Dynamics Response Of Spatio-temporal Variation Of Chlorophyll A Concentration To The Fronts In The Changjiang Estuary

Fronts are the important mesoscale physical process in the estuaries and adjacent seas.The physical properties of the front determine that the water masses on either side of the front are characterized by different environmental factors（e.g.,temperature,salinity,suspended matter concentration,and nutrients）,which generate different habitats for the growth of phytoplankton,and,consequently,impact the spatial patterns of chlorophyll a（Chl-a）.In summer,there were sediment front and plume front formed by the interaction of low-salinity Changjiang diluted water（CDW）and tidal mixing in the Changjiang River Estuary（CRE）and its adjacent waters which was a natural place to study the influence of fronts on Spatio-temporal variation of Chl-a.Previous studies have shown that the Chl-a concentrations in the coastal high turbidity zone and offshore oligotrophic zone were usually lower in summer.Moreover,the high Chl-a concentration usually occurs in the central sea beyond the sediment front.However,the distribution of high Chl-a concentrations varied greatly in the summer of different years.There were many studies on the hydrodynamic and ecological effects of a single front（sediment front or plume front）,but the studies on the combined ecological effects of dual fronts were limited.In addition,most of the studies were based on a single cruise.There was a lack of research on the interannual variation characteristics of the front,especially under the different wind fields in summer.In this paper,the spatial and temporal variation characteristics of environmental factors and Chl-a concentration were analyzed based on 13 cruises observational data in the CRE and its adjacent waters（121°E-124.5°E,28.5°N-33°N）during the summer of 1988-2018.Meanwhile,combined with observational data,satellite remote sensing data,and a three-dimensional physical-biogeochemical model（FVCOM-ERSEM）,the influence mechanism of fronts on the Spatio-temporal variations of environmental factors and Chl-a concentration was explored.The main research contents and conclusions were summarized as follows:1.In summer,the environmental factors and Chl-a concentration on both sides of the sediment front and the plume front in the CRE and its adjacent waters had spatial differences and interannual variations.The multi-year average sediment front was mainly located in 121.9°E-122.5°E,30°N-32°N in the summer of 1988-2018,and the plume front was located primarily at 122.5°E-123.6°E,28.5°N-32.5°N.Based on the boundary of the sediment front and plume front,the CRE and its adjacent waters were divided into three zones with significant differences in environmental factors and Chl-a:the estuarine zone on the shoreward of the sediment front,the transitional zone between the sediment and plume front,the shelf water mass beyond the plume front.The high Chl-a concentration(mean:5.45±5.95μg?L^-1)usually appeared in the transition zone,but it was usually lower in the estuarine zone(2.30±2.81μg?L^-1)and shelf water zone(2.53±2.99μg?L^-1).The interannual variations of environmental factors and Chl-a concentration in the CRE and its adjacent waters were analyzed using the observational data in the summer of 2013-2018,which had relatively consistent sampling sites and time.The highest average sea surface salinity（SSS）and the lowest average surface nutrient concentrations in the whole study region all occurred in the summer of 2013.The lowest average SSS and highest average surface nutrient concentration appeared in the summer of 2016.Those were related to the lowest summer runoff of the Changjiang River that appeared in 2013(34711 m³·s^-1)and the highest in 2016(64281 m³·s^-1).The net heat flux was relatively high in the summer of 2013,2017,and 2018,and the average sea surface temperature（SST）in the corresponding years was also high.While in the summer of 2014 and 2014,the net heat flux and SST were both lower.The average Chl-a concentration was higher in the summer of 2013 and 2016,while the average Chl-a concentration in the summer of 2014 was the lowest.2.Sediment and plume fronts are important“barriers”for constructing the spatial patterns of environmental factors and Chl-a concentration.The high concentration of total suspended matter(TSM:93.8±86.3 