Estuaries are important in coastal ecosystems because they serve as a transition zone between freshwater and saltwater environments. They provide vital habitat for a diverse range of plant and animal species, including many commercially important fish and shellfish. Estuaries also help to filter pollutants from runoff before they reach the ocean, and they serve as nurseries for many marine species. Overall, estuaries play a crucial role in maintaining the health and productivity of coastal ecosystems.