Distributions And Regulations Of Nutrients In The Changjiang Estuary And Its Adjacent Areas In 2017 And The Trend Assessment Of Ecological Effects Of Eutrophication

The Yangtze River is the largest river in China and the third largest in the world.The Yangtze Basin is subject to high population density,the widespread use of chemical fertilizers and the large amount of sewage discharge,delivering a large amount of nutrients into the sea,leading to the serious eutrophication in the Changjiang estuary and its adjacent areas,greatly affecting the ecosystem structure and functions.The observational data are from the survey of the three seasons of winter,spring and summer in the Changjiang estuary and its adjacent areas,in February,May and July of 2017.The results showed that the water body was evenly mixed vertically in February.The dissolved oxygen saturation(DO%)is(104±3)%and most stations are saturated with dissolved oxygen in February.In surface,middle and bottom layers the DO%is 62%?185%in May.Most stations in the bottom water is in unsaturated state.In July,the water body is in serious stratification,the concentration of DO is 1.52?1 3.92 mg/L and the DO%is 20%?179%.The DO%of surface layer in 122°?0123°E and 31°?32°N is 139%?179%,but the stations of bottom layer is in unsaturated state except for the stations near Hangzhou Bay.The authors found a hypoxic zone near 31°N and 123°E with DO as low as 1.52 mg/L.The total alkalinity(TAlk)in the freshwater end member of the Yangtze River is 1 874± 15?mol/kg in February,1698±11?mol/kg in May and 1554±28 ?mol/kg in July,respectively.And the concentration of dissolved inorganic carbon(DIC)is 1883±14?mol/kg.1750±11 pmol/kg and 1590±18 ?mol/kg.respectively.The TAlk and DIC are smaller in flood than in dry season.The TAlk and DIC were lower in summer of 2017 than in previous years.The variation ranges of dissolve silicate(DSi).dissolve inorganic nitrogen(DIN)and dissolve inorganic phosphorus(DIP)were 111.6--134.3?mol/L.156.8?158.4 pmol/L and 1.72 ?mol/L in February;119.2?120.6 ?mol/L,127.0?133.8 ?mol/L and 1.26-1.50?mol/L in May;121.4-122.7 ?mol/L.93.9-98.6?mol/L and 0.83?0.88 ?mol/L in July,respectively.And their seasonal changes are the biggest in February and the smallest in July.According to the relationship between nutrient flux and runoff.the flux of DSi,DIN and DIP are 10.7×1010 mol/yr,11.3×1010 mol/yr and 12.9×108 mol/yr in 2017.respectively.Nutrients in July are mainly affected by physical mixing and biological effects.Physical mixing is mainly manifested by the conservative mixing of DSi and DIN at the study area,which can be analyzed by two elements consisting of the freshwater and seawater ends.The biological interaction is mainly reflected in the coupling relationship between the four parameters DIC,apparent oxygen consumption(AOU),DSi and DIN.In July,the removal of DIC and DSi was 160 ?mol/kg,33.2 ?mol/kg,and the addition of DO was 168.30 ?mol/kg in the region with the surface strongest biological effect.The addition of DIC and DSi was 107 ?mol/kg,24.2 ?mol/kg and the removal of DO is 149.65 ?mol/kg in the low oxygen zone.The contribution of dissolved oxygen saturation(DO%)in the high value area to the low oxygen in the bottom layer is about 70%?90%,and the actual value is more inclined to 90%.It is further proved that most of the nutrients(DSi and DIN)in this area are produced by the degradation of organic matter.Since the 21 st century,DSi and DIN have no obvious upward or downward trend.The DIP has a growth trend,but the change in the past decade is very small.According to the relationship between nutrient output flux and runoff,the influx of DSi and DIN into the sea in the past ten years is calculated.Since the 21st century,the sea fluxes of DSi,DIN and DIP have fluctuated in 8.0-12.0×1010 mol/yr,8.7-13.3×1010 mol/yr and 8.7-15.5×108 mol/yr,respectively,all of which are at a high level,with no obvious upward or downward trend.Since 1980,the indicator of coastal eutrophication potential(ICEP)of the Yangtze River began to be greater than 0,indicating that eutrophication began to occur in the Changjiang Estuary and its adjacent areas.From 1980 to 2005,the occurrence frequency and area of red tide increased,the corresponding ICEP increased gradually,and the eutrophication of the Changjiang Estuary and its adjacent areas strengthened gradually.Since 2005,ICEP fluctuated between 8.7-14.0×109 kg/yr,suggesting that the eutrophication level is relatively stable in the Changjiang Estuary in the recent decade.Although the number and area of red tides in the 2010s were recorded less than the 2000s,however,the field data in 2017 showed that the summertime oxygen deficient conditions were still remarkable.Therefore,so far the Changjiang Estuary and its adjacent western East China Sea were still seriously affected by the negative ecological effects of eutrophication,which was nearly the same as the 2000s.