Using Stable Carbon And Nitrogen Isotopes To Trace The Evolution Of Eco-environment At Xisha Islands,South China Sea

Global changes have had an impact on all the regions and different ecosystems in the world.Coral islands,located in tropics at low latitudes,respond sensitively to climate change and human activities.Studies on the evolution of ecological environment of the islands will help us better predict the fate of the ecological environment and protect the fragile ecosystems.Xisha Islands,most of which belong to coral islands,are famous bird islands in the South China Sea.Previous studies had reconstructed the historical records of seabird population size in some of the islands over the past 2000 years.The key role of seabirds had been identified in the evolution process of coral island ecosystems,however,those possibly caused by seabird population change,e.g.,seabirds' foraging,and biogeochemical cycle of nutrients in the islands,remain unclear.For these issues,stable isotope analyses have been widely used in many other places in the world.Samples used here mainly include coral sand ornithogenic sediments,soils,and plants collected from Yongle Islands and Qilian Yu during scientific expeditions at the Xisha Islands.A large number of biological residues,such as guano pellets,bird and fish bones,bird eggshell and fish scales were picked from the sediments.Modern biological samples such as seabirds,flying fish,and squid were also collected.Principles and methods of stable isotope geochemistry were primarily used in this study,stable carbon and nitrogen isotope analyses were conducted,and novel methods including compound-specific isotope analyses of amino acids,ammonia,nitrate were introduced,with the addition of 210Pb and AMS14C dating and analyses of basic physical and chemical properties.From the perspective of carbon and nitrogen biogeochemical cycles,the ecological environment evolution process of Xisha coral islands is analyzed.The main findings were as follows:1.Dietary changes in seabirds at the Xisha Islands during the past millennium?¹³C and ?¹⁵N analyses of modern flying fish muscle and scale samples show that there is a good correlation between isotopes of fish muscle and scales,and thus fish scale can be used as an alternative material for fish muscle to conduct stable isotope analyses.The main influencing factors of ?¹³C and ?¹⁵N in flying fish are different.A good positive correlation is found between fish ?¹⁵N and mass.The main influencing factor of ?¹³C is the living area characteristics of flying fish,and for those in the waters around Xisha,it is the distance between living area and islands.Flying fish muscle isotope values could be estimated based on the functional relationship between muscle and scale isotopes and fish scale isotope values.In addition,stable isotope analyses of different tissues from ancient and modern flying fish and seabirds show that the isotopes are tissue-specific,which can be attributed to several factors such as different metabolic rates,differences in amino acid composition,this needs to be considerred when using isotopes to indicate diet and trophic position of organisms.Different species of fish have specificities in the morphology and structure of scales,which can be used to identify species,providing a basis for the study of paleo-ecology of seabirds and flying fish using ancient flying fish scale samples.There is no obvious correlation between ?¹⁵N and ?¹³C for modern flying fish,quite different with that for historical flying fish scale samples from the sediments of Qilian Yu?Zhaoshu,Beidao and Nandao Islands?,and there is a significant negative correlation between ?¹⁵N and ?¹³C in historical flying fish scales,attributed to the spawning habits of flying fish.Flying fish spawn in the offshore areas and swim farther away when they grow up.Organism ?¹⁵N increases with age or mass and environmental background ?¹³C decreases with the increase of distance with the islands,so smaller flying fish have lower ?¹⁵N and higher ?¹³C values,while larger flying fish have higher ?¹⁵N and lower ?¹³C values,and tropical seabirds could prey both small and large flying fish.Tropical seabirds bring nutrients to barren coral islands and play a key role in the evolution of the ecological environment in the Xisha Islands.For the typical Xisha bird islands,there were more seabirds in the Little Ice Age?LIA?and less in the Medieval Warm Period?MWP?.The results of ?¹⁵N mixture model at Guangjin Island show that the food composition of seabirds is also related to the number of seabirds.There are more seabirds in the LIA and they tended to prey more squid.In the MWP,there were fewer seabirds and their food is dominated by flying fish and supplemented by squid,as at present.The seabird population size at Nandao Island overall remained at a peak during the LIA,but there were fluctuations.Seabirds might migrate to adjacent island and made seabird polulation in Nandao suddenly decrease,when the seabirds there mainly fed on near-shore smaller flying fish with a low ?¹⁵N and high ?