Dynamics And Bioavailability Of Dissolved Organic Matter In Zhangjiang Estuary-Mangrove System

Mangrove ecosystem is one of the three major blue carbon ecosystems with high productivity,and its carbon storage capacity is strong,so the carbon cycle process of mangrove has become a hot field in recent years.The lateral transmission of dissolved organic matter(DOM)of mangrove is the link of carbon cycle between the intertidal wetland,the estuarine and coastal waters.In recent decades,the anthropogenic impacts on mangrove DOM is very significant,but there are few studies on that.In this paper,we carried out studies that focused on a strongly impacted by human activities Zhangjiang estuary-mangrove system,using UV visible absorption spectrum,three-dimensional fluorescence spectrum and dissolved organic carbon(DOC)as the research means,our objectives were 1)to analyze the uniqueness,the origin and sink process,the seasonal variation of the DOM in Zhangjiang estuary-mangrove system,2)to know the DOM response to the tide and human influence,3)to investigate the DOM bioavailability of estuary-mangrove system and its stimulating factors in the region.The following conclusions were drawn:(1)In Zhangjiang Estuary,the main source of DOM is Zhangjiang river runoff,biological production and surrounding tidal flat.DOM in mangrove Creek mainly comes from mangrove forest.The seasonal variation characteristics of DOM in Zhangjiang estuary and tidal creek are characterized by high wet season and dry season.The biological productivity of the Zhangjiang estuary is the main reason for controlling the change of DOM,precipitation is also an important factor causing the increase of the abundance of CDOM in the estuary;and the change of precipitation and litter is the controlling factor of the seasonal variation of the DOM in the mangrove tidal creek.(2)The changes of the DOM quantity and quality of the mangrove creek have obvious characteristics of tidal cycle changes.Compared with the process of flood tide,the concentration of DOC and the abundance of CDOM are higher in the process of ebb tide,and also have the characteristics of high molecular weight,high aromatic degree and high degree of humification.The main factors that regulate the DOM of the mangrove creek are the soil runoff,the gate effluent,the shrimp pond water and the sea water.Based on the significant difference of the DOM properties of the different sources,the ratio of C1(250,330/416 nm)to C3(240/424 nm)can be used to identify signal of the wasterwater in mangrove tidal creek.(3)The proportion of labile DOM in Zhangjiang estuary is 12.3%,which is greatly influenced by the runoff factors of Zhangjiang river,fresh endmenber(33.0%)of DOM bioavailability was higher than the marine endmenber(13.6%),but it reaches the lowest(2.4%)characteristics in the middle salinity region,which were shown "V" taye.After biological incubation,the abundance of CDOM is increased by 20.6%,and the spatial distribution is opposite to BDOC,indicating that the biological metabolic mechanism was different in different abundance of quantity of labile DOM.In the four fluorescent components,the changes in abundances of C3 were less(1.4%),indicating that C3 were basically not involved in the microbial metabolism,but the abundance of C1,the C2(250,385/476 nm)were increased by 22.8%,15.7%,respectively,and the spatial variation trend of Cl and C2 were similar to that of BDOC.While the C4(<240,280/344 nm)increased by 27.5%which was showed that the biodegradability of the protein components associated with the humic acid structure was limited.At the same time,the average molecular weight of DOM of Zhang iang estuary increased after incubation indicating that the small molecule DOM transformed the large molecule DOM in the metabolic process,and the change amplitude of the fluorescent component structure increased with the increase of salinity,indicating that the sea source microorganism's ability to transform the land source DOM was stronger than that of the terrestrial microorganism.(4)The bioavailability of DOM in the tidal creek is distinct with the variation of the tidal cycle,the BDOC value is about 11.5%,the high tide and low tide are high,and the increase of BDOC during flood periods may be related to the bioavailability of seawater DOM and biological species change.The bioavailability of DOM in the low tide indicates that the DOM of soil runoff can also be utilized by microorganism.After incubation,the abundance of CDOM in the tidal creek increased by 12.4%,indicating that the biological contribution to substance CDOM in the metabolic process.The abundance of fluorescence components of C1,C2,C3 and C4 also showed net production,which increased by 13.3%,7.3%,2.2%and 12.1%respectively.In addition,there are also labile DOM derived from effluent or litter in the tidal creek,such DOM will stimulate the consumption of refractory DOM in the tidal creek.In addition,the average molecular weight of most DOM increased after incubation,but the structure of fluorescent components remained unchanged(<5%).