Spatiotemporal characteristics and remote sensing estimation of colored dissolved organic matter around the Leizhou Peninsula

Colored dissolved organic matter(CDOM)is an important optically active substance in marine environment.Its biochemical conservatism makes it an important indicator to offshore pollution process.Monitoring the content,composition,and diffusion process of CDOM is a good approach to analyze the terrestrial input,optical properties,and ecological environment of offshore areas.The spatiotemporal characteristics of CDOM around the Leizhou Peninsula were analyzed based on field observation data collected in the autumn 2020 and spring 2021,and an empirical inversion model of the ag(355)(absorption coefficient at 355 nm)and spectral slope(Sg)(g stands for gelbstoff/gilvin,which is called CDOM)based on Sentinel-3A ocean and land color instrument(OLCI)images was constructed.The results show(1)the order of average ag(355)value around the Leizhou Peninsula was east coast(0.503/m)>Qiongzhou Strait(0.502/m)>west coast(0.365/m);(2)the best band combinations of CDOM inversion in spring and autumn were(B4+B11)/B3 and(B7-B1)/B6(B stands for the band of spectral images),and the final inversion results are close to the measured results,indicating that the model has good accuracy;(3)the S_(g)value of the CDOM absorption spectrum was fitted to the hyperbolicexponential model.The fitting accuracy of the model was higher than those of the exponential model and the hyperbolic model,and the best S_(g)inversion model was constructed by selecting Sg(275-295)and Sg(250-295)in spring and autumn;(4)the spatial distributions of ag(355)and S_(g)were inverted,and CDOM in the waters around the Leizhou Peninsula originated from terrestrial organic matter carried by coastal aquaculture zones and runoff in the northeast Zhanjiang and the northern Beibu Gulf.

Seasonal Distribution of Observed Colored Dissolved Organic Matter(CDOM)in the Eastern Indian Ocean

Colored dissolved organic matter(CDOM)is a crucial constituent that affects the optical absorption properties of seawater.Owing to the relatively limited measured data on the spatial distribution characteristics of CDOM in the tropical eastern Indian Ocean,this study analyzes the optical absorption characteristics of CDOM in the southeast Indian Ocean using the data collected during four seasons from 2013 to 2017.This work also systematically describes the seasonal horizontal and vertical distribution characteristics of CDOM in this area and conducts a preliminary analysis of the relevant factors affecting CDOM absorption characteristics in this region.Results indicate that the CDOM ag(440)during summer was remarkably lower than that in the coastal waters of Europe and coastal waters of China but slightly higher than that in the western and southeast Pacific.The spatial distribution of surface CDOM shows remarkable seasonal differences,and the spatial distribution characteristics of CDOM in the 5°S,92°E region differ between spring/summer and autumn/winter.The values of ag(400)and ag(440)are weak/strong at a surface/subsurface level of 100 m,with differences found between summer and winter.The correlation of CDOM with temperature,salinity,and chlorophyll-a concentration is relatively low,indicating that CDOM is an independent driving mechanism influenced by phytoplankton degradation,photobleaching,and water mixing.

The satellite reversion of dissolved organic carbon (DOC) based on the analysis of the mixing behavior of DOC and colored dissolved organic matter: the East China Sea as an example

The retrieval of dissolved organic carbon （DOC） distribution by remote sensing is mainly based on the em- pirical relationship of DOC concentration and colored dissolved organic matter （CDOM） concentration in many literatures. To investigate the nature of this relationship, the distributions and mixing behaviors of DOC and CDOM are reviewed in the world＇s major estuaries and bays. It is found that, generally, the C- DOM concentration is well correlated with the salinity in most estuaries, while DOC usually shows a non- conservative behavior which leads to a weak correlation between the DOC concentration and the CDOM concentration. To establish a good satellite reversion of the DOC concentration, the East China Sea（ECS） was taken as an example, and the mixing behavior of DOC and CDOM as well as the influence of biogeo- chemical processes were analyzed except for the physical mixing process with the data from late autumn （November, 2010） and winter （December, 2009） cruises. In the two ECS cruises, the CDOM concentration was found to be tightly correlated with the salinity, influenced little by the photochemical or biological pro- cesses. The data from the winter cruise show that DOC followed a conservative mixing along the salinity gradient, while in the late autumn cruise it was significantly affected by the biological activities, resulting in a poor correlation between the DOC and the CDOM. Accordingly, an improved DOC algorithm （CSDM） was proposed： when the biological influence was significant （Chl a greater than 0.8 μg/dm3）, DOC was retrieved by the conservative and biological model, and if the conservative mixing was dominant （Chl a less than 0.8 μg/dm3）, the direct DOC concentration and CDOM concentration relationship was used. Based on the pro- posed algorithm, a reasonable DOC distribution for the ECS from satellite was obtained in this study, and the proposed method can be applied to the other large river-dominant marginal sea.

