Effects Of Tidal Action,Typhoon And Human Activities On The Cycle Of Biogenic Elements In Zhanjiang Bay

Zhanjiang Bay is located in the northwestern part of the South China Sea,and its ecosystem structure and function have unique characteristics influenced by monsoons,tides,typhoons,and land runoff.At present,there are few studies on the coupling of biogeochemical processes and physical processes in the bay,which is a hot issue in current marine science.In this study,we observe the physical processes and biogeochemical elements of seawater in Zhanjiang Bay,use particulate matter carbon and nitrogen stable isotopes and nitrate nitrogen and oxygen isotopes in combination with physical processes such as typhoons and tides on the Zhanjiang Bay ecosystem,and discuss how these physical processes and human activities affect the marine ecosystem.The main findings are as follows.（1）Through continuous observation of four stations（two stations in the bay and two stations at the mouth of the bay）in Zhanjiang Bay,it was found that in terms of vertical distribution,the source factors of S1,S2,S3 and S4 of four stations in summer and autumn were similar,which indicated that the water in Zhanjiang Bay was vertically mixed strongly and the nutrient distribution of the whole water column was uniform.In terms of temporal variation,the nutrient concentrations at each station in autumn had semi-diurnal fluctuations in each layer,and the nutrient concentrations at each station in summer also had semi-diurnal fluctuations in each layer,but due to the large amount of rainfall in summer would affect the semi-diurnal fluctuation pattern of nutrient salts,while the nutrient concentrations at stations S1and S3 had better semi-diurnal fluctuations in each layer,and the nutrient concentrations at stations S2 and S4 fluctuated significantly on Sundays with relatively.This indicates that the nutrient distribution in Zhanjiang Bay is mainly affected by tidal action,and the nutrient distribution in the stations with deeper water depth is more significantly affected by tidal action.This study showed that the controlling factor of chlorophyll a concentration and diurnal fluctuation of nutrients was tidal action>illumination>biological utilization.（2）Based on a combination of conventional geochemical indicators and isotopic compositions,compared with satellite remote sensing data,the biogeochemical processes in Zhanjiang Bay during Super Typhoon Rainbow and non-typhoon periods were reveal the biogeochemical and ecological processes in the Bay caused by Typhoon Rainbow.The results showed that the chlorophyll a（Chl a）concentrations inside and outside the bay during the super typhoon“rainbow”and non-typhoon periods were in a mirror image relationship:during the non-typhoon period,the Chl a inside the bay was high,while the Chl a outside the bay was low;on the contrary,during the super typhoon“rainbow”period,the Chl a inside the bay was low,while the Chl a outside the bay was low and high Chl a outside the bay.Our results show that the chlorophyll a（Chl a）concentrations inside and outside the bay during these two periods were considerably different.The positive apparent oxygen utilization（AOU）and unsaturated dissolved oxygen（DO）values during typhoon periods suggested significant biological degradation in Zhanjiang Bay,rather than algae blooms after typhoons,as previously reported.Geochemical indicators,such as the particulate organic C（POC）/Chl a ratio and isotopic compositions,indicate that the degradation of organic matter was stronger after typhoon.This phenomenon is because of the shallow depth of the bay,the strong mixing effect caused by super typhoon“rainbow”greatly damaged the fishery resources of the bay and caused the deterioration of the water body.The typhoon induced decomposition processes,rather than algal blooms,in this semi-enclosed bay in the northern South China Sea,providing new understanding of the typhoons impact on marine biogeochemistry and ecology.（3）In this study,the isotopic composition(δ¹³C andδ¹⁵N),total organic carbon content,total nitrogen content,and C/N ratios of suspended particulate organic matter（POM）in Zhanjiang Bay,which is a semi-enclosed bay with concentrated artificial activities in Southern China,were analyzed in order to investigate the seasonal variations in the principal POM sources in the monsoon region.In summer,theδ¹³C andδ¹⁵N values showed a weak correlation with the chlorophyll a（Chl a）,suggesting that terrigenous sources were dominant.However,in winter,the particulate organic carbon and particulate nitrogen values were correlated with the Chl a in the middle bay and bay mouth.Moreover,theδ¹³C values showed a significant correlation with Chl a during the winter,indicating that the contribution of the in situ phytoplankton was relatively important and was affected by the monsoon in winter.Compared with the correspondingδ¹³C values,theδ¹⁵N values exhibited a complex spatial distribution.By using a Bayesian mixing model,in the upper bay,the source of POM was mainly from marine organic matter（49%）in summer,and almost an equilibrated contribution of all sources in winter.In the middle bay and bay mouth,the POM contribution mainly originated from marine organic matter（53%）during the winter.In contrast,the POM source was mainly soil organic matter（63%）in summer,suggesting that the POM was sourced from the runoff from the upstream basin.Our results suggest that the seasonal shifts of the source of POM should be taken into account when estimating C or N mass balance in the monsoon-controlled bay.（4）Nitrate（NO₃^-）dual isotope analysis was performed in Zhanjiang Bay,which is a closed bay with intensive human activities in South China,to investigate seasonal changes in the main NO₃^-sources and their biogeochemical processes in the monsoon-controlled climate.The relatively low N/P ratios in Zhanjiang Bay suggests that nitrogen（N）is a limiting nutrient,which indicates that the increase of N is favorable for phytoplankton proliferation.However,a sufficient amount of ammonium was found in our study area owing to intensive human activities,which can support biological processes.Thus,less NO₃^-biological processes were found,indicating that NO₃^-isotopic characteristics may reveal details of the mixing from various sources.The Bayesian mixing model showed that NO₃^-in the upper bay originated from manure（43%）,soil N（30%）,N fertilizer（17%）,and N precipitation（10%）during winter,which reflects the local human activities;while NO₃^-sources during summer were mainly N fertilizer（36%）,soil N（32%）,and manure（31%）,indicating the source as the runoff from the upper river basin.Our results suggest that nitrate dual-isotope was very useful for tracing the main NO₃^-sources in the condition of the sufficient ammonium,and runoff exerted an important impact on the shift in NO₃^-sources between both the local source and the source from the upper river basin during the two seasons in this monsoon-controlled bay.