mg?L^-1)caused the heavy light-limitation（76%）in the CRE,resulting in the lower Chl-a concentration.The physical-chemical effects（stratification and flocculation）near the sediment front caused noticeable sedimentation of fine sediments.Thus,the light-limitation was distributed on the shoreward of the sediment front.In the transition zone of the sediment and plume fronts,the stratification was strong,and the TSM concentration was lower(15.1±12.1 mg?L^-1).Thus,there were only 3%of stations with light-limitation in the transition zone.Meanwhile,impacted by the nutrient transport of the Changjiang diluted water（CDW）and upwelling,the nutrients supply was sufficient,and the salinity and temperature were suitable in the transition zone.The light,salinity,temperature,and availability of nutrients reached the best balance,resulting in the high Chl-a concentration mainly appearing in the transitional zone.The plume front was the second important“barrier”off the CRE,which was the boundary of the transition zone and shelf water.The absorption of nutrients by phytoplankton in the transition zone and the convergence around the plume front limited the transport of nutrients to the shelf water.The severe phosphorus limitation（44%）and a certain silicon limitation（11%）led to the lower Chl-a concentration in shelf water.3.The observational data in the summer of 2014 and 2015 showed that there was a surface cold-water zone（CWZ,123°E-124.5°E,31.5°N-33°N;SST<22℃）in the Yangtze Shoal in the northeast direction off the Changjiang River Estuary;and the CWZ had an impact on the spatial and temporal variations of Chl-a concentration in the summer.The average SST of the CWZ in the summer of 2014（20.2±0.3℃）and 2015（21.3±0.9℃）was lower 4-8℃than that in the summer of2013,2016-2018.Combining the result of observational data,satellite data,and modeling data,it was found that the formation and disappearance of CWZ in the Yangtze Shoal were mainly related to the change of wind field.Compared with the multi-year average summer wind field,there were more northerly winds during the formation period of the CWZ（July 1-17,2014,and July 1-19,2015）.Infected by the above wind field,the expansion of the CDW to the northeast was hindered,and the plume front was restricted to the nearshore area west of 123°E.At this time,the water depth of the Yangtze Shoal beyond the plume front was relatively shallow（30-45 m）,and the turbulent kinetic energy caused by tidal mixing was intense in the CWZ.The mixing of surface and bottom low-temperature water led to a decrease in SST,resulting in the formation of CWZ in the Yangtze Shoal.Besides,the vertical mixing in the CWZ transported the rich nutrients from the bottom to the surface,relaxing phosphorus limitation in the shelf water zone.However,the enhancement of vertical mixing also increased the TSM concentration and mixed layer depth（MLD）,leading to the vertical average irradiance of the mixed layer(I_m:32.3±11.1μmol photons m^-2 s^-1)being lower than the threshold of the light limitation.The light limited growth of phytoplankton resulted in the low Chl-a concentration in the CWZ during the summer of 2014 and2015(1±0.4μg·L^-1and 1.4±0.7μg·L^-1).During the disappearance period of the CWZ（July 18-31,2014,and July 20-31,2015）,the southerly wind became the dominant wind.Under the southerly wind,the CDW expanded to the northeast and gradually covered the CWZ.Thus,the stratification of CWZ increased rapidly,the SST rose,and the CWZ disappeared.The stratification also reduced the surface TSM concentration and MLD,which caused the I_mto increase significantly(>150μmol photons m^-2 s^-1).The sufficient light and nutrients caused a temporary high Chl-a concentration.Until surface nutrients were consumed by phytoplankton,the Chl-a concentrations decreased again.In summary,the fronts affected the spatial and temporal variations of environmental factors and Chl-a concentration in the CRE and its adjacent waters.The high Chl-a concentration mostly appeared in the transition zone between the sediment and plume fronts.When there were more northerly winds in summer,the CWZ beyond the plume front would also affect the Spatio-temporal variations of phytoplankton.The above results were significant for understanding the spatial and temporal variations of phytoplankton biomass in the CRE and its adjacent waters.