Increases in atmospheric temperature and nutrients from land are thought to be promoting the expansion of harmful cyanobacteria in lakes worldwide, yet to date there has been no quantitative synthesis of long-term trends. To test whether cyanobacteria have increased in abundance over the past ~ 200 years and evaluate the relative influence of potential causal mechanisms, we synthesised 108 highly resolved sedimentary time series and 18 decadal-scale monitoring records from north temperate-subarctic lakes. We demonstrate that: (1) cyanobacteria have increased significantly since c. 1800 ce, (2) they have increased disproportionately relative to other phytoplankton, and (3) cyanobacteria increased more rapidly post c. 1945 ce. Variation among lakes in the rates of increase was explained best by nutrient concentration (phosphorus and nitrogen), and temperature was of secondary importance. Although cyanobacterial biomass has declined in some managed lakes with reduced nutrient influx, the larger spatio-temporal scale of sedimentary records show continued increases in cyanobacteria throughout the north temperate-subarctic regions.

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Phytoplankton have evolved different strategies to minimize the potential damage caused by solar ultraviolet radiation (UVR), including the synthesis of UV-absorbing compounds that act as sunscreens and carotenoids that provide protection against photooxidative stress. The concentration and qualitative composition of mycosporine-like amino acids (MAAs) and photoprotective carotenoids (PPCs) were investigated in natural phytoplankton assemblages of 26 lakes located below and above the treeline in the Alps and the Pyrenees. Among these lakes, exposure to UV varied because of differences in the incident spectral dose, water column transparency, stratification, and maximum depth. Seven MAAs were identified, but the most abundant were shinorine (λ max = 334 nm) and palythine (λ max = 320 nm). The highest concentrations of MAAs (up to 9.6 µg [µg chl a] ) were found in the clearest and shallowest lakes, while phytoplankton from lakes with low UV transparency generally presented low values. However, phytoplankton of some clear lakes located at high altitude did not have high concentrations of these compounds. Consequently, underwater downwelling UVR, UV water transparency, or lake altitude explained only a low percentage (< 26%) of the variability among lakes in MAA and PPC concentration. Within the water column, the concentration of MAAs decreased in most cases with depth, suggesting their photoprotective role. Our results indicate that MAAs and PPCs are widespread among lake phytoplankton assemblages and suggest that other environmental factors besides UV exposure are important in regulating their synthesis.

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