Effects of climate change and episodic heat events on cyanobacteria in a eutrophic polymictic lake

• Published in: The Science of the total environment 2019-11, Vol.693, p.133414, Article 133414
• Main Authors: Bartosiewicz, Maciej, Przytulska, Anna, Deshpande, Bethany N., Antoniades, Dermot, Cortes, Alicia, MacIntyre, Sally, Lehmann, Moritz F., Laurion, Isabelle
• Format: Article
• Language: English
• Subjects: Buoyant cyanobacteria; Carbon dioxide; Carbon Dioxide - analysis; Climate Change; Climate warming; Cyanobacteria - physiology; Eutrophication; Geologic Sediments - chemistry; Harmful blooms; Heat waves; Hot Temperature - adverse effects; Lakes - analysis; Lakes - chemistry; Lakes - microbiology; Phosphorus - analysis; Phytoplankton - physiology; Quebec; Seasons; Stratification
• ISSN: 0048-9697; 1879-1026
• DOI: 10.1016/j.scitotenv.2019.07.220

Mixing regime and CO2 availability may control cyanobacterial blooms in polymictic lakes, but the underlying mechanisms still remain unclear. We integrated detailed results from a natural experiment comprising an average-wet year (2011) and one with heat waves (2012), a long-term meteorological dataset (1960–2010), historical phosphorus concentrations and sedimentary pigment records, to determine the mechanistic controls of cyanobacterial blooms in a eutrophic polymictic lake. Intense warming in 2012 was associated with: 1) increased stability of the water column with buoyancy frequencies exceeding 40 cph at the surface, 2) high phytoplankton biomass in spring (up to 125 mg WW L−1), 3) reduced downward transport of heat and 4) depleted epilimnetic CO2 concentrations. CO2 depletion was maintained by intense uptake by phytoplankton (influx up to 30 mmol m−2 d−1) in combination with reduced, internal and external, carbon inputs during dry, stratified periods. These synergistic effects triggered bloom of buoyant cyanobacteria (up to 300 mg WW L−1) in the hot year. Complementary evidence from polynomial regression modelling using historical data and pigment record revealed that warming explains 78% of the observed trends in cyanobacterial biomass, whereas historical phosphorus concentration only 10% thereof. Together the results from the natural experiment and the long-term record indicate that effects of hotter and drier climate are likely to increase water column stratification and decrease CO2 availability in eutrophic polymictic lakes. This combination will catalyze blooms of buoyant cyanobacteria. [Display omitted] •Enhanced stratification along with reduced wind speed during summers with heat waves, as compared to a more average year, lead to intensified warming of surface waters in a shallow polymictic eutrophic lake.•Surface warming fosters development of high phytoplankton biomass, which, under reduced carbon inputs, is conducive to persistent CO2 depletion in the epilimnion.•Buoyant cyanobacteria produce massive blooms when warm surface waters of strongly stratified shallow eutrophic lake become CO2 depleted.•In polymictic lakes, synergistic mechanisms may catalyze cyanobacterial blooms once a certain threshold in warming is reached, carbon inputs are reduced and the water column is sufficiently stable to allow epilimnetic CO2 depletion.•This effect was responsible for proliferation of cyanobacteria in the lake over the last decades and will likely re