Quantitative Inversion Of Microphytobenthos In Estuarine Tidal Flats By Remote Sensing And The Bottom-up Effect Study

Microphytobenthos（MPB）are the main contributors to the primary production of estuarine tidal flats,are an essential source of bottom material and energy for estuarine food webs,and play a central role in the ecological functions and services of tidal flats.However,under tremendous pressure,such as coastal development,sea level rise,coastal erosion,and the reduction in river sediment flux,the global tidal flat shows a downwards trend,which directly affects the habitat of MPB in the tidal flat.Because MPB are easily affected by the complex interaction of biotic and abiotic factors,it is difficult to efficiently realize large-scale monitoring and ecological function assessment of MPB based on traditional field sampling.Meanwhile,in the application of remote sensing,the impact of the complex environment on the retrieval of MPB on the tidal flat has not been fully considered,the macro estimation of the biomass below the surface has not been fully explored,and the ecological function supply of MPB based on remote sensing means needs to be discussed.Therefore,this study will use multisource remote sensing data to construct high-precision algorithms,quantify the temporal and spatial variations in MPB biomass in estuarine tidal flats,and analyse its bottom-up effect.The results will help to understand the essential ecological status of MPB in tidal flats,optimize coastal management and biodiversity conservation strategies,and provide decision support.This study used the tidal flat of the Changjiang estuary in China and the Westerschelde estuary in the Netherlands as the study areas.We researched MPB biomass and its bottom-up effect through in situ field investigation,laboratory analysis,control experimental design,and optical remote sensing model construction.The two estuarine tidal flats belong to muddy tidal flats,and the study area belongs to Wetlands of International Importance（Ramsar Sites）,with relatively consistent wetland typicality and ecological importance,which can ensure a good undertaking relationship of the research system.The data collected at the two estuarine tidal flats can meet the complementary research content,which can effectively support the research framework of this study.The main research progress and understandings achieved include the following:1.The retrieval of MPB surface biomass in the estuarine tidal flat can be realized by the comprehensive application of optical and radar satellite data,which can effectively weaken the influence of the complex environmental background on the retrieval of MPB.（1）The absorption depth in red light has a more significant correlation with the biomass of surface microphytobenthos,and the regression algorithm constructed based on this characteristic can effectively invert the surface biomass of microphytobenthos on the estuarine tide flat;continuum removal processing can effectively enhance the spectral absorption characteristics of MPB and can eliminate the background effect of sediment more than NDVI.（2）The microphytobenthos and salt marsh in tidal flats have similar absorption characteristics in red light but apparent differentiation in red edge light,which is further strengthened after continuum removal processing;the effective discrimination and masking of salt marsh help to improve the accuracy of the MPB biomass inversion algorithm.（3）The comprehensive application of radar data and optical data with high temporal and spatial resolutions can effectively distinguish turbid pooled water and exposed tidal flats and realize the mask of pooled water.The relative elevation information of the tidal flat can be extracted through the radar data and the associated tidal level information throughout the year,which helps understand the distribution characteristics of surface microphytobenthos biomass in the tidal flat.（4）The case study of Chongming Dongtan in the Changjiang estuary shows that the surface MPB in the supratidal zone and the intertidal zone have different seasonal distribution patterns;the highest value is not limited to a specific season or a specific region,which may be caused by a combination of biotic and abiotic factors.2.Remote sensing inversion based on sediment surface parameters can solve the problem of difficulty in estimating the underground biomass of MPB in the tidal flat at the macroscale and estimate the total biomass of MPB in the tidal flat of the estuary.（1）The tidal flat microphytobenthos biomass has a prominent exponential decay vertical distribution model,and this model has a better fit than other vertical distribution models.The decay rate in the model is variable in different depositional environments;generally,the larger the sediment grain size is,the smaller the water content,the smaller the organic matter content,and the larger the decay rate.（2）The correlation between the biomass of surface MPB in tidal flats and the decay rate is the most significant.However,the best subset regression model shows that the surface MPB biomass,the median grain size and water content of surface sediments and their interaction are the best predictive variables for predicting the decay rate of MPB.They can be used to calculate the vertical distribution of MPB and realize the inversion of MPB total biomass.（3）Indoor control experiments show that higher sediment water content（W₀）and larger median grain size(D₅₀)result in lower reflectance in the visible and near-infrared light;when W₀ reaches a particular value（shown to be 40%in this study）,the reflectance of near-infrared light relative to visible light decreases slowly with increasing W₀ and D₅₀,and it is more sensitive to changes in W₀ and D₅₀,which can be used to develop remote sensing algorithms.（4）The case study of Chongming Dongtan in the Changjiang estuary shows that the seasonal variation in MPB biomass in the estuary tidal flat can be achieved through the vertical integration of surface MPB and biomass decay rate.The total biomass in the study area varies with the seasons,with the highest biomass in summer,and the difference between the north and south is significant.The northern region has a relatively high sediment water content,relatively small sediment grain size,and higher total biomass than the southern region.3.The comprehensive application of hyperspectral remote sensing and multispectral remote sensing can realize the spatial analysis of the bottom-up effect of MPB on macrobenthos and expand the macro understanding of the ecological function of MPB.（1）Pheophytin a（Pheo-a）in sediments is much smaller than pheophorbide a（Pheob-a）and other significant pigments.The process of Chl-a degradation to Pheo-a in the study area can be ignored.（2）Pheob-a/Chl-a has a high correlation with the standing stock of macrobenthos,including the abundance and.It can effectively characterize the abundance of omnivorous（Bivalve,Malacostraca）,filter-feeding and herbivorous macrobenthos.（3）The principal component regression model based on blue light can realize the inversion of Pheob-a/Chl-a and the macroscopic expression of the spatial distribution of macrobenthos abundance indicated by Pheob-a/Chl-a.（4）The MPB has a bottom-up effect on macrobenthos,and the MPB biomass（Chl-a）can be used to predict the distribution of macrobenthos abundance.The time-series spatial distribution of MPB biomass（Chl-a）and macrobenthos abundance（Pheob-a/Chl-a）can be obtained by comprehensively using multispectral and hyperspectral satellite data.The comparative analysis shows that the bottom-up effect of MPB on macrobenthos is distributed in patches.In conclusion,the temporal and spatial distribution of MPB in estuary tidal flats is affected by both abiotic and biotic effects and has a bottom-up effect on macrobenthos.High spatial and temporal resolution multispectral,hyperspectral optical and radar data have strong application potential and complementarity.Our study highlights the importance of applying remote sensing big data in MPB mapping,analysing its spatiotemporal dynamic distribution and bottom-up effect.It can provide an essential reference for future research on the biodiversity of estuarine tidal flats and the material and energy cycle mechanism of the benthic ecosystem.