While there is ample experimental evidence for a role of species diversity in ecosystem performance, the functional significance of genetic diversity is less clear. In fact, many aquatic plant communities are highly productive although they consist of only a few or a single dominant species. In order to shed light on this apparent contradiction, scientists at the MPI of Limnology manipulated the genotypic diversity in the field in the seagrass species Zostera marina. The experiment took place in the Baltic Sea in 2003. During that year, a heat wave caused surface water temperatures to rise above 25°C, leading to widespread heat stress related mortality among shallow water animals and plants. Such conditions may serve as a model for predicted increases in climatic extremes. After the heat wave, genotypically diverse seagrass areas recovered faster, had more shoots and biomass and harboured more associated invertebrates at the end of the experimental period. Positive effects of genotypic diversity were due to true biodiversity effects (complementarity) and not due to the dominance of particularly resistant genotypes. These results provide experimental evidence that not only species diversity but also genetic diversity should be preserved. Genotypic diversity had a similar function as species diversity. This way, the level of genetic diversity can be incorporated into existing ecological theory on biodiversity at the level of species.