Sub-areas compartmentalization of estuary is not only the precondition for developing nutrient criteria, but also the most effective tool for management of estuarine resources. It is an important step in addressing the problem of resource degradation caused particularly by nutrient overenrichment. Sub-areas compartmentalization should reduce variability of ecosystem-related measures within identified classes and maximize variability in interclass. The compartmentalization should be completely based on physical and hydrothermal parameters such as salinity gradient, excluding chemical and nutrient elements. In addition, there should be a distinct boundary between any two sub-areas, one sub-area should be a continuous geographical area.
In the present study, Changjiang Estuary are carved up into sub-areas, based on the monitoring data during 2005-2006, and global relevant research results, as well as the current environmental conditions. The"United States Environmental Protection Agency Nutrient Criteria Technical Guidance Manual: Estuarine and Coastal Marine Waters and European Commission Water Framework Directive" is referred, the hierarchical classification method is performed in the study. The results show that: (1) at first class of compartmentalization, Changjiang Estuary can be compartmentalized into transitional sub-area and coastal sub-area; (2) the coastal sub-area is a very extensive range area, with great transformations in hydrological conditions, biological communities and their different responses to environmental stress, this sub-area can be divided further into three sub-areas at the second class of compartmentalization, according to the hydrological and geographical characteristics, as well as the convenience for administrative management of the sub-areas, the three sub-areas are coastal zone outside of the estuary, Hangzhou Bay and Zhoushan Archipelago. Consequently, Changjiang estuary is divided into four sea sub-areas: transitional zone, coastal zone outside of the estuary, Hangzhou Bay and Zhoushan Archipelago.
In the previous compartmentalization of the Changjiang Estuary based on the characteristics of surface sediment distribution, neither the insular effects from Zhoushan Archipelago to the environment of the estuary, nor the range sizes of the archipelago and Hangzhou Bay were considered. In the present research, the compartmentalization is based on the natural geographical characteristics, the administrative districts, the features of different biota, as well as the effects of marine currents in Zhoushan Archipelago. The bifurcate river channel section of Changjiang Estuary with alternated shoals and troughs is cognized as transitional sub-area of Changjiang Estuary, the Hangzhou Bay and Zhoushan Archipelago are treated as independent sub-areas. Comparing with the previous compartmentalization of the estuary based on the nutrient distribution which reflects the differences of hydrological, chemical, and hydrodynamic elements of the estuary, the present compartmentalization using hierarchical method reflects more natural geographical characteristics of the four sea sub-areas than the previous study. The boundary between any sub-areas has definite geographical importance. The administrative units to which the sub-areas belonging are clear to different administrative districts, so that avoid managing disorder or omission.
The characteristics of sea water and sediment of the four sub-areas are tested by one-way Analysis of Variance (ANOVA). The results show that there were significant differences of the characteristics among the sub-areas.
In conclusion, the present compartmentalization of the Changjiang Estuary is rational. It should be helpful for normalizing human exploitation activities, the conservation of marine resources and the improvement of regional ecological environment. However, because of the complexity of the environment in Changjiang Estuary and its adjacent waters, and the increasing of human activities in the area, the compartmentalization of the estuary should be improved continuously.