| Being widely applied in agricultural production, rare earth elements (REE) accumulated in natural water with the time passing by. Algae, as the primary producer, not only are of great importance in biogeochemical cycle and energy circulation of ecosystem, but also show great sensitivity toward outside incitation. As a result, it is very necessary for us to make research into the influence of REE upon alga-growth-mechanism and ecosystem, and illuminate the impact of REE on the whole aquatic ecosystem and eutrophication of the water body.In this paper, algal growth potential tests (AGP) in which parameters such as chl a. contents, optical density and cell density were analyzed and compared were conducted. In different culture system, two kinds of REE (lanthanum La and neodymium Nd) were investigated on their effect upon the multiplication and competition of two common phytoplanktons in fresh water (Scenedesmus obiquus and Microcystis aeruginosa). With the guidance of the results, scientific evidence was provided for agricultural production, aquatic ecosystem protection, water body eutrophication deceleration or prevention.It was indicated by the AGP results that under middle-low concentration (≤1.0000mg/L), lanthanum and neodymium did not show obvious acceleration or deceleration effect on multiplication of Scenedesmus obiquus. However, intense restraining and poisoning impact was detected under high lanthanum or neodymium concentration (10.0000mg/L).Lanthanum under low concentration (≤0.1000mg/L) slightly accelerate the multiplication of Microcystis aeruginosa and the accelerating influence showed positive correlation with the concentration. Neodymium with low concentration slightly promoted the multiplication of Microcystis aeruginosa, but the impact changed into slightly restraint with the concentrationincreasing. Both lanthanum and neodymium in high concentration (≥1.0000mg/L) exhibited intensively restraining and poisoning effect upon the multiplication of Microcystis aeruginosa. Under middle-low lanthanum concentration, Scenedesmus obiquus held obvious competition predominance in the two co-culture systems where the initial ratio between Scenedesmus obiquus and Microcystis aeruginosa were 1:1 and 1:10.It was shown by further investigation that Microcystis aeruginosa had stronger tolerance ability against low-concentration REE than that of Scenedesmus obiquus. At the same time, Scenedesmus obiquus apparently showed stronger tolerance ability against middle-concentration REE (around 1.0000mg/L) than that of Microcystis aeruginosa.There was no obvious difference between the influences of lanthanum and neodymium upon the alga multiplication, and the effect of lanthanum was appreciably stronger than that of neodymium.Under given lanthanum concentration range (≤ 1.0000mg/L) and competition of other algae, the alga multiplication rate increased with the augment of lanthanum concentration and strengthening of competition.The results could be employed as scientific basis for proper and safe application of REE in agricultural production, and provide reasonable evidence to prevent the poisoning of over-dosed REE on aquatic ecosystem, and offer a new train of thoughts in preventing and decelerating eutrophication in water body.
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