Estuaries, where freshwater mixes with saltwater, are dynamic environments of great complexity and a critical habitat for economically important species. Together with coastal waters, which are affected by much the same environmental pressures, they have long suffered degradation. Indeed, last year the United Nations declared the estuaries of China's two greatest rivers, the Yangtze and the Yellow, "dead zones." The damage affects not only the immediate estuarine and coastal environment but also the oceans.

To assess the extent of the harm to estuaries and coastal seas and to reconstruct their ecological history, an international team headed by Heike K. Lotze of Dalhousie University in Halifax, Nova Scotia, has analyzed hundreds of documents. The group focused on 12 temperate-zone estuarine and coastal ecosystems that have been exposed to intense human development for periods ranging from as little as 150 to as many as 2,500 years. They examined 30 to 80 species in each system plus seven water-quality parameters and data on species invasions.