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Periphyton as a Habitat for Meiofauna : a case of a neglected community

Meiofauna has received much attention of aquatic ecologists during the past decades. Many new and important findings have increased the understanding of meiofaunal biology and ecology and the function of meiofauna in aquatic ecosystem processes. Unfortunately, most researchers have concentrated their attention on meiofauna living in soft sediments of marine and freshwater habitats and have been reluctant to describe and analyse meiofauna attached to hard substrate surfaces. In recent years, periphyton communities in the littoral zone of lakes have received much interest and have been shown to be of great importance by contributing considerably to whole ecosystem primary production and as a key component in benthic littoral food webs. Meiofauna has often been described to be part of periphyton communities, but paradoxically our knowledge of periphytic meiofauna is extremely limited. In the present thesis, I investigated several aspects of periphytic meiofauna, focusing on the often dominating group of nematodes within the meiofaunal community. 
The basis for quantitative and qualitative periphyton community analyses is an efficient and reliable method to make valid comparisons between natural periphyton communities possible. Here, I present construction details and functionality of a new in situ sampling device. The new brush sampler is based on a previously developed sampling device, but has all the required modifications to allow a precise and efficient sampling of natural hard substrates. This new device was tested under ambient conditions in the field and compared to the old sampler and control samples. As the field test revealed reliable results, this brush sampler can be used as standardised method for periphyton community analyses in the field and for monitoring programmes or water-quality assessments. With this new sampling device, an extensive survey in a large number of different lake ecosystems was now possible. I investigated epilithic meiofauna with particular emphasis on nematode species composition in 17 Swedish lakes differing in size, depth, and periphyton biomass. The results of this study revealed that highly abundant meiofaunal assemblages and diverse nematode communities (overall 58 species, ranging from 8 to 34 species per lake) inhabit hard substrates; the abundance of these organisms depend on the amount of organic matter attached to hard-substrate surfaces. These findings contribute fundamental information to meiofaunal ecology and point to the necessity to include meiofauna in studies on periphyton. 
The most important factors influencing periphyton communities in shallow littoral zones of lakes are physical disturbance events (e.g., wave action, water-level fluctuations), which often lead new space being offered for colonisation. In a subsequent field experiment, I investigated the colonisation ability and community development patterns of epilithic meiofauna and nematodes in a lake characterised by large water-level fluctuations throughout the year. I followed the community development of meiofaunal taxonomic groups and nematode species over a 57-day period on artificial hard substrates that either could be colonised by direct infaunal crawling or were prevented from being directly colonised. I included analyses of natural hard substrates (natural species pool) and sediment trap samples. The data presented point to a distinct ability of meiofauna and nematodes to colonise rapidly on littoral hard substrates and also to persist on the stony substrate surfaces. The colonisation of hard substrates by meiofaunal organisms was shown to be mediated by water-column transport and to depend on a source pool of species on the nearby natural hard substrates and on the simultaneously developing periphyton biomass. The results of the nematode species identification allowed a detailed analysis of possible adaptations to a periphytic lifestyle. Some nematode species have morphological adaptations and were found to be dominant on the treatments most exposed to wave action, which was in accordance with previous studies. However, meiofaunal colonisation is a process that is largely influenced by external physical factors and depends on the species-specific colonisation and persistence ability and on the size and physiognomy of the periphyton habitat. 
Periphyton has been shown to be part of various biotic and abiotic interactions. One of the key concepts of periphyton ecology is the top-down regulation of periphyton biomass by benthic herbivores. Grazers are known to reduce periphyton biomass and to change periphyton composition. Therefore, meiofaunal organisms might also be influenced directly by consumption or indirectly by altered habitat structure or resource availability. I investigated this by manipulating grazer access to periphyton in a nested spatial design and found that grazers strongly depressed periphyton biomass, but influenced meiofaunal abundance and community composition only locally. However, grazing is known to alter both the physiognomic complexity and the biomass of the periphyton, and therefore, the presence of macrograzers can also affect periphytic meiofauna and will lead to highly complex and spatially heterogeneous ecological interactions.