Research on Ecosystem Health Assessment Indices and Thresholds of a Large Yangtze-Connected Lake, Poyang Lake

Abstract

Assessing the aquatic ecosystem health status of lakes has been the focus of lake studies for domestic and overseas researchers and water resource managers. Poyang Lake, which is one of the only two remaining large Yangtze-connected lakes in the middle reaches of Yangtze River in China, provides considerable ecosystem service functions, such as flood water storage, water supply for industrial and agricultural production, maintenance of the unique and diverse biota of the entire Yangtze floodplain ecosystem, maintenance of regional ecological balance and so on. However, due to the impact of climate change and human activities, Poyang Lake has experienced drastic hydrological changes, water quality decline and aquatic ecological function degradation in recent years. An assessment of the aquatic ecosystem health of Poyang Lake is of great importance. On the basis of the index system construction principles and a meta-analysis of previous research, this paper puts forward the index system for the ecosystem health assessment of Poyang Lake. The indices contains the three element layers of aquatic organisms, hydrologic characteristics and trophic status and the five indicators of ratio of cyanobacteria biomass to diatom biomass (CB/DB), diversity index of macrozoobenthos, lowest water level anomaly during the dry season, mean water level anomaly during rising season and trophic state index (TLI). Then, the thresholds of indicators are determined using empirical study, frequency analysis, standard specification and so on. Aquatic ecosystem health status is categorised into three groups: 'good', 'moderate' and 'poor'. The study could provide theoretical basis for identifying the causes of the lake aquatic ecosystem health degeneration and protecting the environment of Poyang Lake. This present study provides an attempt to quantify the aquatic ecosystem health in a large floodplain lake with dramatic hydrological variations and will give some impetus for floodplain ecosystem assessment and management worldwide.

Aquatic Ecosystem Health Assessment of Poyang Lake through Extension Evaluation Method

“…The aquatic ecosystem health of a lake should be assessed in terms of the status of biological elements and supporting physicochemical and hydromorphological elements [30]. Therefore, in accordance with references [7] and the major characteristics of Poyang lake, the index system for the aquatic ecosystem health assessment in Poyang Lake was established and the assessment indicator thresholds were determined. The index contained three element layers, namely hydrologic characteristics, trophic status, and aquatic organisms.…”

“…The six indicators (Figure 2) were as follows: lowest water-level anomaly in the dry season (LWLa-D), mean water-level anomaly during the rising season (MWLa-R), trophic state index (TLI), ratio of CB to DB, diversity index of macrozoobenthos (DI) and biological index (BI). The calculation method for BI, DI and TLI was according to references [7,31]. LWLa-D was calculated by the lowest water level in the period of December-March, subtracting the lowest water level mean over multiple years during the dry season.…”

“…MWLa-R equals the mean water level in April-May minus the mean water level over multiple years during the rising season. In accordance with [7], the assessment indicators thresholds (Table 2) were determined based on empirical method, frequency analysis, specification standards and so on. Thus, the health status of the aquatic ecosystem in Poyang lake was divided into three categories: 'good', 'moderate (sub-healthy)' and 'poor'.…”

“…As such, the entire Poyang Lake was divided into the following five zones in accordance with their characteristics (e.g., topography, habitat conditions, and hydrodynamics) to determine Many studies showed significant regional differences in hydrodynamic conditions, water quality, and aquatic species [19,35,36]. For example, big-sized aquatic plants are mainly found in a few branch areas of the lake over the rising season [7]. As such, the entire Poyang Lake was divided into the following five zones in accordance with their characteristics (e.g., topography, habitat conditions, and hydrodynamics) to determine the spatial variation in the health status of the aquatic ecosystem and the major contributors to such variations: the northern region connected to the Yangtze River, the eastern region, the northwestern region, the central lake and the southern region (Figure 9a).…”

“…Such ecological deterioration poses a threat to the use of water resources, particularly drinking water supply and economic development [3,4]. Thus, the lake aquatic ecosystem health evaluation has been a focus of lacustrine studies among domestic and overseas researchers and water resource managers [5][6][7]. Since the 1980s, the health status of lake aquatic ecosystems have been explored through lacustrine studies by highly skilled professionals and managers with water resources at home and abroad.…”