Estuaries contain the most productive ecosystem because these are the area of the transition zone between two lakes or two ecosystems.  Estuaries maintain biodiversity as they provide a vast range of habitat for a variety of species to thrive and live. Many mammals, fishes and birds live and thrive in estuaries. Many fishes and shellfish come toe estuaries to provide nourishment to their spawns and eggs. So, both the statements are correct with respect to an estuarine ecosystem.