The Biogeochemical Study On Biogenic Elements In The Coastal Waters Near The Yangma Island

Essential biogenic elements such as oxygen,carbon,nitrogen and phosphorus are the basic substances of material circulation and energy flow in the marine,and play key roles in marine ecosystems.In recent years,due to the rapid advancement of urbanization and rapid growth of aquaculture in coastal areas,a number of environmental problems have emerged,e.g.coastal eutrophication,hypoxia,seawater acidification,red and green tides,which bring a serious threat to the healthy development of the marine ecosystem and causes huge losses to the national economy.It is thus important to explore the biogeochemical cycle of biogenic elements and its main controlling factors,which has important scientific and practical significance to the regional material cycling and environmental evolution and protection.This study focused on the coastal area near the Yangma Island.The concentration,distribution characteristics and the influencing factors of nutrients in the water were analyzed systematically through a continuous survey of two years?2016²017?.In addition,the characteristics of low-oxygen distribution in the bottom water in summer and its influence mechanism was analyzed preliminary,and the carbon cycle process in the water and sedimentary environment was studied.This work aims to enrich the understanding of the biogeochemical processes of typical marine source elements under human disturbance,and provide the scientific basis on the study of regional biogeochemical processes and land-sea interaction.A series of results and viewpoints were presented as follows:?1?The biogeochemical characteristics of biogenic elements in the coastal waters near the Yangma IslandThe nutrient concentrations in the coastal waters near the Yangma Island were relatively low,and the inorganic nitrogen?DIN?and phosphate(PO₄^3-)values meet the first-class seawater standards.The spatiotemporal variations of nutrients were not obvious due to the influence of complex hydrological and biochemical conditions.For nitrogen compounds,DIN was mainly dominated by NO₃^-,which accounted for 28%⁷4%?mean 58%?during the study period,followed by NH₄⁺,which accounted for21%⁶4%?mean 38%?.According to the potential eutrophication and trophic index?TRIX?assessment model,the nutrition level was characterized in oligotrophic level,and the trophic status was rated at the medium level.It was different from the nutrients,the dissolved oxygen?DO?,inorganic carbon?DIC?,chromophoric dissolved organic matter?CDOM?in the water and the sedimentary organic matter?SOM?exhibited obvious spatiotemporal variations.From May to November,the monthly variation of DO was firstly reduced and then increased with the lowest values appeared in August.In the study area,the hypoxia phenomenon in the bottom water appeared in summer,and the low-oxygen area was mainly distributed in the inshore area.The characteristics are as follows:the low DO characteristics began to emerge from June to July,and a large area of DO<94?mol L^-1 occurred in August,and then disappeared in September.It was similar to DO,the DIC showed a trend of decreasing first and then increasing with the month,with the lowest values appeared in August.From May to November,the study area was the net source of atmospheric CO₂.The flux of seawater to the atmosphere were 7.95±6.94?May?,3.58±3.56?June?,18.98±10.26?July?,12.34±11.85?August?,27.94±21.21?September?and 4.77±2.93 mmol C m^-2 day^-1?November?.However,it was a net sink in March,and the flux of CO₂ absorbed was 3.25±6.04 mmol C m^-2 day^-1.As for the spatial distribution of DIC,it was higher in the nearshore area and decreased towards the offshore area.It was opposite to the seasonal variation of DO,the highest values of DOC,CDOM in the water and the SOM were found in August.Spatially,the CDOM in summer and SOM in autumn were higher in the nearshore area and decreased towards the offshore area.However,the relatively high values of SOM appeared in the offshore areas in summer and spring.?2?The key process for controlling the changes of biogenic elements in the coastal waters near the Yangma IslandThe geochemical characteristics of the biogenic