The depth of ______ usually determines the maximum depth at which underwater plants will grow.

The maximum depth at which underwater plants can grow is primarily determined by light penetration. Aquatic plants rely on photosynthesis, a process that requires light. As water depth increases, the intensity of light diminishes due to absorption and scattering by the water and suspended particles. Therefore, the depth at which light penetrates effectively dictates how deep aquatic plants can thrive.

Beyond a certain depth, typically known as the euphotic zone, light levels become too low for photosynthesis to occur, limiting plant growth. While factors like water temperature, salinity levels, and nutrient concentration do have significant impacts on aquatic ecosystems and plant health, they do not primarily control the maximum depths of plant growth in the same way that light penetration does.