Jochen Vandekerkhove

MULTI-GROUP BIODIVERSITY IN SHALLOW LAKES ALONG GRADIENTS OF PHOSPHORUS AND WATER PLANT COVER

This study aimed at unraveling the structure underlying the taxon-richness matrix of shallow lakes. We assessed taxon richness of a large variety of food-web com- ponents at different trophic levels (bacteria, ciliates, phytoplankton, zooplankton, fish, macro-invertebrates, and water plants) in 98 shallow lakes from three European geographic regions: Denmark (DK), Belgium/The Netherlands (BNL), and southern Spain (SP). Lakes were selected along four mutually independent gradients of total phosphorus (TP), vege- tation cover (SUBMCOV), lake area (AREA), and connectedness (CONN). Principal-com- ponents analysis (PCA) indicated that taxon diversity at the ecosystem level is a multidi- mensional phenomenon. Different PCA axes showed associations with richness in different subsets of organism groups, and differences between eigenvalues were low. Redundancy analysis showed a unique significant contribution to total richness variation of SUBMCOV in all three regions, of TP in DK and SP, and of AREA in DK and BNL. In DK, several organism groups tended to show curvilinear responses to TP, but only one was significantly hump shaped. We postulate that the unimodal richness responses to TP that are frequently reported in the literature for many organism groups may be partly mediated by the unimodal response of macrophyte vegetation to lake productivity.

Dormant propagule banks integrate spatio-temporal heterogeneity in cladoceran communities

The dynamics of populations of short-lived organisms are very patchy, both in space and time. The production of dormant propagules, however, results in an effective increase in generation time. We hypothesize that prolonged dormancy, together with variable regeneration niches, result in integration of temporal variability in community structure. In addition, in aquatic habitats, mechanisms such as sediment focussing can contribute to the integration of spatial variability. We tested the hypothesis that dormant propagule banks integrate spatial and temporal variation in active zooplankton communities. This was done by comparing cladoceran species richness and the community structure of hatchling assemblages retrieved from propagule bank samples collected on a single occasion with assemblages encountered in active community samples covering spatial variation (littoral and pelagic zone), diel (day and night), intra-year (May–October) and inter-year variation (1996–2000). The egg bank community structure differed significantly from the active community structure, but the dissimilarity decreased as spatial and temporal variation was better covered by the active community samples. Furthermore, the identification of all fully grown hatchlings (n=214) yielded an equally high number of species (n=22) to that occurring in all active community samples together (a total of 1,730 individuals were analysed). We conclude that the analysis of dormant propagules may form a cost-efficient alternative tool to the analysis of active community samples for an integrated assessment of cladoceran communities.

Zooplankton, phytoplankton and the microbial food web in two turbid and two clearwater shallow lakes in Belgium

Components of the pelagic food web in four eutrophic shallow lakes in two wetland reserves in Belgium (‘Blankaart’ and ‘De Maten’) were monitored during the course of 1998–1999. In each wetland reserve, a clearwater and a turbid lake were sampled. The two lakes in each wetland reserve had similar nutrient loadings and occurred in close proximity of each other. In accordance with the alternative stable states theory, food web structure differed strongly between the clearwater and turbid lakes. Phytoplankton biomass was higher in the turbid than the clearwater lakes. Whereas chlorophytes dominated the phytoplankton in the turbid lakes, cryptophytes were the most important phytoplankton group in the clearwater lakes. The biomass of microheterotrophs (bacteria, heterotrophic nanoflagellates and ciliates) was higher in the turbid than the clearwater lakes. Biomass and community composition of micro- and macrozooplankton was not clearly related to water clarity. The ratio of macrozooplankton to phytoplankton biomass – an indicator of zooplankton grazing pressure on phytoplankton – was higher in the clearwater when compared to the turbid lakes. The factors potentially regulating water clarity, phytoplankton, microheterotrophs and macrozooplankton are discussed. Implications for the management of these lakes are discussed.

