What impact does pollution typically have on freshwater biodiversity?

Pollution typically exerts a detrimental effect on freshwater ecosystems, leading to a reduction in the variety of species present. Contaminants—such as toxic chemicals, excess nutrients, and pathogens—can harm aquatic organisms directly by causing illness, reducing reproductive success, or even leading to mortality. Moreover, pollution can alter habitat structure and function, making it less conducive to supporting diverse communities. For instance, an increase in nutrient pollution can trigger algal blooms, which deplete oxygen levels in the water, creating "dead zones" where most aquatic life cannot survive.

As various sensitive species are pushed out or eliminated from their habitats due to these adverse effects, the overall biodiversity of the ecosystem declines. In the long term, this loss in biodiversity can destabilize ecosystems, making them more vulnerable to further environmental stresses. Therefore, the correct answer reflects the reality that pollution generally reduces the diversity of species in freshwater environments.