Studies On The Formation Mechanism Of Diatom Blooms In Hanjiang River From Hydrodynamics And Nutrition

Diatom blooms in Hanjiang River have been caused significant ecological and social concerns. However, there are limited reports about its formation mechanism. We isolated several key diatoms from Hanjiang River in order to start a series of laboratory experiments from the view of hydrodynamics and nutrition, systematically researched the physiological and biochemical responses to phosphorous (P) in 3 diatoms under different flowing rates, and compared them with species isolated from other water body. Our results may contribute to further understanding the mechanism of diatom blooming formation in Hanjiang River and provide the significant theory basis for diatom blooming prediction. The main contents and results of this study as following:(1) Isolation, purification and identification were started in 4 key diatoms species. We isolated several diatom species from Hanjiang River and Guanqiao Pond for our experimental study and analyzed their morphology combined with 18S rDNA sequence to classify the diatom species. The result showed that the diatom from Hanjiang River were Cyclotella atomus, Nitzschia palea and Fragilaria crotonensis, and the diatom from Guanqiao Pond was Cyclotella meneghiniana.(2) We comparably studied the effects of hydrodynamics on the growth and phosphorous (P) strategy of riverine diatoms C. atomus, N. palea and lake-derived diatom C. meneghiniana. The result showed that the small-scale turbulence (0.38 m/s) can improve the growth, P uptake and photosynthesis of riverine diatoms, but contrarily in lake-derived diatom. Under the standing condition (0 m/s), the lake-derived diatom has the better growth and P uptake than that under turbulence.(3) The effect of different flow rates (0-0.57 m/s) on the survival and P strategies of three preponderant diatoms, F. crotonensis, N. palea and C. meneghiniana were systematically studied. The results showed that these 3 diatom species have respective growth characteristic under different hydrodynamics regime, and they all expressed the advantages in growth and photosynthesis under certain flowing rate. Compared with other flowing rates, we conclude the specific survival strategies of these 3 diatom species:F. crotonensis and N. palea can adapt well to the turbulence and P fluctuation condition while C. meneghiniana favor the standing and replete P condition.(4) In order to study the succession of preponderant phytoplankton in rivers, the effects of different hydrodynamics regime on the growth and P strategy of green alga Scendesmus obliquus were studied. The results showed that high and light turbulence can both greatly inhibited the growth and chlorophyll a content of S. obliquus; the affinity for P in S. obliquus under turbulent condition was lower than that under standing conditions and the alkaline phosphatase activity (APA) can increase sharply by high-level turbulence. Moreover, the growth and physiological traits of S. obliquus significantly decline by P-limitation under standing condition. Therefore, with the increase of P concentration, succession from diatoms bloom to green algae bloom may occur under low river flow.