Question

what effect does excessive silt or sand have on an aquatic ecosystem?
none
murky water blocks sunlight and affects plant growth.
increased plant growth
improved levels of dissolved oxygen

Explanation:

Excessive silt or sand in water makes it murky. This blocks sunlight from reaching aquatic plants, which need sunlight for photosynthesis. Without sufficient sunlight, plant growth is affected negatively. It does not lead to increased plant growth or improved dissolved - oxygen levels. And it definitely has an effect on the ecosystem, not none.

Answer:

murky water blocks sunlight and affects plant growth.