Geochemical Characteristics And Environmental Indications Of Some Chemical Parameters From Coastal Zone To Deep Sea

The sea areas of China are diverse and have complex ecological environments. Due to the influence of anthropogenic activities, the ecosystem of the Bohai Sea is being rapidly degraded. The discharge of suspended particulate matter by the Yellow River has a strong influence on the environment of the Bohai Sea. According to the Bohai Marine Environment Quality Bulletin, the Laizhou Bay is seriously polluted by nutrients. The East China Sea, which has high primary production, is strongly influenced by both terrestrial inputs and intrusion of open waters, and has obvious seasonal variation characteristics. The waters off eastern Taiwan are deep, and the main stream of Kuroshio flows northwards through this area and then enters the East China Sea after crossing the Ilan Ridge, which has profound effect on the eco-environment of the East China Sea. Trace metals, biogenic elements or other chemical parameters in these areas have complex environmental characteristics. Through the analysis of geochemical characteristics of these parameters, the specific conditions of interactions between land and ocean can be obtained, which has great scientific significance for the revelation of their biogeochemical processes and the protection of marine environment. Beacause of the toxic and non-biodegradable characteristics, trace metals are important environmental parameters which can reflect the health conditions of the environment. Biogenic elements are the material foundations of ecosystems. Chromophoric dissolved organic matter(CDOM), chlorophyll a(Chl a), phosphorus and other biogenic matter, which can be absorbed or produced by various biological processes, play critical roles in the marine environment. In this study, the Laizhou Bay coastal zone, the East China Sea and the waters off eastern Taiwan were selected as research areas and trace metals(Cd, Cr, Cu, Ni, Pb and Zn), CDOM, Chl a, phosphorus, the stable isotopes of carbon and nitrogen and other biogenic matter were selected as the main research objects. Geochemical distribution characteristics, sources, transport processes, ecological risks and controlling factors of these parameters were systemically studied. A series of results and viewpoints were presented as follows:(1) In different regions from coastal zone to deep sea, the influences of land and ocean presented by the geochemical charactersistics of some chemical parameters were various: in the Yellow River Estuary, trace metals in suspended particulate matter were under little influence of ocean; the concentrations of Chl a in nearshore waters of southwestern Laizhou Bay indicated some influence of terrestrial inputs; in the East China Sea shelf, CDOM in waters and different forms of phosphorus and δ13C in sediments presented the influence of both terrestrial inputs and intrusion of open waters; in the areas off eastern Taiwan where the Kurohio Current flows through, waters were less influence by terrestrial inputs and sediments had little influence on the transportation of biogenic elements by Kurohio Current.The total concentrations and geochemical fractions of particulate trace metals Cd, Cr, Cu, Ni, Pb and Zn in the Yellow River Estuary generally showed no notable spatial variations. These phenomena indicated that the studied trace metals influenced little by ocean. In the southwestern Laizhou Bay, concentrations of Chl a in part of the nearshore waters decreased seaward. The coatal rivers that run into the Laizhou Bay contributed much to that phenomenon. High concentrations of CDOM could reach the outer shelf of the East China Sea. Ads-P, Fe-P and OP in sediments of the East China Sea also presented that phenonmeon. Strong influence of terrestrial inputs was reseponsible for the distribution patterns of biogenic elements in waters and sediments of the East China Sea. The distribution pattern of δ13C in sediments of this area showed that open seawaters could intruse into its middle shelf.The Kurohio Current is a major source of water for the East China Sea. In the areas off eastern Taiwan where the mainstream of Kuroshio flows through, CDOM concentrations were generally low, indicating little influence of terrestrial inputs. The exchange fluxes of biogenic elements in sediment-water interface from this area were generally low, indicating that sediments in sea areas off eastern Taiwan may have little influence on the transportation of biogenic elements by Kuroshio Current.