Abstract: The Ecosystem-Service-based Ecological Footprint (ESEF) measures how much land and water area is required by a given population or activity to provide the required ecosystem services. It can be further extended according to the types of ecosystem services provided by land and water ecosystems such as resource supply and waste absorption services. The ESEF method is proposed to be the theoretical basis for the assessment of water ecological carrying capacity as an important step towards effectively evaluating water ecological carrying capacity with footprint methods. The Water Ecological Footprint is therefore put forward, which is comprised of aqua-product provision footprint, water supply footprint and waste absorption footprint. Correspondingly, Water Ecological Capacity is made up of aqua-product provision capacity, water supply capacity and waste absorption capacity. As they provide an integrated consideration of quantity support, quality limitation and ecological stability of aquatic ecosystems, the ESEF-based water ecological footprint and capacity are qualified to evaluate the carrying capacity of aquatic ecosystems through the comparison between them. However, it is an emphasis and also a difficulty for current researches to define the relation between footprints of aquaproduct provision, water supply and waste absorption and that between their capacities. In this paper, the authors have constructed local models of water ecological footprint and capacity and applied these models to evaluate the water ecological carrying capacity in Huzhou City located in the upper reaches of the Tai Lake Basin. Two methods were used to define the relationship between different footprints as well as that between different capacities:the union method when water quality standards and environmental functions were taken into consideration and the averaging method when they were not considered. Both methods revealed that human demand for water ecosystem services in this city exceeded the supply capacity of local aquatic ecosystems and the gap between demand and supply jeopardized the sustainability of local aquatic ecosystems. Considering aqua-product provision, water supply and pollutant absorption in an integrated way, the population and economy that aquatic ecosystems in Huzhou can account for 95% of their current size using the averaging method. However, when water quality standards and environmental functions were considered, the carrying capacity of local aquatic ecosystems can only reach 79% of the current status. Although the two methods illustrated a similar picture of the water ecological carrying capacity in Huzhou City, the results obtained from the union method were more persuasive, as the way it identifies the water area available to provide water ecosystem services is more in line with the attribute of the ESEF-based water ecological footprint. The way is relatively simple that the averaging method deals with the relation between different capacities or footprints, however it provides a possibility for researchers to study the water ecological carrying capacity at a smaller scale when data of water quality standards and environmental functions are not accessible. Both methods have strengths and shortcomings, which call for more case studies to provide supportive evidence in the future.

Key words: water ecological carrying capacity, Water Ecological Footprint, Ecosystem-Service-based Ecological Footprint (ESEF), water quality standards and environmental functions, averaging method, union method, Tai Lake Basin