1. Introduction
Carrying capacity has been applied to describe the importance of the limiting factors of environment on human material progress and to make the calculation and prediction of the upper limits of population or economic growth for a healthy eco-environment [
1]. In the 1980s, widespread discussion about the drought problem initiated the employment of carrying capacity in resolving the constraint of water resources from which human society was suffering in China [
2,
3]. Simultaneously, rapid increases in nutrient input and environmental deterioration due to the intensified agricultural and industrial production around rivers and lakes have given rise to the applications of water environmental carrying capacity, which originated from the concept of environmental maximum load [
4-
6]. Recently, ecological or eco-environmental carrying capacity were highlighted as people became more aware of the need to detect and predict changes in ecosystem functioning in situations where human activities had profoundly affected ecosystems [
7-
9]. In order to integrate all the definitions of carrying capacity related to water, aquatic ecological carrying capacity was proposed in an attempt to assess the degree of pressure from human activities on ecological resilience.
Tai Lake region is one of the most densely populated and developed areas in China, situated in the lower reaches of the Yangzi River. During the past twenty years, rapid industrialization and urbanization, combined with a lack of effective management and technical measures, have caused many serious environmental problems such as water eutrophication, organic pollution and destruction of aquatic ecosystems [
10,
11]. Comprehensively, there are seven large- and medium-sized cities (namely, Shanghai, Suzhou, Wuxi, Jiaxing, Huzhou, Changzhou, and Kunshan) and 31 counties in the Tai Lake basin, in which Wujin District is located in the core of Changzhou city. The point pollution in this area is mainly attributed to dispersed rural industries such as township and village enterprises (TVEs), which were underpinned by the implementation gap in the toughest environmental laws [
12,
13]. The other prominent cause of water ecosystem degradation is non-point pollution triggered by intensive use of organic fertilizer (nitrogen, phosphorus) and other agricultural activities [
14-
16]. As a result, with the aim of evaluating the carrying capacity of the aquatic ecosystem in the context of diversified human activities and better protection of the aquatic environment, the National Environment Protection Agency in China initiated the “Water Special” project, in which aquatic ecological carrying capacity assessment was carried out in the Wujin District in the Tai Lake basin.
In the past, many methods were explored for assessing water resource, water environment or ecological carrying capacity, in which the multi-indicator assessment approach was the most widely used [
17-
19]. It followed a common procedure of indicator selection, weight determination, indicator (variables) estimation and integrated assessment. But the limitation arose when it was attempted to identify and estimate indicators reflecting ecological resilience in aquatic ecological carrying capacity. Previous research mainly focused on water consumption and water environment loading, and ecological indicators were often ignored in assessments [
3,
20]. For better reflection of nutrient cycles and energy flows in aquatic ecosystems, indicators representing both aquatic and terrestrial ecosystems should be incorporated [
21,
22]. With the accessibility of remotely sensed images and wide use of geographical information system, ecological indicators were capable of being quantified and the assessment of aquatic ecological carrying capacity could be more efficient and comprehensive [
11,
23,
24].
This paper proposes an integrated approach to assess aquatic ecological carrying capacity of the Wujin District in the Tai Lake basin using MODIS (Moderate Resolution Imaging Spectroradiometer)- NDVI (Normalized Difference Vegetation Index) time-series images and GIS technology. Firstly, an operational indicator system was established in terms of human pressure and ecological resilience. Next, the integration of Delphi and the AHP (Analytic Hierarchy Process) was used to determine the weight of each indicator in different layers. Then based on empirical equations, indicators were calculated using statistical data, field data, land use maps and MODIS-NDVI time-series products. Finally, a comprehensive assessment of aquatic carrying capacity was performed based on defined criteria from 2000 to 2008, producing a final result for decision makers.
4. Conclusions
In our research, aquatic ecological carrying capacity assessment was implemented using RS and GIS in the Wujin District. The work concerned can be illustrated as:
Social, economic and ecological context was explicitly investigated in our case, formulating the background for the identification of indicators and the establishment of indicator hierarchy.
The spatial and temporal variation of NDVI was carried out and the extension of vegetation cover into Ge Lake was observed, which should be paid more attention by local authorities.
Ecological resilience assessment was made with the integrated consideration of aquatic and terrestrial ecosystems, in which RS and GIS technologies were of great assistance in obtaining the final result.
The eventual result of the integrated aquatic ecological carrying capacity assessment included all the indicators and their quantitative values. Also, maps were produced and spatial analyses were made accordingly. In the future, geo-spatial technologies should be incorporated in a more comprehensive, deep and quantitative manner into AECC. Generally, based on the work we have carried out, several suggestions could be made for decision makers.
Vegetation cover has increased in central and eastern parts of Wujin District in the past few years, which could be observed in NDVI products. However, the phenomena of cyanobacteria bloom and the decreasing of water area make it imperative to reduce agricultural activities around Ge Lake, since it will impair the aquatic ecological balance and aggravate water eutrophication.
Concentrations of COD, TN and TP were considerably higher than the national standards for pollutant emission; thus water eutrophication was still diagnosed as the main problem in Wujin District. As a result, it is suggested that pollutant emission reduction should be strengthened, especially for industrial waste water discharge and agricultural activities.
In the long run, aquatic ecological carrying capacity has increased slightly, which reveals that government planning on land use, economic growth, population growth and technological levels was able to improve water management and could be reasonably conducted.