What organisms began to grow in large numbers in lake Washington as a result of the release of effluent from sewage treatment plants?

Algae

Explanation:

Eutrophication is the enrichment of water bodies by nutrients. Examples of such nutrients include phosphates and nitrates from inorganic fertilizers and organic matter like manure and untreated sewage. It causes algal bloom (greenish-slimy growth on the water surface) or rapid growth of aquatic plants. The algae prevent light penetration causing death and decay of submerged aquatic plants and animals. This increases organic matter in water causing proliferation of aerobic bacteria that quickly deoxygenates the water leading to suffocation of aquatic animals.