi-and univariate analysis showed a marked difference in chemical, physical and biological properties between ponds deliberately used for fish stocking and ponds that were not. Macrophyte cover was significantly negatively correlated with turbidity and plankti-benthivorous fish abundance. No such correlation was observed with piscivorous fish abundance, except for pike that were associated with a charophyte vegetation in the study ponds. The strong relationship found between fish abundance and turbidity, its negative effect on submerged vegetation cover, and the importance of submerged vegetation in controlling phytoplankton abundance, should be taken into account when selecting ponds for fish stocking. It also suggests that the study ponds have a good potential for ecological quality restoration by biomanipulation.