What are some common aquatic pests in freshwater environments?

Eurasian watermilfoil and purple loosestrife are indeed common aquatic pests in freshwater environments. These species are often regarded as invasive because they can outcompete native flora, disrupt local ecosystems, and affect water quality. Eurasian watermilfoil is a submerged aquatic plant that can form dense mats on the water's surface, which can hinder recreational activities, reduce biodiversity, and alter the habitat for aquatic life. Purple loosestrife is a wetland plant that can take over disturbed areas, leading to the displacement of native plant species and degradation of habitat for wildlife.

In contrast, the other choices represent organisms or conditions that do not fit the classification of common aquatic pests in freshwater ecosystems. Sharks and sea turtles are marine species not typically found in freshwater environments, while goldfish and koi, although they can be problematic in some contexts, are domesticated fish rather than true pests. Nutrient-rich sediments themselves are not an organism but rather a condition that can lead to excess growth of algae when nutrient levels are unbalanced, but they do not represent individual pest species. Therefore, the choice correctly identifies the invasive species that pose significant threats to freshwater systems.