Freshwater Ecology Practice Test

Warmth and nutrients interact to drive cyanobacterial blooms. Cyanobacteria grow fastest in warm water, especially when light is plentiful, and their metabolism ramps up as temperatures rise. At the same time, available nutrients—especially phosphorus, and to a lesser extent nitrogen—remove growth limits and allow rapid population expansion. When both conditions are met, cyanobacteria can rapidly outpace other phytoplankton, form dense surface blooms, and often produce toxins like microcystins.

So the combination of warm temperatures with high nutrient loads is the strongest driver of bloom formation and toxin production, more so than temperature or nutrients alone. The other options misstate the relationships: higher temperatures do not inherently reduce blooms, excess nutrients do not suppress growth, and temperature does have an effect on cyanobacteria.