Effects Of Nitrogen And Phosphorus Nutrients On Phytoplankton Community Structure In Jiaozhou Bay

In recent years,the discharge of land-based pollutants has posed a potential threat to the stability of the coastal water environment,and the distribution and content of nitrogen and phosphorus nutrients in seawater have also changed greatly,which can be directly or indirectly utilized by Marine organisms.Phytoplankton,as the most important primary producer in the Marine ecosystem,mainly uses nitrogen and phosphorus nutrients to carry out life activities,and plays a key role in the material cycle and energy transfer of the Marine ecosystem.Due to its poor hydrodynamic capacity and weak self-purification capacity,the semi-closed gulf is prone to eutrophication,which will lead to changes in the balance of the ecosystem,such as changes in the structure of phytoplankton community.As a typical semi-closed bay,it is of great significance to study the effects of nitrogen and phosphorus changes on phytoplankton community structure in Jiaozhou Bay.Current studies have focused on the effects of specific forms of nitrogen and phosphorus nutrients on phytoplankton community structure,while the effects of complex nitrogen sources from seat-flowing rivers on coastal phytoplankton community structure are still unknown.Therefore,in this study,the dominant phytoplankton species in Jiaozhou Bay,Skeletonema costatum and Pseudo-nitzschia multiseries,were selected as the research objects for indoor simulation experiments.Using Na NO₃ as the nitrogen source,the effects of different N/P ratios on the growth and succession of the two species of algae were studied.Using Na NO₃,NH₄Cl and CO（NH₂）₂ as nitrogen sources,the effects of different nitrogen sources on the growth and succession of these two species of algae were studied.Jiaozhou Bay was selected as the study area to conduct the field nitrogen source enrichment culture experiment.The effects of different nitrogen sources on the phytoplankton community structure and pigment concentration in Jiaozhou Bay were studied by using the water of Licun river,the water of Moshui river,Sewage from the outfall,Na NO₃,NH₄Cl and CO（NH₂）₂ as nitrogen sources,in order to provide a theoretical basis for Marine ecological environment protection.The main results are as follows:1.The results of laboratory simulation showed that the optimum N/P ratio was 32for Skeletonema costatum and 16 for Pseudo-nitzschia multiserie.Under the separate and co-culture conditions with different nitrogen and phosphorus ratios,the maximum growth rate of Pseudo-nitzschia multiserie was significantly lower than that of Skeletonema costatum,which had a strong competitive advantage.Compared with the experimental group without nitrogen source,the addition of nitrogen source could significantly increase the Fv/Fm of Skeletonema costatum and Pseudo-nitzschia multiseries.2.When the N/P ratio was 32,Na NO₃,NH₄Cl and CO（NH₂）₂ were used as nitrogen sources,and CO（NH₂）₂ had the most obvious promotion effect on the growth of Skeletonema costatum and Pseudo-nitzschia multiseries.The promotion effect of single nitrogen source on the growth of Skeletonema costatum and Pseudo-nitzschia multiseries was as follows:CO（NH₂）₂>NH₄Cl>Na NO₃.Under the separate and co-culture conditions of different N sources,the maximum growth rate of Pseudo-nitzschia multiseries was significantly lower than that of Skeletonema costatum,indicating that Skeletonema costatum had a strong competitive advantage.3.A total of 69 taxa which belong to 52 genera of 4 phyla were identified in Jiaozhou Bay.Diatoms accounted for about 28.53%of the phytoplankton species abundance and dinoflagellates accounted for 70.11%of the phytoplankton species abundance.The cell abundance of dinoflagellates was significantly higher than that of diatoms.The cell abundance of Carunella mirii in dinoflagellates was the highest,about 17.61×10² cells/L.The cell abundance of Alexandria tamarensis was the next highest,about 8.56×10² cells/L.4.CHEMTAX results showed that the phytoplankton communities in the waters of Jiaozhou Bay were mainly divided into four phyla:dinoflagellates,diatoms,chlorophyta,and chlorophyta,accounting for 85.55%of the total biomass,and dinoflagellates were the main contributors of Chl a biomass,accounting for about 46.26%.The contribution ratio of dinoflagellates and diatoms to Chl a biomass increased after the enrichment of different nitrogen sources in Jiaozhou Bay,accounting for more than half of Chl a biomass.The contribution proportion of diatoms to Chl a biomass increased significantly.5.Nitrogen enrichment in the sea water of Jiaozhou Bay showed the most obvious promoting effect on the growth of Diatom cells.Skeletonema costatum and Leptocylindrus danicus can effectively use Na NO₃,NH₄Cl,CO（NH₂）₂ and the complex nitrogen sources in river water as nitrogen sources to maintain growth and have competitive advantages.Na NO₃ and CO（NH₂）₂ as nitrogen sources promoted the cell abundance of Pseudo-nitzschia multiseries most obviously.In the single nitrogen source,CO（NH₂）₂ has the most obvious promoting effect on the growth of phytoplankton,indicating that organic nitrogen has a greater promoting effect on the growth of phytoplankton than inorganic nitrogen.6.The linear regression analysis between the results of microscope and CHEMTAX showed that there was a significant positive correlation between the abundance of diatom cells and the biomass of Chl a in the culture system with different nitrogen sources（r=0.674,P<0.01）.The cell abundance of dinoflagellates was positively correlated with the biomass of Chl a（r=0.656,P<0.01）.In conclusion,the enrichment of nitrogen and phosphorus nutrients can promote the rapid reproduction of phytoplankton,and the promotion effect of organic nitrogen on the growth of phytoplankton is greater than that of inorganic nitrogen,and diatoms in phytoplankton have a strong competitive advantage.