Empirical and theoretical results indicate that high biodiversity increases the adaptability to change, the predictability of ecosystem services, and the resistance to biological invasions. Lakes are ideal systems for testing predictions and responses rel ated to biodiversity since they are units with well-defined boundaries, as inverted islands isolated by land. A strong east-west gradient in both phytoplankton and zooplankton species richness makes a longitudinal transect across Norway and Sweden a natur al laboratory for investigating effects of biodiversity on natural lake ecosystem functioning. We will approach the relationships between species pool saturation, ecosystem functioning, and vulnerability to bioinvasions by field sampling of natural biodiv ersity gradients, classical and molecular measures of phyto- and zooplankton biodiversity, and up-scaling by predictive modeling tools. Statistical modeling will also be used to disentangle effects of multiple stressors like eutrophication, climate change , and invading species on the ecosystem services of lakes.