Spatial and temporal variability of colored dissolved organic matter absorption properties in the Taiwan Strait

Spatial and temporal variability of the absorption properties of colored dissolved organic matter （CDOM） in the Taiwan Strait was investigated in summer （July to August of 2006） and winter （from December 2006 to January of 2007） seasons. The CDOM absorption coefficient at 280 nm （a 280 ） showed a decreasing trend from nearshore to offshore areas while the spectral slope coefficient parameter calculated between wavelengths 275–295 nm （S 275 295 ） showed an increase, indicative of decreasing aromaticity and molecular weight of the CDOM. The average a 280 in winter （1.47 ± 0.50 m ^-1 ） was significantly higher than in summer （1.10 ± 0.41 m ^-1 ）, while the average S 275 295 in winter （26.7 ± 5.2 μm^- 1 ） was significantly lower than in summer （30.6 ± 5.5 μm^- 1 ）, demonstrating clear seasonal variation in CDOM abundance and properties in the Taiwan Strait. A three-end- member conservative mixing model showed that local terrestrial CDOM inputs from several rivers along the western coast were small （〈5%）. However, the distribution of CDOM in the Taiwan Strait is mainly controlled by water mass movement [i.e., the Zhe-min Coastal Current （ZCC） and the Kuroshio Branch Current （KBC） in winter and the South China Sea Water （SCSW） in summer]. Biological activity was also an important factor affecting the distribution of CDOM in the offshore region in summer months.

Chemical properties of colored dissolved organic matter in the sea-surface microlayer and subsurface water of Jiaozhou Bay, China in autumn and winter

The distribution and chemical properties of chromophoric dissolved organic matter （CDOM） in the Jiaozhou Bay, China were examined during four cruises in 2010-2011. The influence of freshwater and industrial and municipal sewage along the eastern coast of the bay was clearly evident as CDOM level- s （defined as a30s）, and dissolved organic carbon （DOC） concentrations were well correlated with salinity during all the cruises. Moreover, DOC concentrations were significantly correlated with chlorophyll a con- centrations in the surface microlayer as well as in the subsurface water. The concentrations of DOC and CDOM displayed a gradually decreasing trend from the northwestern and eastern coast to the central hay, and the values and gradients of their concentrations on the eastern coast were generally higher than those on the western coast. In addition, CDOM and DOC levels were generally higher in the surface microlayer than in the subsurface water. In comparison with DOC, CDOM exhibited a greater extent of enrichment in the microlayer in each cruise, with average enrichment factor （EF） values of 1.38 and 1.84, respectively. Four fluorescent components were identified from the surface microlayer and subsurface water samples and could be distinguished as peak A, peak T, peak B and peak M. For all the cruises, peak A levels were higher in the surface microlayer than in the subsurface water. This pattern of variation might be attributed to the terrestrial input.