¹³C.However,they must fly to distant seas to catch larger flying fish when there are more seabirds in Qilian Yu.At this time,there is an Ashmole's Halo with fewer flying fish around Qilian Yu.Besides,compound-specific analyses of isotopes in amino acids show that a single nitrogen isotope trophic discrimination factor method can be used to calculate the trophic level of flying fish,but not for seabirds.The multi-nitrogen isotope trophic discrimination factor method derived from penguin feeding experiments is an effective method to calculate the trophic position of tropical seabirds in Xisha,and the results are very close to those based on their diets.Compared with seabird bulk ?¹⁵N,the analyses of amino acid isotopes not only can indicate trophic position,but also can quantify.2.Changes of ?¹³C in sediments and evolution of island ecological environmentStable carbon isotope analyses of guano pellets and plant residues from the sediments in Guangjin and Jinqing Islands show that the two have significantly different organic ?¹³C values,which is relatively stable after burial.The main components of coral sand ornithogenic sediments are coral sands,guano,and plant residues.Among them,coral sands contain almost no organic matter?OM?,so the OM of sediments is only from guano and plant debris.The ?¹³C end-member method can quantitatively analyze the proportions of the two OM sources in the sediment samples,which can be used to analyze the evolution process of the islands'ecological environment.Seabirds landed the islands 800-600 years ago and the input of seabird guano brought a lot of nitrogen,phosphorus and other nutrients to the islands.Plants began to develop properously on the islands in the past 200-100 years ago.However,the number of seabirds in most of the islands at Xisha had decreased rapidly over the past 100 years.This result is consistent with those from the bio-element geochemical method and near-infrared reflectance spectroscopy,proving the effectiveness of the ?¹³C end-member analysis.In addition,the method can also be used to analyze the source of OM of offshore marine sediments,the results indicate that the input of terrestrial OM on marine sediments in the Wanquan River estuary and the eastern continental Hainan Island has decreased over the past half century,mainly due to the increase of human activities and decreases of runoff and sediment discharges from the Wanquan River.3,Soil-plant nitrogen cycle during the evolution of coral island ecosystemThere are a group of coral islands at different degrees of development in Qilian Yu Islands,including barren Xishazhou,Beishazhou,Zhongshazhou;Zhaoshu,Beidao,Zhongdao and Nandao Islands covered by dense shrubbery,and Nanshazhou between the two.The results of carbon and nitrogen content and isotope analyses of soils,plants and guano show that sparse and low vegetation and corresponding soils on barren coral islands are N-limited before a large number of seabirds landed,the main nitrogen source of islands is atmospheric deposition.With the increase of seabird guano input,a large amount of vegetation began to grow.In addition to a small amount of vegetation on the margin of the coral islands,the nitrogen in the plants and soil in the central areas of the islands become sufficient,and the nitrogen source become seabird input.The results of ammonia and nitrate isotope analyses indicate that plants use more nitrate in the soil,due to the higher nitrate content,when the islands' nitrogen source is mainly atmospheric deposition,and plants mainly utilize ammonium in the soil when the nitrogen source is mainly guano input.In a word,seabird guano inputs have greatly changed the nitrogen cycle of soil-plants system in the Xisha coral islands,and the core role of seabirds in the evolution of the islands' ecological environment was proved once again.4.Carbon and nitrogen losses and isotopic characteristics of sediment and guanoThe input of seabird guano had brought a large number of nutrients to the islands,it is necessary to evaluate the material loss after guano inputs,a preliminary analysis is conducted in this study.After guano is buried,its nutrients can be used by plants or mixed with soil and thus to participate in the nutrient cycle in the islands.This part of nutrients would loss from the guano but not the islands.The loss of carbon and nitrogen is also accompanied by isotope fractionation,especially for nitrogen,?¹⁵N in guano gradually decreases with time and depth for several profiles.The variation of ?¹⁵N in different inorganic forms is complex.Overall,the proportion of inorganic nitrogen is very low,and nitrogen with higher ?¹⁵N is preferentially discharged from guano.The rate of elemental loss from guano in different islands varies,and depends on the specific environment.Nutrients from guano might also be lost from the soil-plant system of islands,which needs more data to quantify.Considering the core role of guano input in the evolution of the islands' ecological environment,it is very important to fully assess the loss of nitrogen and other nutrients from the guano in the sediments of the islands,and more related researches are needed.