Turnover’s ecological role includes which of the following?

• A. Stabilizes thermal stratification
• B. Re-oxygenation of bottom waters and nutrient redistribution, potentially triggering phytoplankton blooms
• C. Reduces nutrients permanently
• D. Increases sedimentation rate only

Answer: (B)

Turnover refers to the mixing of a lake’s water column during seasonal transitions. This process re-oxygenates bottom waters by pushing oxygen-rich surface water downward, helping prevent anoxic conditions in the deep layers. It also redistributes nutrients, moving them from sediments and deeper waters into the upper, light-rich layer where phytoplankton live. When these nutrients reach the photic zone at the same time as favorable light and temperature, phytoplankton can rapidly grow, potentially triggering blooms. The other statements don’t fit because turnover breaks rather than stabilizes stratification, nutrients aren’t permanently reduced by turnover, and its effects aren’t limited to increasing sedimentation rate.