Spatial And Temporal Variations Of Dissolved Organic Nitrogen In Typical Seas And Their Effects On Phytoplankton Communities

Dissolved organic nitrogen(DON)plays an important role in coastal waters and oceanic nitrogen cycle.Based on the relatively weak problem of DON research in Xiamen sea area and the western Pacific Ocean,In this paper,DON research was carried out in Xiamen sea area in spring,summer and winter,and DON research was carried out in the western Pacific Oceanin the summer of 2016.The temporal and spatial distribution of the DON from the coastal waters to the open ocean and its possible influencing factors are systematically discussed.The effects of DON and Urea-N and different nitrogen source structure on the growth of phytoplankton were studied by field investigation and simulation.It has the scientific research foundation to explore the global marine environment change and its ecological effect,the long-term evolution of harmful red tide and its outbreak mechanism,and has more scientific significance in the law of ocean nitrogen cycle and nitrogen nutrition ecology.The main results are as follows:(1)The DON concentration in Xiamen sea area is the seasonal variation of summer> spring> winter.The concentration of DON in spring and summer is higher than that in Inner Bay and estuary,In winter,the estuaries and inner bay areas are higher than those of the outer sea.DON is an important component of TDN in Xiamen sea area.The DON / TDN mean is summer> spring = winter,DON / TDN is the distribution of the outer sea than the estuary and inner bay area.The average DON/ TDN of the outer sea in spring and summer is larger than that in Inner Bay and estuary.In winter,the distribution of estuarine and inner bay is larger than that of the outer sea.The distribution of DON concentration in the spring area is more complicated by the environmental factors.The summer is mainly affected by the generation or decomposition of the sea source.The decomposition of the organic matter in the Jiulong River and the decomposition of the organic matter in the seaarea is an important factor affecting the DON concentration in winter.(2)Urea-N is an important component of DON in Xiamen sea area.The Urea-N concentration is the seasonal variation of summer>winter> spring,and the Urea-N concentration in the estuary and the inner bay is higher than that of the outer sea.The seasonal variation of the Urea-N / DON mean is summer> winter> spring.In spring and summer,the Urea-N / DON mean in the estuarine and the inner bay is larger than the outer sea,and in winter,the distribution of the inner bay area is greater than that of the estuary and the outer sea.The distribution of Urea-N in spring waters may be mainly affected by the decomposition of suspended organic matter and the absorption and utilization of phytoplankton.The land source input of Jiulong River is one of the main sources of Urea-N in sea area.Water temperature is the main factor affecting the distribution of Urea-N concentration in summer waters.The concentration distribution of Urea-N in winter waters is mainly affected by the decomposition of suspension of organic matter and the input of land source in Jiulong River.(3)DON is the main component of TDN in the western Pacific Ocean,and the concentration of DON in the western Pacific Ocean is significantly lower than that in the summer of Xiamen sea area.The distribution of DON concentration is South of Taiwan and its adjacent waters>East China Sea> Japan South Sea> Japan East Sea.DON / TDN mean distribution of South of Taiwan and its adjacent waters> Japan South Sea> Japan East Sea>East China Sea.The distribution of DON concentration in the East China Sea is mainly due to the input of the sea source.The decomposition of organic matter may be one of the sources of DON.The decomposition of organic matter may be an important source of DON in the waters Japan South Sea.The distribution of DON concentration in the Japan East Sea is influenced by the input of the cold end in the northern part of the section,and the decomposition of organic matter may be one of the sources of DON in the sea area.The distribution of DONconcentration in the south of Taiwan and its adjacent waters is more complicated by the environmental factors.The decomposition of DON may be one of the reasons for the distribution of DON concentration in the sea area.(4)The distribution of Urea-N and DON concentrations has an important effect on phytoplankton community structure.The responses of dinoflagellates and diatoms phytoplankton(water mining and net mining)to Urea-N and DON were different.Dinoflagellates phytoplankton may release DON while absorbing Urea-N,while diatoms phytoplankton may release Urea-N when absorbing other components of DON,The effects of Urea-N and DON on net mining phytoplankton species and abundance were more significant than water mining phytoplankton.Urea-N had a certain effect on the distribution of Pico-phytoplankton,Nano-phytoplankton and Micro-phytoplankton.while DON had no significant effect.Pico-phytoplankton and Micro-phytoplankton may release Urea-N in their life activities,while Nano-phytoplankton may absorb Urea-N as a nitrogen source.(5)The waters Japan South Sea belong to the nitrogen restricted area.NO3--N and Urea-N can promote the growth of phytoplankton as a single or mixed nitrogen source.and low concentration of Urea-N has a certain effect on the growth of phytoplankton.High concentration of Urea-N is not conducive to the growth of diatomaceous phytoplankton.In the mixed nitrogen source culture group,Urea-N may be a supplementary nitrogen source,and in the presence of Urea-N and NO3--N,the oceanic phytoplankton may preferentially absorbNO3--N as nitrogen Source,in the process of phytoplankton growth may exist the release of DON phenomenon.