Study On The Influential Factors Of Phytoplankton In Shallow Lakes And Restoration Assessment For Aquatic Ecosystem

Lake eutrophication, a syndrome of ecosystem responses to human activities thatfertilize water bodies with nitrogen (N) and phosphorus (P), has become a worldwideenvironmental issue. Most of the urban shallow lakes in China had been functioned asreceiving water bodies of sewage effluent for long time, together with urban runoff,leading to increasing trophic status. Many urban lakes in the middle-lower reaches ofYangzte River has been in eutrotrophic status, characterized with frequent outbreak ofMicrocystis blooms. Due to the complexity and unstability of an ecosystem, themechanism of Microcystis bloom in urban eutrophic lakes, however, has not beenidentified.To characterize eutrophication and the influential factors for algal growth in typicalurban shallow lakes, the shallow lakes in Wuhan city were taken as instances in this study,and samples from15shallow lakes in Wuhan City were taken and analyzed during theperiod from March2004to March2006. By conducting field monitoring and indoorcultivation experiments, the regularity of growth and distribution of non-filamentous algae,typically Microcystis aeruginosa, under both circumstances were compared, and thelimiting effects of the concentration and ratio of nitrogen and phosphorus on algal growthwere discussed. Algae inhibition effect by various macrophytes was analyzed through theecological remediation project in the Ziyang Lake, in order to investigate appropriateengineering techniques for algae and bloom control. A set of remediation process-basedassessment index system was also proposed to assess and quantify the effect of anecological restoration project.Main conclusions of the study are listed as follows:(1) Majority of the lakes in Wuhan are in the highly eutrophic status, with extremelyabundant biomass of phytoplankton, averaged456.87mg/L in summer and189.24mg/Lduring annual period, respectively. The phytoplankton community was dominated byMicrocystis aeruginosa and Euglena caudate in summer and Cryptomonas ovata andCyclotella meneghiniana in winter. Low TN:TP ratios were detected accompanied withfewer occurrences of nitrogen-fixing cyanobacteria and other filamentous algae.81 percent of the lakes in summer had the ratio of nitrogen to phosphorus content of less than10, with no nitrogen-fixing cyanobacteria dominant the algea community, indicating thatlow TN:TP ratio does not always result in the preferential growth of nitrogen-fixingcyanobacteria. Temperature and TP content were found to be the principal limiting factorsfor phytoplankton growth on an annual basis, due to the maximum positive relationshipbeween TP concentration and phytoplankton biomass (r=0.817, p<0.001). The results ofredundance analysis (RDA) show that the biomass of Microcystis and Euglena caudatehad significant positive correlation with TP. The results of variance analysis (ANOVA)demonstrated that the density of phytoplankton increased with the increase of TPconcentrations, and reached the maximal with the TP concentration in the range of0.2-0.5mg/L. This phenomenon implies that the threshold of TP concentration for themaximum algae biomass is probably between0.2mg/L and0.5mg/L in the studied lakes.On the other hand, total nitrogen showed no similar relationship. Thus we concluded thatneither TP nor TN is the limiting factor of phytoplankton biomass in urban eutrophic lakeswith extremely high nutrient concentrations, since the nutrient concentration is high abovethe assimilating capacity of algea.(2) A set of bench-scale completely random combination experiments were designedto test the effects and limitation of nitrogen and phosphorus under different ratios whenMicrocystis aeruginosa were cultured. The results showed that the maximum stockingbiomass of Microcystis increased significantly with the increase of total phosphorusconcentration ranging from0.5mg/L to0.8mg/L, but were less influenced when the TPconcentration was greater than0.8mg/L. Thus, the optimal phosphorus threshold forMicrocystis aeruginosa is probably in the range of0.5-0.8mg/L, which is different to theresults of field tests in these lakes. This phenomenon can attributed to the lowerassimilation ability of Microcystis aeruginosa to the phosphorus, which was caused by thedispersive cells in pure cultured conditions.(3) The effect of different macrophytes on the physico-chemical composition andcommunity of algea was investigated during the process of ecological restoration projectin the Ziyang Lake. The results showed that submersed macrophytes, e.g., Ceratophyllumdemersum, more effectively inhibit the growth of surrounding phytoplankton by directly releasing allelopathic substances through stalk and leaves, as compared to emergedmacrophytes (e.g. Vallisneria natans) and floating macrophytes(e.g. Nymphaea alba). Thestudy also found that the ratios of nitrogen to phosphorus were all less than10in the lakeswhere algae blooms broke out. On the other hand, TN:TP ratios were less than10duringmost of the monitoring periods at the four stations in the Ziyang Lake, but the algaedensity was far less than the degree of the outbreak of water bloom. Hence we concludedthat low TN:TP ratio is not a “sufficient condition”, but a “necessary condition” for waterbloom.(4) Based on the published results and study, a set of restoration process-basedassessment index system for ecosystem restoration of lake is proposed. The index system,named integrated restoration index (IRI), includes a “restoration score” and the weightedcoefficient factors. The restoration score equal the sum of change ratios of thelogarithm-indicators during the periods of assessment unit, which could denote the effectof an ecological restoration project. By using IRI, we can not only get information on thestatus of an ecosystem, but also quantify long-range variation of the eco-system of a lakefollowing ecological remediation project.