Study On Nitrogen Source Tracing Using Stable Isotopes Of Macroalgae In Intertidal Zone

As an important primary producer in the marine intertidal ecosystem,macroalgae has important economic and research value.Stable isotopes have the ability to integrate long-term geochemical changes and connect different system components.They are widely used in biology,soil,environment,ecology and other fields.The chemical method of direct detection of seawater can not reveal the land and ocean source of nitrogen,unable to reveal bioavailable concentration.In addtion,the impact of nitrogen nutrient input on the ecosystem cannot be accurately evaluated when seawater nutrients are diluted or absorbed in large quantities,Biological samples can better record the changes of environment for a long time,and have obvious advantages compared with non biological samples?¹⁵N value is an effective method to study the source of n.In this paper,two intertidal zones,Heishi reef and xiaochangshan Island,were selected for investigation,and the contents of TC and TN,C/N value and?¹⁵N value was analyzed,and the physical and chemical factors and nutrients of intertidal seawater were measured.In order to provide reference for the study of human activity N nutrient input in nearshore water,the specific results are as follows:(1)The variation range of seawater temperature in the intertidal zone of Heishijiao is 6.52?28.96?;The variation range of salinity is 28.85?31.95;The pH range is 7.91?8.13.The concentration of nutrients in the intertidal zone is higher in autumn and winter(June?November)than in spring and summer(December?May).The average concentration range of PO₄-P at the five sampling points is 0.014±0.01mg/L?0.039±0.033mg/L.The concentration of PO₄-P at S2sampling point is the highest.The range of DIN concentration in intertidal seawater is0.045mg/L?0.588mg/L,and the relationship between DIN concentration at each sampling point is S5>S4>S1>S2>S3.The order of Don concentration in S1?S5seawater from high to low is S2>S5>S1>S3>S4.A total of 56 species of macroalgae(33 species of red algae,11 species of brown algae and 12 species of green algae)were collected in the intertidal zone of Heishijiao.The species of macroalgae at S2sampling point are the least,and the species of macroalgae at S3 sampling point are the most abundant.There are many kinds of macroalgae in spring and autumn.The water content of brown algae is higher than that of green algae and red algae.The variation range of TC of macroalgae in the intertidal zone of Heishijiao is20.16±2.14%?40.53±0.06%.The TC content of brown algae and green algae is greater than that of red algae.The TC content of macroalgae in summer(June?August)is lower than that in other months.The TN content of macroalgae varied from2.53±0.6%to 8.32±0.17%.The TN content of macroalgae in April,may,June and July was lower than that in other months.The TN content of brown algae is lower than that of red algae and green algae.The C/N range of macroalgae is4.42±0.35?10.2±1.07,and the average value is 6.65±1.1.The variation range of?¹⁵N value is 6.47±0.08‰?27.55±0.03‰,among which red algae The?¹⁵N value is less than that of brown algae and green algae,and the latter has little difference.Of macroalgae at S5 sampling point The?¹⁵N value was the highest,and the growth rate of macroalgae in summer and autumn was the highest?¹⁵N was higher than that in winter and spring.(2)The variation range of seawater temperature in the intertidal zone of xiaochangshan island is 6.80?21.00?;The salinity range is 30.00?31.95;The pH range is 8.03?8.18.The concentration range of PO₄-P is 0.009?0.14mg/L,the concentration range of DIN is 0.081?0.143mg/L,and the concentration range of DON is 0.014?0.494mg/L.There are 31 species of macroalgae and 15 species of small algae in the intertidal zone.The largest number of macroalgae species was collected in June,and red algae accounted for the majority of macroalgae collected every month.The order of TC and TN contents of macroalgae each month is March>October>June>December and June>October>December>March,respectively.The relationship between the C/N value of macroalgae in four months is March>December>October>June,and the C/N value of macroalgae in December The?¹⁵N value is the lowest,higher in March,June and October,and the difference is small.In December,March and June,the TC content of red algae and green algae was higher than that of brown algae.(3)Effects of season and location on TC and TN contents,C/N and The influence of?¹⁵N value were significant.The contents of TC and TN,C/N and?¹⁵N values were compared and the results showed that,the C/N value of macroalgae ranged from 6.38±1.88 to 7.28±2.30,indicating that the N sources at each sampling point in each season was sufficient.TN content is higher,C/N value is lower,and the macroalgae with high?¹⁵N value includes Ulva pertusa,Ahnfeltiopsis flabelliformis,Undaria pinnatifida and Chondrus nipponicus.The contents of TC and TN,C/N and?¹⁵N value and seawater nutrient content were analyzed by regression.The results showed that The R² values of?¹⁵N and the contents of NO₃-N,NO₂-N,NH₄-N,DIN and DON in seawater were higher(0.324,0.277,0.310,0.364 and 0.218,respectively)The R² values of?¹⁵N value and seawater NH₄-N and DIN content regression analysis reached 0.319 and 0.264 respectively.In the tissue of Mazzaella japonica The R²values of?¹⁵N value and seawater NH₄-N and DIN content regression analysis reached 0.213 and 0.203 respectively.Based on the above results,Ulva pertusa and Ahnfeltiopsis flabelliformis were selected as indicator species of Heishi reef and xiaochangshan island.(4)By comparing the C/N value and The analysis of?¹⁵N value combined with the composition of seawater nutrients judged the input of nitrogen sources at different sampling points.The results show that the nitrogen source in the intertidal zone of Heishijiao is sufficient and is affected by aquaculture sewage,domestic sewage and excreta.The S1 sampling point of Heishijiao is affected by aquaculture sewage or domestic sewage,and the pollution is more serious in summer and autumn.The seawater at S2 sampling point may be greatly affected by human domestic sewage in spring,autumn and winter,and the nitrogen source in summer may be domestic sewage and excreta.S3 sampling point may be affected by human domestic sewage in winter and spring,and domestic sewage,excreta or agricultural fertilizer in summer and autumn.S4 sampling point is affected by domestic sewage throughout the year,and may also be affected by excreta in autumn.S5 sampling point may be affected by domestic sewage in winter and spring,and by domestic sewage or excreta in summer and autumn.The intertidal mining in xiaochangshan island may be affected by a low degree of domestic sewage and excreta.