What common problem can large numbers of aquatic weeds cause?

Large numbers of aquatic weeds can significantly displace native vegetation within an ecosystem. This displacement occurs because invasive or fast-growing aquatic weeds compete for resources such as light, nutrients, and space, ultimately overshadowing and outcompeting native plants. When native vegetation is reduced or eliminated, it can have cascading effects on the entire aquatic ecosystem, affecting not only plant diversity but also the animals that rely on those plants for habitat and food.

In contrast, the other options do not accurately reflect the impact of excessive aquatic weeds. They do not typically improve water clarity, but rather can cause murkiness by proliferating in a dense manner. While some species of fish might temporarily benefit from the presence of certain aquatic vegetation, in excess, aquatic weeds can disrupt fish populations by altering habitats. Lastly, aquatic weeds do not contribute positively to soil erosion; instead, excessive growth often leads to structural changes in water bodies that may exacerbate erosion issues elsewhere.