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Dominance by cyanobacteria hampers human use of lakes and reservoirs worldwide. Previous studies indicate that excessive nutrient loading and warmer conditions promote dominance by cyanobacteria, but evidence from global scale field data has so far been scarce. Our analysis, based on a study of 143 lakes along a latitudinal transect ranging from subarctic Europe to southern South America, shows that although warmer climates do not result in higher overall phytoplankton biomass, the percentage of the total phytoplankton biovolume attributable to cyanobacteria increases steeply with temperature. Our results also reveal that the percent cyanobacteria is greater in lakes with high rates of light absorption. This points to a positive feedback because restriction of light availability is often a consequence of high phytoplankton biovolume, which in turn may be driven by nutrient loading. Our results indicate a synergistic effect of nutrients and climate. The implications are that in a future warmer climate, nutrient concentrations may have to be reduced substantially from present values in many lakes if cyanobacterial dominance is to be controlled.

1.A logical way of distinguishing functional groups of phytoplankton is to cluster species according to their functional traits, such as growth rate and nutrient assimilation constants. However, data for such an approach are lacking for the vast majority of the species. 2. In this study, we show that a classification based on simple morphological traits may capture much of the variability in functional properties among the phytoplankton. We used information on more than 700 freshwater species, from more than 200 lakes situated in climate zones ranging from subpolar to tropical. 3. Morphological characteristics correlated well with functional properties, such as growth rate and sinking rate, and also with the population size and biomass attained in the field. This suggests that morphology is a good predictor of the functional characteristics of species. 4. Cluster analysis was used to define seven species groups based on morphology. Although some of the clusters are taxonomically homogeneous, others include species of several separate divisions. Functional traits (not used for the classification) differed significantly among the clusters, suggesting that the clusters may indeed represent meaningful functional groups. 5. Advantages of our morphological approach to classification include its objectivity, its independence from taxonomic affiliations, and the relative ease of its application to the majority of species for which physiological traits are unknown and are not readily determined.

Summary 1. The hypothesis that cyanobacteria have higher optimum growth temperatures and higher growth rates at the optimum as compared to chlorophytes was tested by running a controlled experiment with eight cyanobacteria species and eight chlorophyte species at six different temperatures (20–35 °C) and by performing a literature survey. 2. In the experiment, all organisms except the chlorophyte Monoraphidium minutum grew well up to 35 °C. The chlorophyte Chlamydomonas reinhardtii was the fastest‐growing organism over the entire temperature range (20–35 °C). 3. Mean optimum growth temperatures were similar for cyanobacteria (29.2 °C) and chlorophytes (29.2 °C). These results are concordant with published data, yielding slightly higher mean optimum growth temperatures for cyanobacteria (27.2 °C) than for chlorophytes (26.3 °C). 4. Mean growth rates of cyanobacteria at 20 °C (0.42 day−1) were significantly lower than those of chlorophytes at 20 °C (0.62 day−1). However, at all other temperatures, there were no differences between mean growth rates of cyanobacteria and chlorophytes. 5. Mean growth rates at the optimum temperature were similar for cyanobacteria (0.92 day−1) and chlorophytes (0.96 day−1). However, analysis of published data revealed that growth rates of cyanobacteria (0.65 day−1) were significantly lower than those of chlorophytes (0.93 day−1) at their optimum temperatures. 6. Although climate warming will probably lead to an intensification of cyanobacterial blooms, our results indicate that this might not be as a result of higher growth rates of cyanobacteria compared with their chlorophyte competitors. The competitive advantage of cyanobacteria can more likely be attributed to their ability to migrate vertically and prevent sedimentation in warmer and more strongly stratified waters and to their resistance to grazing, especially when warming reduces zooplankton body size.