Untersuchung zur Diversität und Funktion benthischer Mikroalgen und Protozoen im Nahrungsnetz mariner und limnischer Sedimente

Abstract

This thesis aimed at achieving a deeper understanding of the dynamics and structure of microbenthic communities with a focus on protists. The most important discovery concerning protozoa in recent years is the fact that they play a much more important role in for example carbon and energy flow in nature than previously believed. In this thesis several studies were presented which focus on the seasonal dynamics and the spatial distribution of components of the small benthic community in marine and freshwater environments in temperate and arctic climates and which aim at establishing a generalised model of relationships and controlling factors in small benthic food webs. Samples of the upper 3 mm of the surface of soft sediment were collected in coastal freshwater and marine sites in Germany, USA and North East Greenland. The abundance and biomass of bacteria, cyanobacteria, diatoms and nanoflagellates (phototrophic and heterotrophic) were enumerated by means of epifluorescence microscopy. Ciliate species and meiofauna were enumerated with the help of light microscopy after QPS (quantitative protagol staining). Environmental data, such as temperature, salinity, sediment water content, grain size distribution, total and organic carbon- and nitrogen content as well as chlorophyll-a values were determined in order to study their influence on the community structures and total biomass. The investigation of a habitat in a mudflat in Dorum (Germany) revealed that the benthic microbial food web varied with the season with a primacy of bottom-up control during winter and spring, an increasing influence of top-down forces mainly by meiofauna grazing towards summer, decreasing again towards autumn, accompanied by a recovery of microphytobenthos.Investigations of soft sediment communities in different climate and geographic positions of freshwater and marine sites in Germany, North America (USA; Pennsylvania, New Jersey) and in North East Greenland were carried out. The results revealed considerable differences in the biomass of the small benthic components as well as in ciliate species composition and richness. The study of the ciliate species composition also exhibited a high variability of feeding types between the investigated sites, but the herbivorous feeding type seemed to be dominant in all sediments. Differences in total microbenthic biomass, abundance and ciliate species richness could not be attributed to the differences measured in carbon content, salinity or climatic parameters. The amount of chlorophyll-a was the only factor influencing the small benthic biomass. The sources of chlorophyll-a remained unclear. The hypothesis of Manini et al. (2003) stating that the composition of available carbon has a strong influence on the structure and biomass of microbial communities in shallow sediments seemed to hold true for the investigated sites. In a series of laboratory experiments the influence of the absence of all macrograzers as well as the presence of a single common abundant small macrograzer such as Hydrobia ulvae, Corophium sp. as well as Chironomid larvae and polychaets on the components of the microbenthic communities in sediments from all study sites were investigated. The structuring influence of Arenicola marina on components of the small food web was investigated in a long term field experiment on the island of Sylt. The results of the laboratory experiments and those of the field study revealed that the small macrofauna tended to have positive stimulating effects on the microbial community by bioengineering, rather than negative ones by grazing. The exclusion of Arenicola marina in a long term experiment revealed the fact that the role of this grazer was adopted by other grazers.The results of this thesis and literature data led to the conclusion that in small communities of soft sediments the trophic levels are blurred and most species rather feed opportunistically on what is most available. The dominance of the omnivourous feeding type, detritus feeding, stimulating bioengineering effects and feed back mechanisms compose a highly networked food web. The fundamental attributes of microbes such as large absolute population sizes, short generation times and high dispersal capabilities, form a system with rapid changes in species composition.The outcome of this thesis verified the model of Fretwell (1977) for benthic microbial food webs in coastal regions, predicting that top-down forces form the trophic structure, but the bottom-up attributes of the ecosystems, such as nutrients availability, temperature and light, determine the fundament of the community as total biomass, abundance and production