Role of dispersal for maintenance of diversity in experimental plankton communities

An increasing number of observational studies report regional patterns in communities of aquatic protists and zooplankton, pointing at the existence of dispersal limitation in the microscopic world. Moreover, recent studies also show that natural phytoplankton communities exhibit diversity functioning relationships comparable to those known from vascular plants and other higher organisms. The existence of such patterns is in stark contrast to the conventional assumption that communities of microscopic organisms are constantly saturated. Given the importance of diversity for ecosystem functioning, a better understanding on the controls of local and regional factors on plankton diversity is mandatory. Yet, in spite of finding spatial patterns which are in qualitative agreement with metacommunity theory, we are lacking a mechanistic understanding how local and regional factors interactively control diversity in plankton communities. The proposed study aims at studying the role of dispersal for maintenance of diversity in experimental communities. To this end, mesocosms will be connected to a species rich source pool in a gradient design. Diversity and community turnover of bacterio-, phyto-, and zooplankton will be monitored, employing both microscopcic as well as molecular analyses. Measurements of resource use efficiency will be performed for all functional groups. The project will closely collaborate with mathematical ecologists who will use the data for parameterizing a dynamic model on the maintenance of diversity through dispersal.

Unicellular organisms like bacteria and phytoplankton play a key role in the functioning of lake ecosystems. In contrast to larger organisms, it is not clear to what extent physical connectivity of their habitats may limit the diversity of the microbial communities inhabiting them. In this project, we tested to what extent diversity of aquatic microorganisms (bacteria and phytoplankton) may be limited by the connectivity of their habitats. We performed an experimental test where we incubated small aquatic ponds (mesocosms) with a diverse microbial assemblage communities from natural lakes. After thus initial inoculation, the control treatments did not receive further inoculation, while the connected treatments received small additions of water from the same lakes in weekly intervals. We followed the diversity of the communities and studied a number of state parameters (like biomass production and nutrient use).We found that a small dose of connectivity had a strong impact not only the diversity of the experimental communities, but also on their functioning. Interestingly, especially unicellular consumers (ciliates) responded very sensitive to the manipulation of connectivity, and had strong effects on the dynamics of the experimental communitiesThe results clearly refute the conventional theorem, that dispersal of microbes is unlimited. And provides strong support for the idea that communities of microbial organisms are subject to similar rules as macrobial communities (metacommunity dynamics). The connectivity of a habitat to habitats affects the diversity its biota. Among others, these results provide strong support for the perspective that the functioning of a local habitat (here lake or pond) depends on the regional species pool the habitat is embedded in.