Research On Spatial And Temporal Variation Of Nutrients In The Yangtze River Estuary And Adjacent Waters Based On GOCI Imagery

Nutrient concentration is an important basic substance of phytoplankton energy metabolism,and it is also one of key points of water quality parameter evaluation.The content and proportion of nutrients can largely determine the primary productivity level of the ocean,the population structure and biological community of phytoplankton.With the large-scale development and utilization of marine resources by human beings,marine environmental problems have gradually become prominent,especially the eutrophication of seawater.Therefore,reasonable monitoring of nutrient concentration is of great significance to marine environmental protection.Traditional water quality monitoring is time-consuming and labor-intensive,and is limited by weather,time scale and scale.The development of remote sensing technology provides a new method for water quality monitoring,which can not only monitor water quality on a large scale,but also reflect the temporal and spatial distribution characteristics and changes of water quality,and can also study the influence range of pollution sources and the migration characteristics of pollutants.It has been widely used in the field of water quality monitoring.In this paper,the Yangtze Estuary were taken as the research area,and the measured data of nitrate,phosphate and silicate in the surface layer from December 8 to December13,2019,and the total suspended solids concentration and synchronous GOCI images were taken as experimental data.Firstly,the correlation between the reflectance of GOCI and measured nutrients data was analyzed,and the band and band combinations with high correlation were selected for regression analysis with measured data,and then the best phosphate and silicate inversion model ware selected and verified.Then,based on the GOCI image data from 2011 to 2020,the phosphate concentration and silicate concentration were estimated,and the inter-annual,intra-annual(monthly and seasonal),semi-monthly tidal cycle and intra-day distribution and variation characteristics of the sea area were analyzed.The temporal and spatial variation of the four sub-study areas,namely the Yangtze River Estuary,Yangtze River Channel,Hangzhou Bay and Subei Shoal,were quantitatively analyzed.Finally,the influence mechanism of the corresponding hydrological factors on the spatial and temporal distribution of nutrient concentration was discussed by combining the hydrological factors such as the inflow runoff,coastal flow,Taiwan warm flow,wind wave and tide.(1)Based on the correlation between measured data and nutrients,the inversion model was verified.The verification results showed that the MAPE of phosphate and silicate inversion models were 21.65% and 6.73%,respectively.(2)According to the spatial distribution of phosphate and silicate concentrations,the gradient changes obviously in each sea area,which is low near the shore and high outside the sea,and oligotrophic salt phenomenon appears in the sea area to the east of 123,and obvious high value areas appear in the shoal of northern Jiangsu,the Yangtze River estuary and Hangzhou Bay.(3)There is no obvious interannual variation of phosphate and silicate concentrations from 2012 to 2020,but on the whole,they show a downward trend.The long-term variation trend of phosphate and silicate concentrations in different sea areas is different,with the Yangtze River channel decreasing the fastest,while the annual variation in Hangzhou Bay is the smallest.In terms of seasonal variation,the concentration of phosphate and silicate decreased continuously from winter to spring,reached the lowest in summer,and rose again in autumn.From the perspective of semi-monthly tidal cycle,there is no obvious interannual variation of phosphate concentration and silicate concentration under different tidal conditions,but there is obvious seasonal variation.The seasonal variation of spring tide,middle tide and small tide is consistent,which is the highest in winter and the lowest in summer,with little difference between spring and autumn.In addition,the seasonal average value of the four sub-study areas is the highest on spring tide day and the lowest on neap tide day.From the diurnal variation of single tidal cycle,the concentrations of phosphate and silicate are the highest in spring tide day,the second in normal tide day,and the lowest in neap tide day.Different sea areas show different variation characteristics.Under different tidal conditions(spring tide,ordinary tide and small tide),the concentration of phosphate and silicate in the Yangtze River estuary and northern Jiangsu shoal changes slightly within 8 hours,and shows a slow upward trend as a whole.However,the concentration of rivers in Hangzhou Bay and the Changjiang Estuary varies greatly.(4)The interannual variations of phosphate and silicate concentrations in the surface water of the Yangtze River Estuary and its adjacent waters are mainly affected by the physical factors such as diluted water of the Yangtze River,wind waves and surface currents,and are also affected by the annual variation of the growth,reproduction and aggregation density of phytoplankton,benthos and algae.The seasonal variation of nutrient concentration is the highest in winter and the lowest in summer.There are two main reasons for this phenomenon.On the one hand,the terrestrial input of nutrients is the largest in summer and the photosynthesis of phytoplankton is strong,and the terrestrial input in winter is the smallest,and the intake of phytoplankton is also small,which shows that the terrestrial input of nutrients only affects or balances the seasonal variation of nutrients.On the other hand,the average wind speed in winter is the strongest,while that in summer is relatively weak,and the upwelling intensity in summer is far greater than that in winter.The strong winter monsoon makes the vertical mixing of water body strong,and the change of wind speed will lead to the resuspension of sediment,thus releasing nutrients.The enhancement of water stratification in summer will also lead to the lower content of surface water body.Different sea areas are affected by tides in the half-moon tidal cycle.In addition to neap tide,the concentrations of phosphate and silicate in the northern Jiangsu shoal and the Changjiang Estuary show obvious regularity with the tidal changes,that is,the nutrient concentrations gradually decrease at high tide,and rapidly rise at low tide.There is a large tidal range in Hangzhou Bay.In normal tides,there is a positive correlation between the changes of nutrient concentration and tidal changes,while the days of small tides and large tides are basically irregular.The phosphate concentration and silicate concentration in the Yangtze River channel are less affected by tides.