What effect can large numbers of plankton algae have on water color?

Large numbers of plankton algae can significantly change the color of water due to their pigmentation and density. When algae proliferate, they can impart hues such as green, brown, or red to the water, depending on the specific types of algae present and their concentrations.

In freshwater and coastal ecosystems, the presence of algae can create what is known as a "bloom," which can drastically alter the appearance of the water. This phenomenon is not only a visual change but can also indicate shifts in the water's ecological health, sometimes signifying nutrient overload and leading to consequences like hypoxia or water quality issues.

The fact that the water may become clear, have no effect on color, or brighten significantly does not accurately represent the impact of algal blooms. Instead, the characteristic color changes that occur during algal blooms are a well-documented effect, affirming that the correct interpretation of the situation is that they indeed change the color of the water.