How does zooplankton grazing influence phytoplankton community composition and bloom dynamics; what evidence supports top-down control?

How does zooplankton grazing influence phytoplankton community composition and bloom dynamics; what evidence supports top-down control?

• A. Grazing reduces all phytoplankton equally.
• B. Grazing has no impact on phytoplankton dynamics.
• C. Grazing increases dominance of edible taxa.
• D. Grazing preferentially removes edible phytoplankton, shifting community toward less-grazed taxa; evidence includes experiments showing reduced phytoplankton biomass when zooplankton densities increase, and changes in community structure.

Answer: (D)

Top-down control in lakes occurs when predators shape prey communities. Zooplankton grazing tends to be selective for the most edible, nutritionally favorable phytoplankton, so those easily eaten taxa are removed more rapidly. As a result, the phytoplankton community shifts toward taxa that are less edible or harder to digest, such as larger cells or colonial forms. This change in the community not only alters which taxa dominate but also how blooms develop, because the taxa that trigger blooms under low grazing can be suppressed when grazing pressure rises. The strongest evidence for this pattern comes from manipulative studies and mesocosm experiments where increasing zooplankton densities leads to a drop in overall phytoplankton biomass and clear changes in community structure—fewer edible taxa and more of the less-grazed forms. In short, grazing removes the readily eaten phytoplankton, reshaping the community composition and influencing bloom dynamics.

Top-down control in lakes occurs when predators shape prey communities. Zooplankton grazing tends to be selective for the most edible, nutritionally favorable phytoplankton, so those easily eaten taxa are removed more rapidly. As a result, the phytoplankton community shifts toward taxa that are less edible or harder to digest, such as larger cells or colonial forms. This change in the community not only alters which taxa dominate but also how blooms develop, because the taxa that trigger blooms under low grazing can be suppressed when grazing pressure rises.

The strongest evidence for this pattern comes from manipulative studies and mesocosm experiments where increasing zooplankton densities leads to a drop in overall phytoplankton biomass and clear changes in community structure—fewer edible taxa and more of the less-grazed forms. In short, grazing removes the readily eaten phytoplankton, reshaping the community composition and influencing bloom dynamics.