Phytoplankton Community Phosphate Stress In The Jiulong River Estuary And Changjiang River Plume

Much more significant changes have happened in the estuaries and costal areas under the influence of global climate changes and human activities.Phosphorus(P),with its significant impact on the structure and function of the ecosystem,is an important factor limiting the phytoplankton growth in these sea areas.Alkaline phosphatase(AP),as the phytoplankton ectoenzyme,is a good indicator for the status of both phosphate limitation and DOP(dissolved organic phosphorus)use in the waterbody.Since there are few studies that focus on the variation and impacting factors of AP in the variable and complex estuaries and plume area,more efforts should be done.This study was carried out to study the spatial and temporal changes of the phytoplankton community AP activity(APA)and phosphate stress in the estuary and river plume,discussing their influencing mechanism.The main results were as follows:1.The spatial APAchla performed higher value in the upper estuary(oligohaline area),followed by the middle estuary(mesohaline area at the edge of freshwater and saltwater),and APAchla rised again in the lower estuary(polyhaline area)and Xiamen Bay(euhaline area).The temporal APAchla performed higher value in summer in 2014,while it had no significant difference among the seasons of 2017.The spatial percentage of phytoplankton APA(3-200 ?m)and bacterial APA(0.22-3 ?m)performed the gradual decline from the upper estuary to the lower estuary,but no significant difference in the lower estuary and Xiamen Bay;while the free APA(0-0.22 ?m)performed the opposite trend,and performed higher value in the Xiamen Bay than the lower estuary.The APAchla in 2014 in the Jiulong River Estuary performed higher value(average 97.3 nmol/?gChl a/h,range 3.6-547.3 nmol/?gChl a/h)than that in 2017(average14.5 nmol/?gChl a/h,range 0.9-59.4 nmol/?gChl a/h).The APAchia in Xiamen Bay in 2017 performed no significant difference with the value in the Jiulong River in 2017.2.The spatial ELF-labeling rate was higher in the lower estuary in the Jiulong River Estuary,and the seasonal ELF-labeling rate performed no significant difference.But the high value 55.0%happened in the spring of 2017 with the bloom of Alexandrium sp.in the middle saltwater.The ELF-labeling rate was higher for the Bacillariophyta and Dinophyta,but almost zero for the Chlorophyta and Cyanophyta.3.Phytoplankton hardly suffered from phosphate stress in the upper estuary,with the particle-attached bacteria contributing the high APA value by degrading the particulate P;phytoplankton was still in low phosphate stress with replete phosphate in the middle estuary;phytoplankton suffered from higher phosphate stress with decreasing cells growth limited by deficient phosphate in the lower estuary;and phytoplankton was in long-termed phosphate stress in the Xiamen Bay.4.Comparing the diluted water(salinity<31)and high-salinity(salinity>34)water in the Changjiang River Plume and its adjacent sea area,we found that spatial APAchia performed no significant difference for both diluted water and high-salinity seawater,except for autumn during which APAchia in the diluted water(average 13.2 nmol/?gChl a/h,range 7.2-19.6 nmol/?gChl a/h)was higher than the high-salinity seawater(average 77.7 nmol/?gChl a/h,range 3.7-549.1 nmol/?gChl a/h).As for the spatial ELF-labeling rate,in spring,its value were similar between the diluted water(47.5%)and mixed seawater(31<salinity<34,36.5%);in summer,the value was higher in the diluted water(45.6%)than high-salinity seawater(8.9%);in autumn,the value was higher in the diluted water(100.0%)than mixed seawater(17.8%);and in winter,the value was higher in high saltwater(72.8%)than diluted water(35.1%).In spring,phytoplankton phosphate stress became more severe due to the input of Taiwan Warm Current(TCWW)and was relieved in summer with the phosphate supply from the Nearshore Kuroshio Branch Current(NKBC).Then in autumn,phytoplankton was in long-termed phosphate stress in the diluted water and high saltwater,while in winter,phytoplankton in the high-salinity seawater got rid of phosphate stress.5.Comparing the diluted water and high-salinity seawater in the Changjiang Plume water and its adjacent sea area,we found that APAchla in the diluted water orderly;while in the high-salinity seawater,APAchla performed higher value in spring and summer,followed by autumn and winter.Seasonally,the ELF-labeling rate in diluted water performed higher value in autumn,followed by spring and summer,winter orderly;in high saltwater,APAchla performed higher value in winter followed by summer,and in mixed seawater,APAchla was higher in spring than autumn.In spring,the warmer seawater brought higher biomass and nutrient need,so phytoplankton became in phosphate stress;in summer,the nutrient input with high N/P ratio from the Changjiang Plume water aggravated the phytoplankton phosphate stress,and the phosphate supply from NKBC relived phosphate stress;in autumn,phytoplankton phosphate stress was relived with the lower stratification effect and nutrient limitation;and in winter phytoplankton became getting rid of phosphate stress due to the light limitation and low temperature.