Spatial And Temporal Distribution Of Urea In Typical Coastal Waters And Response Of Akashiwo Sanguinea To Different Nitrogen Sources

In recent years,due to the rapid development of modern agriculture,industry,tourism and mariculture,a large amount of urea has been imported into coastal waters,resulting in the degree of eutrophication increased and algae blooms frequently erupted.In view of the importance of urea nitrogen(Urea-N)in coastal ecosystems,we plan to study the spatial and temporal distribution of Urea-N and its influencing factors in Xiamen Bay,Xinghua Bay and Qinzhou Bay and the response of Akashiwo sanguinea to four nitrogen sources was also explored through culture experiments.The purpose of this study is to understand the spatial and temporal distribution of Urea-N in study area and its effects on the growth and reproduction of phytoplankton.The results are as follows:During the investigation period,the concentration of Urea-N in Xiamen Bay ranged from 0.36 to 6.15 mol·L-1.The concentration of Urea-N in the surface layer was highest in summer and lowest in winter,there is no significant difference between the four seasons(p>0.05,n=16);The concentration of Urea-N in the bottom layer was the highest in spring and the lowest in winter,there is no significant difference between the four seasons(p>0.05,n=16).Except for the bottom layer of Dadeng sea area in spring and the surface layer of southeastern sea area in autumn.High concentration of UreaN always occurs in areas that are significantly affected by land runoff or have poor water exchange,such as the Jiulong River Estuary,Tong'an Bay and the western waters.Low concentration Urea-N always appears in open sea areas,such as the southeast sea area and Dadeng sea area.The plane distribution of Urea-N is high concentration in the inner bay and estuary,and low concentration in the outer bay.Compared with other seasons,Urea-N accounts for the highest proportion of available nitrogen sources in spring.Based on the spatial and temporal distribution of Urea-N and its correlation with environmental factors.The spatial and temporal distribution of Urea-N is affected not only by land runoff and coastal sewage discharge,but also by mariculture,marine biological activities and tidal current.(2)During the investigation period,the average concentration of Urea-N in the surface layer of Jiulong River was 1.77 ?mol·L-1,ranging from 0.43 ?mol·L-1 to 5.00 ?mol·L-1.The horizontal distribution of Urea-N concentration is higher in the upstream than in the estuary,and decreases gradually from the upstream to the estuary.According to the spatial and temporal distribution of Urea-N and its correlation with environmental factors,the source of Urea-N may be located on the upper reaches of the Jiulong River.Agricultural fertilizer application,livestock breeding and domestic sewage discharge along the coast may be factors affecting the spatial and temporal distribution of UreaN in Jiulong River.(3)During the investigation period,the average concentration of Urea-N in the surface layer of Xinghua Bay(1.84 ?mol·L-1)was higher than that in the bottom layer(1.28 ?mol·L-1).The concentration of Urea-N in surface and bottom layer range from 0.59 to 3.39 ?mol·L-1 and 0.39 to 2.04 ?mol·L-1,respectively.The plane distribution of Urea-N concentration in surface and bottom layers showed a trend of low in Northwest and high in Southeast,and gradually increased from northwest to southeast.According to the spatial and temporal distribution of Urea-N and its correlation with environmental factors,industrial and agricultural production,mariculture and livestock farming along the eastern coast of Xinghua Bay may be the main factors affecting the spatial and temporal distribution of Urea-N.The results of 24-hour monitoring shows that in addition to industrial,agricultural and aquaculture activities along the coast,physiological activities of microorganisms,phytoplankton and swimming animals and tidal currents may have a significant impact on the spatial and temporal distribution of Urea-N.(4)During the investigation period,the average concentration of Urea-N in the surface layer of Qinzhou Bay(0.65 ?mol·L-1)was higher than that in the bottom layer(0.50 ?mol·L-1).The range of Urea-N concentration was 0.00~2.66 ?mol·L-1 and 0.00~1.55 ?mol·L-1,respectively.Urea-N concentration high values are located in the inner and outer bay.It shows that Urea-N not only has a high concentration,but also accounts for a high proportion of available nitrogen sources in Qinzhou Bay.According to the characteristics of salinity distribution in Qinzhou Bay and the current situation of aquaculture,industrial and agricultural production,domestic sewage discharge,shrimp culture and inadequate vertical exchange of seawater may be the main factors of affecting the distribution of Urea-N in Qinzhou Bay.(5)The results of culture experiment showed that NO-3-N,NH+4-N,Urea-N and GluN could promote the growth of Akashiwo sanguinea in a certain concentration range,and the growth of Akashiwo sanguinea might be inhibited if the concentration exceeded a certain level.The optimum concentration ranges are 0~500 ?mol·L-1,0~80 ?mol·L-1,0~100 ?mol·L-1,and 0~100 ?mol·L-1,respectively.In this range,the maximum algae density increases with the increase of nitrogen source concentration.According to Fig.21,the growth kinetics of Akashiwo sanguinea in all four experimental groups conformed to the Monod model.When Urea-N was used as a single nitrogen source,the maximum specific growth rate was lower than that of Glu-N and NH+4-N,slightly higher than that of NO-3-N.The minimum Urea-N concentration needed to maintain growth was lower than that of NH+4-N,higher than that of NO-3-N and Glu-N.The affinity of Akashiwo sanguinea to Urea-N was the same as that of NO-3-N,much lower than that of Glu-N and higher than that of NH+4-N.