Record?setting algal bloom in polymictic Lake Balaton (Hungary): A synergistic impact of climate change and (mis)management

After 25 years of apparently successful eutrophication management, a record-breaking mixed bloom Ceratium furcoides and Aphanizomenon flos-aquae developed unexpectedly in 2019 the large, polymictic Lake Balaton. The peak concentration chlorophyll exceeded 300 mg/m3, 1.5 times higher than pre-management maxima. external load was insufficient to provide phosphorus (P) required support this bloom, its taxonomic composition radically different from blooms 1980s. We hypothesised: (1) that unusually long periods intermittent stratification led anoxic P release sediments, providing P; (2) surprising indicated potential regime shift. To test hypothesis 1, we analysed decade-long high-frequency data on drivers lake metabolism recorded at shallow near-shore observatory. explore likely dissolved oxygen (DO) conditions deep water 2019, performed sensitivity analysis one-dimensional DO model driven by General Ocean Turbulence Model, latter being calibrated against hydrometeorological measured open water. Hypothesis 2 examined sequential t-test applied multidecadal (1976–2020) phytoplankton biomass composition, nutrient loads summer mean air temperature. found depletion (<1 g/m3) immediate precursor large bloom. synergistic impact climatic shift multiple coincident management actions provoked 2013 forced ecosystem into state internal eutrophication. In present state, similarly could develop any depending sequence weather events. Within present, warmer climate identified here, effectiveness control may be limited changes thermal structure. Therefore, it is vital supplement with strategies, such as level regulation, manage prevent future blooms. Large, continuous lakes are susceptible small level, because likelihood (and thus likeliness event) increases steeply non-linearly increasing depth. With continues warm, associated expected processes, possible will see shifts (such Balaton) across more globally. Managing these require case-specific modifications plans.