Abstract:Research on the first and second levels of aquatic ecological function regionalization (AEFR), a new regionalization framework, is now being conducted in several representative river/lakes watersheds in China. How to regionalize the target watersheds scientifically and logically is an important and urgent question. In this paper, we proposed understandings for regionalization objectives, guidelines and methods, through discussing the relationships between AEFR and aquatic ecological regionalization, the roles of aquatic organisms and selections of quantitative methods. Based on summarizing the general characters of ecological regionalization schemes from various government departments, countries and regions, we firstly gave the definition and objectives of AEFR: developing and improving from existing ecological regionalization theories, it is a task that divides the target watershed into several independent and intact units by distinguishing the spatial heterogeneity of aquatic ecosystems in terms of patterns and functions, and discriminating the coupling relationship between aquatic and terrestrial ecosystems. Regionalization units provide basic management element for watershed pollution control, water quality, the benchmark and standard of ecological health and ecological capacity. By implementing regionalized, hierarchical, classified and sequential management strategies in various regionalization units, it is possible to ensure watershed health and aquatic ecological functions, so that to achieve the objectives of water quality safety and ecological integrity within target watershed. By comparing the existing regionalization methods, we suggested to combine the up-bottom deductive method and bottom-up induction method in AEFR research. Namely, environmental driving factors, including climate, hydrology, geomorphology, soil, vegetation and land use/land cover indices, etc., should be used to divide the watershed into several units. Whereafter, habitat based variables such as biological community compositions, functional groups, biodiversity indices, physicochemical factors, etc. can be used to verify and validate the regionalization results. The above two steps are both essential and indispensable. Considering the different responses of diverse aquatic organisms to environmental stress, aquatic invertebrates and algal assemblages, which are normally used as bio-indicators, we highly recommend using as testing organisms in Chinese waters. Being a subjective task, after all, it is impossible to accomplish AEFR using quantitative methods completely. To achieve more objective results, we proposed to integrate expert judgments with quantitative results. The procedure will be as followings: correlation analysis or dimensionality reducing techniques such as principal component analysis (PCA) should be used firstly to find independent factors that contribute mostly to spatial heterogeneity of aquatic ecosystems. And then, such small collection of independent factors will be used to differentiate spatial patches of the watershed, with the assistance of expert judgments. Thirdly, at the stage of regionalization verification, classification and ordination methods such as clustering (unweighted pair group method with arithmetic mean (UPGMA), Ward′s method, k-means clustering, etc.), two-way indicator species analysis (TWINSPAN), detrended correspondence analysis (DCA), canonical correspondence analysis (CCA), redundancy analysis (RDA), and artificial neutral network (ANN) etc. can be used to analysis spatial patterns of aquatic organisms and other habitat variables. The results, in turn, will be used to verify or modify regionalization results, assisted with expert judgments, again. However, special attention should be given to applicability of each quantitative method. Besides, it is necessary to use and unify quantitative methods in regionalization as possible, so that to get comparable results and lay solid foundation for national regionalization work in the future.