Latitudinal Chemical And Hydrogen Isotope Profiles Of Fatty Acids In Intertidal Sediments Along Chinese East And South Coasts

The intertidal zone,as part of coastal wetland,is a special and complex ecosystem,an important channel for the interaction between the sea and the land,and also a transitional zone where the terrestrial and aquatic ecosystems intersects.It has great ecological and resource values.Due to frequent material exchange with the surrounding complex ecosystem,the intertidal zone constantly and rapidly experiences a series of biochemical,geochemical and physical processes that simultaneously control the organic matter and metals in this area.Understanding of the mechanism of occurrence,migration and transformation of organic matter(and metals)in the intertidal zones has gradually become a research hotspot in the past two decades.Although significant progress has been made in understanding the dynamics of organic matter in the intertidal zones,the factors governing these processes are still unclear.Past work has usually focused on an individual intertidal zone and little attempt has been made to build an integrated picture of the spatial,temporal and tidal variations of the organic matter and metals over long lengths of coastline.Such information is necessary for the development of coordinated and effective management strategies of these intertidal zones to protect their ecology.It is therefore the purpose of this study to investigate the organic matter and metal distribution in the sediments of 14 intertidal zones distributed longitudinally along the Chinese east and south coasts.A total of 84 samples representing 14 intertidal zones(with a latitudinal span over 20 degrees),two seasons(summer and winter)and three tidal levels(high,mid and low)were collected and analyzed for bulk organic carbon content(TOC),bulk organic nitrogen content(TON),TOC carbon composition isotope composition(TOC-?13C),TOC nitrogen isotope composition(TON-?15N),carbon-nitrogen molar ratio(C/N)and sediment grain size.Together with the paired hydrogen isotope data of fatty acid in sediments,the results were used to make inference about the contributors of intertidal organic matter and the causes of seasonal changes in their relative contributions along latitudinal gradient.The main steps and results are as follows:TOC of the studies intertidal zone sediments ranges from 0.06%to 1.35%,with the highest value occurring in the Pearl River and Jiulong River Estuaries,and the lowest value occurring in the Qinhuangdao intertidal zone;The C/N molar ratio of TOC to TON of the studies sediments fell in the range of 6.3-17.3,with the exceptionally value of 104.6 for Yantai(a reflection of extensive aquaculture in this area).The relatively low C/N ratio in most of the studied samples may indicate that the source of sediment organic matter is more likely to be dominated by marine source such as macrobenthos(benthic microalgae);The ?13C of sediment TOC in the intertidal zone ranges from-26.00 mUr to-19.47 mUr,with an average value of-22.73 mUr,indicating that organic matter in the sediments has both marine and terrestrial sources;The high correlation between ?13C of TOC and ?15N of TOC of sediments implied that C and N production and consumption in intertidal zone is mostly coupled and proceeds in a stoichiometric way.The higher ?2H values of fatty acids in winter(relative to summer)sediments in mid-latitudinal zone must be mainly due to larger inter-seasonal seawater temperature difference;the reversed trend observed in high latitudinal zone such as Bohai Bay(semi-closed water body)must be due to a reduced autotrophic contribution from algae while the observed lower ?2H value in winter(relative to summer)in low latitudinal zone is most likely caused by a reduced terrestrial input to the intertidal zone by riverine transport in winter(relative to summer).The abundances of metals V,Cr,Mn,Co,Ni,Cu,Zn,Pb,As,Fe in the studied sediments in most cases highly correlated.The highly correlated metal concentrations in the sediments indicate that life processes in the intertidal zone pose minimal impact on the concentrations in the sediments.The concentrations of metals in the sediments are mostly a result of partition between sediments and waters governed by simple physicochemical principles.