Factors Regulating Plankton Community Respiration In The Pearl River Estuary

In this study,we investigated plankton community respiration by on-deck incubation in the Pearl River Estuary and nearshore waters in December 2016 and July 2017,and analyzed the spatial distribution and seasonal variation of community respiration rate in the Pearl River Estuary.In addition,we analyzed the correlation of community respiration with dissolved organic carbon?DOC?,chlorophyll-a?Chl-a?,dissolved oxygen?DO?,bacterial abundance?BA?,nutrients,salinity and other environmental factors.We also evaluated the effect of community respiration on seasonal hypoxia zone formation at the bottom water of the lower Pearl River Estuary.The main conclusions are as follows:?1?Significant spatial and seasonal variations of community respiration rate were observed in the Pearl River Estuary.In winter,the community respiration rates of the surface and bottom waters ranged from 1.6 to 37.7?mol O₂ L^-11 d^-11 and 3.7 to 14.3l?mol O₂ L^-11 d^-1,respectively.The high-value areas were located in the upper reaches of the estuary.While in summer,the community respiration rates of the surface and bottom waters ranged from 4.3 to 45.9?mol O₂L^-11 d^-11 and 0.67 to 11.24?mol O₂ L^-11 d^-1,and the high value areas of community respiration rate were located at the inner Lingdingyang.The community respiration rates in the surface water ranged from 4.1 to 28.9?mol O₂ L^-1d^-1(arithmetic mean±SD:13.9±7.7?mol O₂ L^-11 d^-1)in the coastal waters outside the Pearl River Estuary,and the high-value areas were located on the west side of the coastal waters.The community respiration rates in the bottom water ranged from 0.3 to 29.2?mol O₂ L^-1d^-1(rithmetic mean±SD:6.2±7.5?mol O₂ L^-11 d^-1),and the high value areas were located in the waters near Hong Kong.The community respiration rates were generally high in near shore,and gradually decreased with the increase of offshore distance.The community respiration rates in the algal bloom area were significantly higher than that in the non-algal bloom area,and the community respiration rates of surface water were significantly higher than that in the bottom water.The average community respiration rate of surface water was 2.5 times of that of bottom water.?2?The statistical analysis of the correlation between environmental factors and community respiration rate in the surface water showed that phosphate concentration was the strongest?r²=0.48,p<0.05?,followed by DO?r²=0.34,p<0.01?,then DOC?r²=0.24,p<0.05?.Environmental factors such as BA and salinity have weak effects on community respiration changes,especially the correlation between BA and surface water community respiration is the weakest?r²=0.09,p>0.05?.The above results indicated that the factors affecting the respiration rate in surface water of the Pearl River Estuary were complicated and diverse,and were controlled by multiple environmental factors.There was a significant negative correlation between bottom water DO and community respiration rate?r²=0.37,p<0.01?,indicating that DO is an important factor affecting the respiration rate of the bottom water community.In addition,a large area of hypoxic zone in the bottom water was observed on the west side nearshore,indicating that the community respiration was an important factor resulting in the hypoxia of the bottom water in the coastal waters of the Pearl River Estuary in summer.?3?Salinity and alkalinity?TA?were used as conservative mixing parameters to establish a"three-terminal mixed"model,so as to predict conservative dissolved inorganic carbon?DIC?,dissolved inorganic nitrogen?DIN?,and dissolved inorganic phosphate?DIP?.Than?DIC,?DIN,and?DIP were calculated based on the differences between the observed and conservative values.The?DIC,?DIN,and?DIP showed signifficantly positive correlation with apparent oxygen utilization?AOU?,and the ratio of?DIN/AOU,?DIP/AOU was basically consistent with the ratio of Redfield.This indicated that the community respiration was the main factor for the decrease of DO and the increase of DIC in the bottom water of the Pearl River Estuary and adjacent coastal water in summer.This paper reveals the spatial and temporal distribution characteristics of plankton community respiration rate in the Pearl River Estuary,discusses the environmental factors affecting the respiration rate of the surface and bottom communities in the offshore waters of the Pearl River Estuary,and clarifies the influence of community respiration on the formation of underlying hypoxia.This result provides an important basis for understanding the ecological functions of plankton community respiration in estuarine and coastal water ecosystems.