Turnover-related phytoplankton responses are most evident during which times?

• A. During drought only
• B. Seasonal transitions
• C. Daily cycles
• D. During volcanic events

Answer: (B)

Turnover is the mixing of the water column that brings nutrients into the sunlit layer and redistributes phytoplankton. In many temperate lakes, this mixing happens most clearly during seasonal transitions—spring and autumn—when the lake shifts between stratified and mixed states. The break-up or formation of stratification changes nutrient availability and light conditions, so phytoplankton communities respond noticeably as the entire column mixes. Daily cycles mainly affect light within the existing mixed layer rather than causing whole-column turnover, while drought or volcanic events don’t consistently drive the broad mixing patterns that define turnover.