The Role Of Dissolved Phosphatase And Microbial Activity In The Process Of Lake Eutrophication

In this thesis, temporal and spatial variations in activity and stability of dissolved alkaline phosphatase (DAP) and microbial activity in the lakes and ponds with the different trophic levels were described in the middle and lower reaches of the Yangtze River Catchment, the roles of DAP and microbes in the process of eutrophication were also explored. The horizontal, vertical and temporal heterogeneities in DAP were observed in more than ten lakes, indicating its specific role in P cycling. Higher alkaline phosphatase activity (APA) with different kinetic characteristics and anoxic status, induced by organic matter in overlying, interstitial water and sediment, accelerated the release of bioavailable phosphorus and initiated bloom in the suburb ponds of Wuhan. Hence, high APA was closely related with the occurrence of bloom. The study on stability of DAP is difficult because of its lability. Introduction of micellar enzymological method can effectively avoid the microbial disturbance when studying the activity and stability of DAP. This method was experimentally proven to be feasible in studies on ecological enzymology of lakes. In term of the kinetics and responding modes to varying temperature, pH and inhibitors in micelle, DAP showed the similarity in surface and interstitial water of Lake Donghu, which was markedly different from that in overlying water, probably due to difference in the origin of the enzyme. In the lakes and some hypolimnion with higher trophic levels, DAP showed higher activity and stability. The variations in DAP stability in micelle, as affected by different W0, temperature and pH conditions, reflected the characteristics of molecular size, conformation and active residues of the enzyme, suggesting the existence of isozymes in different lakes. With higher activity and stability, it could accelerate the eutrophication by stimulating the release of internal phosphorus loading. The APA in bacterial sizes fraction was usually very lower in all lake studied, while the algae one accounted for the majority of total APA. On the other hand, the shape of bacteria was more diversiform in sediments than in water column. Free-living bacteria were dominant in water column and sediment, while the attached one was more abundant in sediment than in water column. Bacteria attached to larger particles, even with less abundance, may show significantly higher APA than that given by the dominant free-living bacteria in water column. In Lake Taihu, the undredged zone had the eutrophic symptoms. The activity of DAP was closely related with the degree of eutrophication. DAP with higher activity and stability, coupled with the mobility of aerobic, inorganic and organic bacteria, promoted the formation and fluxes of the internal loading, supporting the excess growth of algae (including the microcystic bloom). The changes in content of organic matter caused by dredging influenced the growth of different microbial functional groups, consequently regulating the nutrient level of water column. An observation on the microbial morphology indicated a sharp decrease of bacilliform bacteria in water column and sediments after dredging. Dredging removed a great deal of attached bacteria, resulting in decreases of the microbial decomposition and nutrient releases. This provided an explanation for the lower nutrient level in water column shortly after dredging. However, microbial community, in terms of shapes, living states and composition would recover by some time after dredging, which would determine its ultimate effectiveness.In short, activity and stability of DAP and microbial community and activity play a crucial role in the process of lake eutrophication.