Hatching rate and hatching success with and without isolation of zooplankton resting stages

Zooplankton resting egg banks accumulate resting stages of various zooplankton species that are active in different habitats and different periods of the year. As such, hatching of resting eggs from lake sediments may potentially be very useful in zooplankton diversity studies. In this study, we tested whether the efficiency of the cost-effective technique is increased by isolating the resting eggs from the sediment prior to incubation. Isolation of the eggs was advantageous for the overall hatching success (+26% after 36 days of incubation compared to incubation of sediment). Furthermore, isolation of resting eggs makes egg bank diversity analyses less time consuming in two ways. (1) It reduced the time needed for the eggs to hatch with on average 35%. In the isolation treatment all responsive resting eggs hatched within the first 4 weeks of incubation, while in the non-isolation treatment neither the cumulative number of macrozooplankton hatchlings nor the cumulative number of hatched cladoceran species levelled off after 36 days of incubation. (2) In contrast to the non-isolation treatment, where large differences occurred between taxa in incubation time, isolation reduced such inter-specific differences, so that even very short incubation periods kept bias within acceptable limits.

Development of cladoceran egg banks in new and isolated pools

The development of cladoceran egg banks was monitored in new and iso- lated freshwater pools during their first year of existence. Sediment samples of 24 newly created pools were collected six months and one year after creation, and screened for dormant eggs of cladocerans. The build-up of the egg bank was compared with the patterns in the corresponding active community as presented in Louette & De Meester (2005). In all but one of the pools, a cladoceran egg bank started to build up within the first year. Up to 10 4 dormant eggs were found per square meter (average: 1,710 eggs/m 2 ), mainly originating from Daphnia species, Chydorus sphaericus and Simocephalus vetulus. The production of extensive dormant egg banks in the first year after colonization may have substantial consequences, both for the ecology and the evolution of the recently founded populations. Yet, our results suggest that the direct numerical impact of spring-time hatching on the dynamics of active populations is likely to be limited in the young pools.

Ostracoda (Crustacea) as indicators for surface water quality: a case study from the Ledra River basin (NE Italy)

Wastewater discharges associated with urbanisations, farming activities and industry may dramatically reduce the ecological health of river ecosystems. During the reconstruction of the Friuli Venezia Giulia region following the 1976 earthquake, a lot of resources were used to build large numbers of wastewater treatment plants to minimize the impact of human activities on lotic ecosystems. Their efficiency is usually assessed through monitoring of the physical and chemical environment near the discharge point. However, discontinuous monitoring of the abiotic environment may fail to detect periodic malfunctioning and do not recognize indirect effects on the ecosystem. We assessed the potential of an alternative approach to assess the impact of wastewater discharges, based on the monitoring of ostracod density, richness and community composition. We repeatedly measured physical, chemical and microbial parameters and collected ostracod samples at stations up- and downstream from wastewater discharge points scattered over a 21-km stretch of the Ledra River (NE Italy). The results indicate that monitoring ostracods is a potentially valuable approach, for two reasons. Communities appeared to be well differentiated even in the small spatial area of this study, indicating that they can provide sufficient resolution to pick up even minor impacts. Secondly, despite the seasonal succession in species composition, spatial differentiation was consistent over time, suggesting that ostracods provide a time-integrated picture of the water quality. The traditional approach failed to detect any consistent impact of wastewater discharges, apart from an ambiguous increase in nutrient levels. The density and/or richness of the ostracod communities was altered by some wastewater discharges, but not by others. We identified a general trend for wastewater discharges to systematically replace regionally rare ostracod species with common species. In particular, the species Ilyocypris inermis was very sensitive to discharges, and may be used as an indicator species for good ecosystem health.

Biodiversity in European Shallow Lakes: a Multilevel-Multifactorial Field Study

This chapter is based on the premise that the precipitous decline in freshwa- ter wetlands and species can only be arrested through conservation and sus- tainable management at a large scale, based on water (usually river) basins. A number of approaches to large-scale freshwater wetlands conservation are presented and assessed to draw conclusions on future conservation priori- ties.