ALBERT

Description: In periphyton communities, autotrophic algae and prokaryotes live in close spatial proximity to heterotrophic components such as bacteria and micro- and meiofauna. In factorial field experiments, we manipulated grazer access and nutrient supply to periphyton communities and measured the effects on algal, ciliate, meiofaunal, and bacterial biomass. We tested whether grazing macrozoobenthos affects all periphytic components (generalist consumption), whether nutrient effects propagate through the community, and whether interactions between the different periphyton groups allow for indirect feedback mechanisms. The experiments were conducted during three different seasons in a meso-eutrophic lake in Sweden (Lake Erken) and at an adjacent coastal marine site (Väddö) of similar productivity, but with contrasting grazer fauna. We found strong direct effects of nutrients and grazing on algae at both sites. Algal biomass increased in fertilized treatments and was significantly reduced when grazers were present. The algae clearly dominated the system quantitatively and were positively correlated to the biomass of ciliates and meiofauna. The effects of grazing and nutrients were more complex for heterotrophs than for algae. Generally, the presence of grazers tended to increase the biomass of bacteria, ciliates, and meiofauna. Thus, macrograzers were not generalist consumers of the entire community, but mainly reduced algae. Furthermore, the results suggested strong indirect effects of grazing, presumably through changes in nutrient supply and algal size structure. Nutrient enrichment had weak and inconsistent effects on bacterial, ciliate, and meiofaunal biomass. There was thus no complete propagation of bottom-up effects through the community, and strong internal feedback mechanisms within the periphyton mediated the effects of macroconsumers and nutrient enrichment.

Description: Recent experiments, mainly in terrestrial environments, have provided evidence of the functional importance of biodiversity to ecosystem processes and properties. Compared to terrestrial systems, aquatic ecosystems are characterised by greater propagule and material exchange, often steeper physical and chemical gradients, more rapid biological processes and, in marine systems, higher metazoan phylogenetic diversity. These characteristics limit the potential to transfer conclusions derived from terrestrial experiments to aquatic ecosystems whilst at the same time provide opportunities for testing the general validity of hypotheses about effects of biodiversity on ecosystem functioning. Here, we focus on a number of unique features of aquatic experimental systems, propose an expansion to the scope of diversity facets to be considered when assessing the functional consequences of changes in biodiversity and outline a hierarchical classification scheme of ecosystem functions and their corresponding response variables. We then briefly highlight some recent controversial and newly emerging issues relating to biodiversity-ecosystem functioning relationships. Based on lessons learnt from previous experimental and theoretical work, we finally present four novel experimental designs to address largely unresolved questions about biodiversity-ecosystem functioning relationships. These include (1) investigating the effects of non-random species loss through the manipulation of the order and magnitude of such loss using dilution experiments; (2) combining factorial manipulation of diversity in interconnected habitat patches to test the additivity of ecosystem functioning between habitats; (3) disentangling the impact of local processes from the effect of ecosystem openness via factorial manipulation of the rate of recruitment and biodiversity within patches and within an available propagule pool; and (4) addressing how non-random species extinction following sequential exposure to different stressors may affect ecosystem functioning. Implementing these kinds of experimental designs in a variety of systems will, we believe, shift the focus of investigations from a species richness-centred approach to a broader consideration of the multifarious aspects of biodiversity that may well be critical to understanding effects of biodiversity changes on overall ecosystem functioning and to identifying some of the potential underlying mechanisms involved.

