Predictive Limnological Researches On Small-To Medium-Sized Lakes Along The Mid-Lower Yangtze River

The Yangtze floodplain is one of the most important wetlands of the world. However, many lakes in the region have suffered from fishery over-exploitation and man-made eutrophication for decades. To establish a general and quantitative platform of lake ecosystem management, predictive limnological researches on mid-lower Yangtze shallow lakes were carried out systematically in 46 small- to medium-sized lakes, using data from field investigations and references. The results are mainly as follows: (1) The ratio of Secchi depth to water depth (ZSD/ZM) was found to be the most important factor regulating submersed macrophyte biomass (BMac). A series of key-time models of BMac were generated using ZSD/ZM during the key time (Mar.-Jun.) as the driving variables. According to the models, ZSD/ZM should reach over 0.66, 0.47, 0.55 and 0.45 respectively in the four months during March-June to enable a normal growth of submersed macrophytes. (2) BMac was found to be the important factor regulating diversity and standing crops of epiphytic gastropods. Accordingly, a series of predictive models were generated. Further analyses found that pulmonates prefer the macrophytes which can spread their terminal parts on the water surface, while prosobranchs prefer the macrophytes which are entirely submersed. (3) Water depth, Secchi depth, water temperature, phytoplankton chlorophyll a and BMac were found to be the important factors regulating the standing crops of bottom zoobenthos. Accordingly, a series of predictive models were generated using these factors as the driving variables. (4) BMac was found to be the important factor regulating the yield of Chinese mitten crab (CY). For convenient applications in crab culture, the ratio of Secchi depth to mean depth (ZSD/ZM), a parameter easy to be accurately measured and close to both BMac and CY was selected as the driving variable. ZSD/ZM during the crab planting season (Dec.-May) was used as the driving variable instead of the annual mean to generate a series of maximal yield models. Based on the theory of MSY (Maximum Sustainable Yield), a general optimal-stocking model was formulated. According to the models, the optimal stocking rates of crab juveniles (yearlings, about 10±5 g/ind) in macrophytic lakes are generally 700±60 ind/ha. (5) Traditionally accepted index of TN/TP ratio was found to be inappropriate to divide phytoplankton as nitrogen- or phosphorus-limited. TP is the primary limiting factor regulating Chl a for all the cases. Accordingly, a series of predictive models were generated using TP as the driving variables. Further analyses showed that the increase in BMac could result in a significant reduction of Chl a per unit TP. However, planktivorous fishes (silver and bighead carps) failed to reduce Chl a and enhance Secchi depth. For lake management concerning total phytoplankton control, nutrient abatement should be carried out first; after that, try to recover submersed macrophytes and ecosystem integrity. (6) TP was determined as the primary factor triggering the regime shift of shallow lake ecosystem. The threshold of TP was tentatively determined as 70-100 mg/m3 for the clear-turbid shift, and 20-30 mg/m3 for the turbid-clear shift. After the research, a suit of special modelling methods were proposed: (1) After the factor limiting the target variable is determined, the modeler should seek for factors close to the limiting factor and easy to be accurately measured as the driving variables. Models generated like this are expected to have higher predictive capacity and be more practical. (2) A new concept"key-time model"was proposed. Models based on key-time driving variables have significantly higher predictive capacity than traditional synchronic models and enable us to predict annual growing tendency well in advance. Such models are of more value in practical application.