Impact of climate change on the vulnerability of lakes for surface blooms of harmful cyanobacteria (CLIMCYANO)
2005 - 12 / 2012
[Objectives]: The summer of 2003, one of the hottest summers ever recorded in Europe, was accompanied by numerous surface blooms of cyanobacteria ( blue-green algae ) in eutrophic lakes. Cyanobacteria are potentially harmful phytoplankton species that can produce a variety of toxins. For instance, in Lake Volkerak-Zoommeer and the Oostvaardersplassen, The Netherlands, dense surface blooms of toxic cyanobacteria were accompanied by mass mortalities of birds (especially gulls and ducks). Several lakes were closed for recreation. Was this an extreme event? Or will global warming lead to an increased incidence of harmful cyanobacteria in the years to come? The cyanobacteria that form these surface blooms have gas vesicles, providing them with buoyancy. Buoyant cyanobacteria compete for light and nutrients against non-buoyant (often slowly sinking) phytoplankton species, such as green algae and diatoms. These phytoplankton groups respond differently to changes in hydrodynamic processes like lake stratification and turbulent mixing. These hydrodynamic conditions, in turn, are to a large extent driven by meteorological forces like wind stress, air temperature, and light conditions. During calm weather with only weak vertical mixing, buoyant cyanobacteria float upwards and form surface blooms, thus out-competing the other phytoplankton. During rough weather with strong wind-mixing, buoyant cyanobacteria are fully mixed throughout the water column and often lose the competition against green algae and diatoms. Furthermore, phytoplankton is affected by grazers (e.g., zooplankton, zebra mussels), and the impact of grazers may vary with climatic conditions as well. It may thus be hypothesized that gradual changes in meteorological conditions may lead to sudden shifts in phytoplankton species composition, especially in the competitive balance between buoyant cyanobacteria and other phytoplankton species. Our main research questions are: Under which climate scenarios and in which geographical regions can we expect increased formation (or suppression) of surface blooms of harmful cyanobacteria? In this project, we will build on recent innovations to study the link between climate, hydrodynamics, and the dominance of buoyant cyanobacteria.The PhD will be mainly stationed at NIOO and will: 1) run laboratory experiments in advanced Limnotrons to study the formation of surface blooms of harmful cyanobacteria, under controlled hydrodynamic conditions. 2) organize together with the postdoc three field campaigns in which a microstructure profiler and moored temperature loggers will be used to analyze turbulent mixing and flows in eutrophic lakes dominated by surface blooms of harmful cyanobacteria. This multidisciplinary approach will enable improved prediction of the potential threats imposed by harmful cyanobacteria under future climate scenarios.