Description: For unicellular organisms, a lack of effects of local species richness on ecosystem function has been proposed due to their locally high species richness and their ubiquitous distribution. High dispersal ability and high individual numbers may enable unicellular taxa to occur everywhere. Using our own and published data sets on uni- and multicellular organisms, we conducted thorough statistical analyses to test whether (1) unicellular taxa show higher relative local species richness compared to multicellular taxa, (2) unicellular taxa show lower slopes of the species:area relationships and species:individuals relationships, and (3) the species composition of unicellular taxa is less influenced by geographic distance compared to multicellular taxa. We found higher local species richness compared to the global species pool for unicellular organisms than for metazoan taxa. The difference was significant if global species richness was conservatively estimated but not if extrapolated, and therefore higher richness estimates were used. Both microalgae and protozoans showed lower slopes between species richness and sample size (area or individuals) compared to macrozoobenthos, also indicating higher local species richness for unicellular taxa. The similarity of species composition of both benthic diatoms and ciliates decreased with increasing geographic distance. This indicated restricted dispersal ability of protists and the absence of ubiquity. However, a steeper slope between similarity and distance was found for polychaetes and corals, suggesting a stronger effect of distance on the dispersal of metazoans compared to unicellular taxa. In conclusion, we found partly different species richness patterns among uni- and multicellular eukaryotes, but no strict ubiquity of unicellular taxa. Therefore, the effect of local unicellular species richness on ecosystem function has to be reanalyzed. Macroecological patterns suggested for multicellular organisms may differ in unicellular communities.

Description: The investigation of the benthic food web took place in the Nationalpark Niedersächsisches Wattenmeer (Dorumer) and in Königshafen (Sylt).Experiments were made to test the influence of macrofauna on the microbial benthic community. Natural sediment was placed into cellwells and incubated under controlled conditions for 30 minutes to 24 hours with and without macrograzers. The snail Hydrobia ulvae, the crustacean Corophium volvutator and a mixture of different polychaete species were used as macrograzer. The number of bacteria, cyanobacteria, flagellates (heterotrophic and autotrophic) and diatoms were enumerated after the incubation. Additionally, FLBs (fluorescently labelled bacteria) were added to the experimental samples described above to estimate bacterivory.The results showed no detectable influence of the macrograzers on the abundance of bacteria, cyanobacteria, flagellates and diatoms. The FLB experiment revealed that about 50 to 60% of the bacteria standing stock were consumed each day. There was no difference between the FLB loses in the treatments with and without macrograzers, therefore the consumption of bacteria was mainly due micro- and meiobenthic organisms like flagellates, ciliates, nematodes and copepods.

Description: This study concentrates on the seasonal composition and trophic dynamics of the benthic microbial community in freshwater and marine systems and on the effect of macrozoobenthos grazing on these communities. Laboratory experiments with and without added macrograzers were carried out with sediments from the North Sea, temperate lakes of Northern Germany and arctic lakes of North East Greenland. Incubation time varied between 30min and 5d. In further experiments, fluorescently labeled bacteria (FLBs) were added as food tracers in order to estimate bacterivory. The abundancec of bacteria, cyanobacteria, protists and meiofauna were determined at the beginning and in the end of all experiments. First results indicate no effect of the macrograzers Hydrobia ulvae (Gastropoda), Corophium volutator (Crustacea), Potamopyrgus antipodarum (Gastropoda), mixed polychaets (Annelida) and chironomid larvae (Insecta) on the abundance of bacteria and flagellates. However, FLB experiments revealed a removal of 40 to 60% of the standing bacterial stock per day, with no differences between the treatments with and without macrograzers. These results point a minor effect of macrograzers as bacterivores. Implications on the structure of the food web will be discussed.

Description: Our knowledge of the functioning of the benthic microbial food web still lags considerably behind that of its pelagic counter part. In addition, there is increasing evidence that the taxonomic composition of the various functional groups within a community may strongly influence the trophic relationships and the carbon flow. We conducted an investigation of the microbial community (bacteria, flagellates, ciliates, meiofauna) of marine, freshwater, and intertidal sediments. Samples were analysed in terms of spatial (between sites) and temporal (samples collected during different seasons in the same site) differences in abundances and community composition. Our study revealed large differences in ciliate species composition. The occurrence of dominant ciliates and their allocation to feeding types indicated that herbivory was the most common and important feeding strategy, while the importance of bacterivory varied significantly among the sediments investigated. Ciliate diversity and species richness, and biomass of ciliates and meiofauna were largely controlled by chlorophyll a concentrations. In contrast, abiotic factors, such as temperature and salinity appeared to have little impact. The ecological implications of our findings will be discussed.