Abstract: Iron is an essential trace element for the biological requirements of photoplankton.It can be involved in chlorophyll and phycobilin pigment biosynthesis,in many components of photosynthetic (PSI and PSII) and electron transport systems,and in n-itrate assimilation as an enzyme co-factor(nitrate reductase and nitrite reductase).In recent years,many researchers focus on effects of iron on cyanobacterial ecophisiology,but limited experiments have been done.The aim of this study is to find out the relationships between different iron forms and physicochemical factors.From the results we can present some basic evidences for controlling cyanobacterial blooms.Experimental enclosures were used in this study. Three encloseures were devised nearby Lake Dianchi, in which we planted aquatic plants and bred fishes except for 1# enclosure.Aquatic plants were the same in other two enclosures,but the density of fishes was different.In 2# enclosure and 3#enclosure,the density of fishes was 100g/m3 and 75g/m3, respectively.We sampled periodically from three enclosures and the concentrations of total iron,ferrous iron and filtered iron(< 0.45祄)of the water samples were determined.The relationships between the dynamic changes and physical and chemical factors were also analyzed.The results showed that under the conditions of pH7-9 and water temperature 1715-2015 e,photoplankton thrived and uptaked ferrous iron,so the concentrations of ferrous iron decreased highly.Dissolved oxygen,phosphate and dissolved total phosphate had no significant influences on the concentrations of ferrous iron and other iron forms.Under serious cyanobacterial bloom,total iron and dissolved iron had no significant variances,while significant relationship between ferrous and population density of phytoplankton was found(r=-018391,P<0105).Based on the results we can make a conclusion that total iron is not a limiting factor in eutrophic freshwater lakes,but iron bioavailability plays an important role in waterbloom formation and disappearance.