How can some weeds negatively impact the environment?

Some weeds can negatively impact the environment by inhibiting fish growth and reproduction. This occurs because certain invasive weed species can disrupt aquatic ecosystems. They may alter water quality, block sunlight from reaching aquatic plants, and deplete oxygen levels in the water. These changes can lead to unfavorable conditions for fish and other aquatic organisms, ultimately affecting their growth and reproductive capabilities. By impeding the natural balance of these ecosystems, such weeds can create significant challenges for biodiversity and aquatic health.

The other options discuss issues that may not directly relate to the environmental impact of weeds. While some weeds might cause respiratory issues due to pollen, this is more a health concern than a direct environmental impact. Competition for sunlight with crops does happen, but it primarily affects agricultural output rather than the broader ecosystem. Enhancing soil fertility is typically a positive effect rather than negative, as most weeds do not contribute beneficially to soil health.