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 of Ceratium furcoides and Aphanizomenon flos-aquae developed unexpectedly in 2019 in the large, polymictic Lake Balaton. The peak concentration of chlorophyll exceeded 300 mg/m(3), 1.5 times higher than the pre-management maxima. The external load was insufficient to provide the phosphorus (P) required to support this bloom, and its taxonomic composition was radically different from blooms in the 1980s. We hypothesised: (1) that unusually long periods of intermittent stratification led to anoxic P release from the sediments, providing the required P; and (2) that the surprising bloom composition indicated a potential regime shift. To test hypothesis 1, we analysed decade-long high-frequency data on drivers of lake metabolism recorded at a shallow near-shore observatory. To explore likely dissolved oxygen (DO) conditions in deep water in 2019, we performed a sensitivity analysis of a one-dimensional DO model driven by the General Ocean Turbulence Model, the latter being calibrated against high-frequency hydrometeorological data measured in the open water. Hypothesis 2 was examined by a sequential t-test analysis applied to multidecadal (1976-2020) data on phytoplankton biomass and composition, nutrient loads and summer mean air temperature. We found that DO depletion (<1 g/m(3)) was the immediate precursor of the large 2019 bloom. The synergistic impact of a climatic regime shift and multiple coincident management actions provoked a shift in phytoplankton composition in 2013 and forced the ecosystem into the state of internal eutrophication. In the present ecosystem state, similarly large blooms could develop in any summer depending on the sequence of weather events. Within the present, warmer climate regime identified here, the effectiveness of external nutrient control in eutrophication management may be limited by changes in the thermal structure. Therefore, it is vital to supplement external nutrient control with internal nutrient control strategies, such as water level regulation, to manage eutrophication and prevent future blooms. Large, continuous polymictic lakes are susceptible to small changes in water level, because the likelihood of stratification (and thus the likeliness of a eutrophication event) increases steeply and non-linearly with increasing depth. With a climate that continues to warm, and the associated changes expected in internal lake processes, it is possible that we will see regime shifts (such as that identified in Lake Balaton) across more lakes globally. Managing these conditions will require case-specific modifications of lake management plans.