The Influence Factors For Spatio-temporal Changes Of Nutrients And The Quantitative Reduction Of Terrestrial TDN In Qinzhou Bay

Eutrophication is the main eco-environmental problem in many coastal waters worldwide,and accurate understanding of its causes is a prerequisite for further ecological protection and governance.Qinzhou Bay(QZB),a small bay with an area of?380 km²,composed of inner and outer bay,located at the northern Beibu Gulf is the largest genetic resources area of ostrea rivularis(oyster germplasm)in South Asia.In recent years,frequent algal blooms and other eutrophication characteristics have occurred in QZB,but the causes of the eutrophication and the relationship between land-nutrients and sea-nutrients is still unclear.In this study,survey data on different time and space scales of water environment of sea area,river and sewage outlet were acquired in QZB.An on-site enrichment cultivation experiment of five nitrogen sources(i.e.industrial organic nitrogen,agriculture organic nitrogen,fluvial organic nitrogen,NO₃^-and NH₄⁺)was also carried out during the dry and wet seasons.The main reasons and ecological risk for spatio-temporal changes of nutrients in recent years,and the migration-transformation kinetic process as well as the quantitative reduction of terrestrial total dissolved nitrogen(TDN)were explored,through multivariate statistics,correlation analysis,box-type balance model,analysis of ecological dynamics,and box-type numerical models.The main conclusions are as follows:(1)Influence factors of spatio-temporal variations in nutrient concentrations were revealed.Seasonally,the"non-point source"factors such as agricultural fertilization and"point source"factors such as urban sewage,jointly affected the concentrations of DIN,while the concentrations of DIP was mainly affected by"point source"factors.Spatially,the river-sea mixing was the main factor affecting the concentrations of DIN and DIP in QZB.They can be also effected in season by groundwater input,oyster farming discharge,and seasonal re-suspension process in the outer QZB,which may be driven by tropical cyclone.Interannually,the worse eutrophication could be attributed to rapid urbanization rather than changes in agricultural fertilization activities in recent thirty-five years.In terms of input source,the contributions of rivers to DIN and DIP load in the inner QZB were>59.3%and63.0%,respectively.There was also a strong DIP input(>7.3 kmol/d)at the Maoling River mouth probably from the nearby industries.(2)Relationship between the frequent occurrence of Phaeocystis globosa bloom and the changes in the nutrient ratio was discussed.Seasonal characteristic with the lower water temperature,high N and the alleviation of P limitation(DIN/DIP close to 16:1),may be important for the frequent occurrence of Phaeocystis globosa blooms in recent years.DIN was bio-limiting in the 1980s and was enriched relative to DIP(DIN/DIP>16:1)after the1990s in QZB,and there was a trend towards a lower DIN/DIP ratio,which may further aggravate the risk of Phaeocystis globosa blooms in the sea area,in future.(3)The migration-transformation kinetic process and the influencing factors of five kinds of terrestrial nitrogen species were revealed in QZB during this study.Firstly,under the influence of microorganisms and/or light,?63.4%?100%of dissolved organic nitrogen(DON)from industrial,riverine,and agricultural breeding sources was effectively degraded and remineralized into NO₃^-..These dynamic processes were affected by the source,light,microorganisms,temperature,and dissolved oxygen concentration,while nitrification process of NH₄⁺was mainly influenced by microorganisms rather than light,and was promoted by high temperature.Secondly,the order of the maximum growth rate of phytoplankton to different nitrogen sources was different in the wet and dry seasons.This may be closely related to the seasonal changes of phytoplankton population structure.Lastly,the death rates of phytoplankton and the degradation rates of particulate nitrogen(PN)produced by the death of phytoplankton in the wet season were significantly higher than those in dry season(p<0.05),indicating that high temperature accelerated phytoplankton death and promoted PN regeneration into DON in the inner QZB.(4)A box-type numerical model coupled with nitrogen migration-transformation dynamics was constructed,which accurately quantifies the relationship between the average annual DIN concentration and the average annual input of TDN from rivers into QZB.The maximum carrying capacity of TDN to meet the DIN control target(?21.42?mol/L)calculated by this model was 5488.4 t/y,and it is the most efficient dual"differential"reduction of different forms of nitrogen,and rivers regarding the current emissions(7816.7t/y).