(2) In different regions of the Laizhou Bay coastal zone, the environmental conditions presented by different chemical parameters were different. Particulate trace metals in the Yellow River Estuary generally showed a good environmental condition; Chl a concentrations in the southwestern Laizhou Bay coatal zone indicated that this area was eutrophic to some extent.Total concentrations of particulate Cd, Cr, Cu, Pb and Zn in the Yellow River Estuary generally fell into the Class I marine sediment category, indicating a good environmental condition. Particulate Ni in this area had some potential risks to environment. Except for Cd and Ni, the studied metals in the most labile fraction having the highest potential risk to biota on average accounted for < 2% of their respective total concentrations, manifesting that they had low ecological risks. The combination of the six studied trace metals in the suspended particulate matter of the Yellow River Estuary had a 21% probability of being toxic to the environment. The annual fluxes of particulate Cd, Cr, Cu, Ni, Pb and Zn in the Yellow River Estuary calculated in this study were only 2.5%–5.3% of the correspongidng fluxes in the 1980 s. The most important reason is that the previous data were based on the measurements at a study site upstream from the mouth of the Yellow River, and a major proportion of the suspended particulate matter measured at that station had settled on the river bed before they passed through the river mouth and finally reached the sea. The particulate trace metal fluxes obtained in this study were more close to the real situation.The southwestern coastal Laizhou Bay and the rivers it connects with presented different eutrophication situations. In the riverine region, 33.3% and 41.7% of the sampling stations were in the status of serious and high eutrophication, respectively. While in the marine region, 16.7% and 83.3% of the sampling stations were in the status of moderate and low eutrophication, resplectively. The discharge of riverine waters, which had high nutrient concentrations, could make the coastal Laizhou Bay more suitable for the growth of phytoplankton which could aggravate the extent of eutrophication in this area. The nutrient concentrations in the riverine region should be controlled in order to protect the environment of the Laizhou Bay.(3) The seasonal variations of TOC, TN, δ15N and different phosphorus(P) fractions in surface sediments of the East China Sea shelf were obvious. Primary production, terrestrial inputs, hydrodynamic conditions and other environmental conditions were responsible for that phenonmenon.In the surface sediments of the East China Sea shelf, concentrations of TOC, TN, Ads-P and OP were generally high in spring, while concentrations of Au-P and De-P were generally high in autumn. The average value of δ15N in autumn was obviously high in autumn than in spring. These seasonal variations were caused by many factors relating to the interactions between land and ocean.In spring, high primary production resulted in generally high concentrations of sedimentary TOC, TN and OP in the East China Sea shelf. Part of them could be decomposed during summer and autumn, leading to generally low concentrations of TOC, TN and OP in autumn. The values of δ15N in surface sediments of the East China Sea shelf changed during the decomposition of sedimentary TN. The decomposition of sedimentary TOC, TN and OP needed much oxygen. However, the water column of the northern part of the East China Sea shelf became stratified in summer and the oxygen could not be completely replenished, leading to hypoxia in bottom waters and surface sediments in that area. Part of Fe-P could be reduced under low-oxygen condition. Therefore, generally low concentrations of Fe-P were found in the northern part of the East China Sea shelf in autumn.Owing to the limited nutrient availability in the surface water after the consumption by phytoplankton during spring and summer, phytoplankton tended to grow in deeper waters where nutrients were relatively abundant due to sediment release. Therefore, some of Ads-P could be released in order to keep balance with P in water column, thus resulting in generally low concentrations of Ads-P in autumn. The increase of Au-P in the East China Sea shelf in autumn was related to the transformation of P and the settlement of biogenic skeletal debris. Strong resuspension of surface sediments in autumn in the East China Sea shelf and strong precipitation in summer and autumn in southeastern China may be responsible for the increase of De-P in autumn. The obvious increase of Fe-P, OP and Bio-P in autumn occurred in the coastal areas of the East China Sea shelf. The main reason was that strong rainfall in southeastern China during summer and autumn could cause strong surface runoff and much Fe-P, OP and Bio-P could be brought to the coastal areas of the East China Sea shelf during that period.