elements?nutrients,DO,DOM,DIC and SOM?in the coastal waters near the Yangma Island were mainly affected by the complex physical and biochemical processes.However,the main controlling factors of different biogenic elements were different.Phytoplankton reproduction,atmospheric deposition,the adjacent sea input,the degradation of organic matter,scallop aquaculture and the exchange between sediment-water interfaces were the main factor affecting its spatiotemporal variations of nutrients.Relatively,the river input contributed less to nutrients.A preliminary estimate of nutrient budgets demonstrated that,the DIN load in the coastal waters near the Yangma Island was mainly from atmospheric deposition and scallop excretion,contributing 49.3%⁶3.5%?mean 56.4%?and 27.4³8.1%?mean 32.8%?of the total influx.Scallop excretion and sediment release were the major source of PO₄^3-,contributing 51.5%⁵4.4%?mean 53.0%?and 23.6%²5.0%of?mean 24.3%?total influx,while the silicate?DSi?mainly came from the sediment release,accounting for 94.7%⁹5.0%?mean94.8%?of total influx.From the perspective of the whole year,about 41.3×10⁶⁷4.7×10⁶ mol?mean 58.0×10⁶ mol?for DIN,4.74×10⁶⁵.03×10⁶ mol?mean4.89×10⁶ mol?for PO₄^3-and 205.5×10⁶²06.3×10⁶ mol?mean 205.9×10⁶ mol?for DSi were converted to other forms?e.g.phytoplankton reproduction and removed by scallop harvest?.As for DO in the bottom water,the formation of the thermohalocline and salinity front was an important condition for the maintenance and development of hypoxia.In this study,the thermohalocline appeared from June to August,and disappeared in September.Spatially,the water-stratified dense area was basically consistent with the distribution of the low-oxygen area.The experiment of DO budget shows that,the aerobic decomposition of the organic matter in the bottom water was the main biochemical factor for the formation of hypoxia,which accounted for 84.6%of the total oxygen consumption,while the sediment oxygen demand contributed to 15.4%of total oxygen consumption.The reproduction of phytoplankton in the surface water in summer was the main source of oxygen-consuming organic matter.The carbonate system in the water was affected by multiple physical and biogeochemical drivers,i.e.the physical mixing of water mass,temperature changes,biological respiration and photosynthesis.In summer?from June to August?,the high primary production in the surface water caused large seasonal decreases in DIC.In comparison,DIC concentrations in the bottom water were higher than those in the surface water,especially in summer,as consequence of marked stratification,restricted mixing and organic matter respiration,and about 42.5?mol kg^-1 in June,62.5?mol kg^-11 in July and 62.4?mol kg^-1 in August of DIC,which accounting for1.91%,2.77%and 2.83%,could come from respiration in the bottom water.The CDOM in the water was mainly from autochthonous and microbial decomposition.In the surface water,phytoplankton was assessed as the contributor to11.6%³5.2%of the CDOM,and 9.0%³7.4%,9.1%³7.4%,7.8%¹8.7%and11.4%¹9.9%of the CDOM were identified as C1,C2,C3 and C4,respectively.In the bottom water,microbial aerobic degradation of sinking biogenic particles is an important source of humic-like components,which contributed 9.1%,18.7%and 48.5%to C3 in July,August and September,and 14.6%,16.3%,18.0%and 26.6%to C4from June to September,respectively.The primary productivity in the surface water and the concentration of DO in the bottom water are the main factors affecting the preservation of SOM.In summer,the high primary productivity in the surface water and hypoxia condition in the bottom water led to high SOM values.In comparison,SOM contents were relatively low due to sufficient DO in the bottom water and relatively low supply of organic matter in autumn and spring.Dissolved nutrients could affect the accumulation of autogenous organic matter by impacting upon primary production.In summer,the NO₃^-in the surface water had the most obvious effects on autogenous organic carbon?AOC?and may be the principal factor of limiting the growth of phytoplankton.In autumn,the NO₃^-as well as DSi had more effects on AOC storage.However,PO₄^3-had the most obvious influence on AOC storage in spring.