The dynamic observation of dissolved organic matter in the Zhujiang(Pearl River)Estuary in China from space

The distributions of estuarine colored dissolved organic matter（CDOM）are the combined results of physicalbiogeochemical processes.Remote sensing is needed to monitor highly dynamically estuarine CDOM.Using in situ data from four seasonal cruises,an algorithm is developed to estimate CDOM absorption coefficient at 400nm（aCDOM（400））in the Zhujiang（Pearl River）Estuary（ZJE）.The algorithm uses band ratios of Rrs（667）/Rrs（443）and Rrs（748）/Rrs（412）.By applying it to moderate resolution imaging spectroradiometer onboard Aqua satellite（MODIS/Aqua）data from 2002 to 2014,seasonal climatology aCDOM（400）in the ZJE is calculated.CDOM distributions are majorly influenced by water discharge from the Zhujiang River and underwater topography.Along the section vertical to a water depth gradient,the seasonal aCDOM（400）exponentially decreased（y=aebx,b〈0）,but with great differences among seasons.Riverine fresh water is the primary source of CDOM in the ZJE.Fulvic acid fraction decreases with increasing salinity.Using developed algorithms,conservative CDOM mixing equation,and river discharge,effective riverine end-member concentration and flux of dissolved organic carbon（DOC）in summer and winter from 2002 to 2014 are first estimated from the MODIS/Aqua data.Both effective riverine end-member DOC concentration and flux are positively related to the river discharge,significantly in summer with R-2 of 0.698 for concentration and 0.965 7 for flux.

The proportions and variations of the light absorption coefficients of major ocean color components in the East China Sea

The East China Sea （ECS）, one of the largest continental seas, has dynamic hydrology and complex optical characteristics that make ocean color remote-sensing retrieval difficult. The distributions and proportions of the light absorption coefficients of major ocean color components based on two large-scale investigations in the ECS are presented, showing these features in typical summer and winter seasons. The absorption coefficient aCDOM, aNAp and aphy of colored dissolved organic matter, non-algal particle, and pigment of phytoplankton show a decreasing trend from the coast to the outer shelf. According to the aeDOM distribution at 440 nm, the Changjiang River plume shows an abnormal southeastward transport. An extremely high aNaP value patch at 440 nm is present in the middle coast. The chlorophyll-a-specific phytoplankton pigment absorption （a^hy） is much higher in winter than in summer, which may cause serious underestimated results when applying the averaged aphy into remote-sensing algorithms for chlorophyll concentration retrieval. The importance of phytoplankton size was evident in outer shelf waters. The absorption of aCDOM （440） is a dominant component accounting for over half of the total seawater absorption in summer. The aNAP（440） accounts for 64% of the absorption of the ECS coastal area in winter.

Feasibility Analysis of MERIS as a Tool for Monitoring Lake Guiers (Senegal) Water Quality

ENVISAT/MERIS scenes of Lake Guiers covering the period 2003-2010 were processed for concentration retrieval of chlorophyll a (CHLa), suspended particulate matter (SPM) and colored fraction of dissolved organic matter (CDOM), i.e. the three main parameters relevant to the water quality management of the lake. Estimates in the range of 30 - 117 μg CHLa L-1 (average 62.13 μg·L-1), 0.10 - 29.0 mg SPM L-1 (average 22.01 mg·L-1), and 1.10 - 1.90 CDOM m-1 (average 1.33 m-1) were recorded, suggesting the possibility of occasional poor quality waters in some compartments of the lake. The values calculated as part of this study are consistent with literature data. On the basis of these estimates, interpretations were made as to the feasibility of applying MERIS data for synoptic environmental monitoring purposes. The data were subjected to statistical analysis, including regression analysis and significance tests. Estimates of CHLa and CDOM revealed some level of correlation, which suggests that phytoplankton biomass degradation may account for nearly 47% of the dissolved optical compounds CDOM. Notable areas of high CHLa and CDOM concentrations are found in the southern inshore zone, an environment with less water agitation. In contrast, SPM concentrations tend to increase in environments of very shallow water marked by high water turbulence and bottom mobility. However, it was not possible to fully assess the model performance and detection accuracy of the results due to lack of ground truths. Nonetheless, the results show concentrations that compared well with the insitu data from earlier studies and data reported elsewhere from other lacustrine systems. Therefore, it can be inferred from this study that MERIS data present a useful low-cost (i.e. cost effective and readily available) approach for environmental monitoring of Lake Guiers waters with excellent spatial coverage. In addition, the study highlighted the minimal effect of the so-called “bottom effect” on model predictions, despite the small depth of the lake.