Resource limitation in phytoplankton is routinely documented by comparing batch culture population growth rates measured under environments varying in available nutrients and/or irradiance. Rarely is resource limitation documented for individual algal cells, filaments, or colonies. Using a technique that integrates microscopy and digital image analysis, we determined how varying nutrient (four nitrogen:phosphorus (N:P) ratios [5, 8, 16, and 30]) and irradiance (25 or 45 µmol photons/m²/sec) conditions influence the growth rate of individual colonies of two genotypes of the bloom-forming cyanobacterium, Microcystis aeruginosa, over four days. The two M. aeruginosa genotypes used in this study were isolated from Lake Erie or a eutrophic aquaculture pond in Alabama and allowed to acclimate to the four N:P treatments for two months prior to the start of the experiment. 

Results/Conclusions

In general, colony growth rates were shown to be highest when exposed to higher irradiance (P = 0.049) and the lowest N:P ratio (P = 0.003). The effect of genotype on growth rate was insignificant (P = 0.159). Moreover, colony growth rates were not affected by interactions between N:P ratio, irradiance, and/or genotype (P > 0.059). Past allometric studies have shown that individual growth rate is inversely related to body size across a broad spectrum of organisms, including colonial cyanobacteria. Despite our efforts to target a similar, narrow size range of colonies for the M. aeruginosa genotypes at the start of the experiment (measured as equivalent spherical diameter (ESD); Lake Erie ESD range = 0.14 to 0.28 mm; Alabama pond ESD range = 0.11 to 0.32 mm) to reduce the influence of size on our results, initial colony sizes varied between genotypes (P < 0.0001) and among N:P ratios (P = 0.003) and were shown to be negatively related to colony growth rates (P < 0.00001).  Including initial colony size as a covariate (P < 0.00001), main effects of irradiance, N:P ratio, and genotype significantly influenced colony growth (P < 0.05). In addition, significant interactions between irradiance and genotype, N:P ratio and genotype, and irradiance, N:P ratio, and genotype were also observed (P < 0.05). Given that colony size varies widely within populations of M. aeruginosa and strongly mediates phytoplankton growth under a variety of environmental conditions, as well as interactions with other phytoplankters and consumers, our results show that the size structure of phytoplankton populations will need to be considered when forecasting the promotion or control of toxic blooms dominated by colonial species, like M. aeruginosa.