Which combination of factors would most likely increase cyanobacterial dominance in a lake?

• A. Cold water, strong mixing, low phosphorus
• B. High phosphorus and cool temperatures
• C. High phosphorus and warm temperatures with stratification
• D. Low nutrients and high turbidity

Answer: (C)

Understanding cyanobacterial dominance hinges on how temperature, nutrients, and water-column structure shape growth. Cyanobacteria tend to flourish when surface waters are warm, nutrients are abundant, and the lake is stratified so the warm, light-rich layer remains relatively undisturbed. High phosphorus removes a common growth bottleneck, enabling rapid increases in population. Warm temperatures boost their metabolic rate, and stratification creates a stable epilimnion where light is plentiful and nutrients stay concentrated near the surface, allowing cyanobacteria to form blooms and outcompete other phytoplankton.

The other scenarios don’t favor blooms as strongly: cold water slows growth and strong mixing dilutes surface populations, making it hard for cyanobacteria to dominate even with some nutrients; high phosphorus with cool temperatures still limits growth rate due to temperature; and low nutrients with high turbidity reduces both nutrient availability and light penetration, limiting cyanobacterial growth.