Turnover affects the phytoplankton community by altering which factors?

• A. Dissolved oxygen only
• B. Nutrient availability and light conditions
• C. Water temperature only
• D. Sediment texture

Answer: (B)

Turnover is the mixing of the water column, driven by wind and seasonal temperature changes. This mixing moves nutrients from deeper, typically nutrient-rich layers up toward the surface, making them available to phytoplankton in the illuminated zone. At the same time, by transporting cells through different depths, turnover changes the light environment those phytoplankton experience, altering photosynthesis and which taxa thrive. Since phytoplankton growth is often limited either by nutrients or by light, shifting the balance of these two factors leads to changes in the community composition and productivity. Sediment texture isn’t altered by mixing, and while turnover can influence oxygen and temperature, the clearest, most direct changes for the phytoplankton are nutrient availability and light conditions.