| The scientific hypotheses of hypereuthrophication and seasonal shift of nutrient limitation on algal growth in Dianshan Lake were tested from the following five aspects:1) historical data analyses,2) limnological survey on nutrient inputs and responses,3) lab and field Nutrient Enrichment Bioassays (NEBs),4) mathematic stimulation on algal growth, and 5) statistical analyses on the relationships between nutrient inputs and their response. The results showed that:(1) After more than 15 years of nutrient accumulation, the trophic state of Dianshan Lake transitioned from eutrophic to hypereutrophic in 1999-2000. Despite short residence time of the lake, rapid nutrient accumulation is possible to induce algal blooms;(2) Although the seasonal succession has been fitted into the model of Plankton Ecology Group (PEG) basically, the phytoplankton community of the lake is dominated by green algae and cyanobacteria, Microcystis in summertime, which may threaten the safe of drinking water;(3) In situ nutrient enrichment bioassays (NEBs) on algal community level suggested phosphorus (P) additions significantly stimulated phytoplankton growth in spring and autumn when the algal composition was dominated by diatoms and green algae. In contrast, significant phytoplankton responses to nitrogen (N) additions occurred only in the summer (July to September) when cyanobacteria dominated the phytoplankton. The pattern of shift nutrient limitation is that, P is the primary limiting factor for the lake but that nitrogen deficiency in the water column, which always occurred in summer, may cause N limitation.(4) The early algal growth of the lake can be well described by Logistic model. The stimulation with recent data showed that in spring the algae started its exponential growth at late February, doubled it density within 15 days, reached its maximum growth rate,29.4μg/L.Month, in the middle March, and bloom might occur during the period of May-July. In summertime, cyanobacteria started its exponential growth at the late of June, doubled its density within 26 days, reached its maximum growth rate,22.8μg/L.Month, and cyanobacteria bloom might occur during Augest-September.(5) The statistical analyses on the relationships between nutrients and their response Ch1 a showed that means (μ) and deviations (σ) of TP, TN, and Ch1 a increased with the eutrophication process. In the state of hypereutrophication, even a little variations of mean and/or deviation of nutrients might cause significant increment of Ch1 a. The TP-Chl a regression relationship fitted OECD model well, slopes increased with the eutrophication process. The empirical equation of TN-Chl a was not statistically derived from the existing data set.Based on the above results, some extrapolations were concluded:(1) The trophic state of Dianshan Lake has been transitioned to hyper eutrophication, and algal growth, including early growth is limited by TP and TN alternatively. Better understanding of seasonal shifts in nutrient limitation may help with the development of management strategies for successful lake restoration.(2) In the state of hyereutrophication, even the nutrient concentrations in the water column stop increasing, algae may not change its growth rate simultaneously, and the events of algal bloom may continually occur in a certain period.(3) Prediction of algal early growth is expected. The fundamental way out for controlling harmful algal bloom is early warming and prevention of algal growth.According to these results, a framework was designed for warming system of algal early development:The First Alert Level (Early Warming):Ch1 a=15μg/L, algal exponential growth starts;The Second Alert Level (Bloom Warming):Ch1 a= 35μg/L, algal growth reaches its maximum rate.Under the normal meteorological and hydrological conditions, if there is no action taken, it may take about 20 days that the concentration of Ch1 a be raised from the First Alert to the second.
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