Surface Suspended Particulate Matter Concentration Research Articles

An Algorithm to Estimate Suspended Particulate Matter Concentrations and Associated Uncertainties from Remote Sensing Reflectance in Coastal Environments

Suspended Particulate Matter (SPM) is a major constituent in coastal waters, involved in processes such as light attenuation, pollutant propagation, and waterways blockage. The spatial distribution of SPM is an indicator of deposition and erosion patterns in estuaries and coastal zones and a necessary input to estimate the material fluxes from the land through rivers to the sea. In-situ methods to estimate SPM provide limited spatial data in comparison to the coverage that can be obtained remotely. Ocean color remote sensing complements field measurements by providing estimates of the spatial distributions of surface SPM concentration in natural waters, with high spatial and temporal resolution. Existing methods to obtain SPM from remote sensing vary between purely empirical ones to those that are based on radiative transfer theory together with empirical inputs regarding the optical properties of SPM. Most algorithms use a single satellite band that is switched to other bands for different ranges of turbidity. The necessity to switch bands is due to the saturation of reflectance as SPM concentration increases. Here we propose a multi-band approach for SPM retrievals that also provides an estimate of uncertainty, where the latter is based on both uncertainties in reflectance and in the assumed optical properties of SPM. The approach proposed is general and can be applied to any ocean color sensor or in-situ radiometer system with red and near-infra-red bands. We apply it to six globally distributed in-situ datasets of spectral water reflectance and SPM measurements over a wide range of SPM concentrations collected in estuaries and coastal environments (the focus regions of our study). Results show good performance for SPM retrieval at all ranges of concentration. As with all algorithms, better performance may be achieved by constraining empirical assumptions to specific environments. To demonstrate the flexibility of the algorithm we apply it to a remote sensing scene from an environment with highly variable sediment concentrations.

The dynamics of suspended particulate matter (SPM) and chlorophyll-a from intratidal to annual time scales in a coastal turbidity maximum

The analysis of 1.8years of data gives an understanding of the response to varying forcing of suspended particulate matter (SPM) and chlorophyll-a (CHL-a) in a coastal turbidity maximum zone (TMZ). Both temporal and vertical concentration variations in the near-bed layer (0–2m) in the shallow (11m deep) coastal zone at 1km off the Dutch coast are shown. Temporal variations in the concentration of both parameters are found on tidal and seasonal scales, and a marked response to episodic events (e.g. storms). The seasonal cycle in the near-bed CHL-a concentration is determined by the spring bloom. The role of the wave climate as the primary forcing in the SPM seasonal cycle is discussed. The tidal current provides a background signal, generated predominantly by local resuspension and settling and a minor role is for advection in the cross-shore and the alongshore direction. We tested the logarithmic Rouse profile to the vertical profiles of both the SPM and the CHL-a data, with respectively 84% and only 2% success. The resulting large percentage of low Rouse numbers for the SPM profiles suggest a mixed suspension is dominant in the TMZ, i.e. surface SPM concentrations are in the same order of magnitude as near-bed concentrations.

Surface suspended particulate matter concentration in the Taiwan Strait during summer and winter monsoons

The Taiwan Strait (TS), situated between Taiwan and China, is shallow, relatively turbid, and characterized by strong tidal currents and winter and summer monsoon seasons. The aim of this study was to use images from the Moderate Resolution Imaging Spectroradiometer (MODIS) on board the Aqua satellite to investigate how local sediment sources in addition to the seasonality in wind, oceanographic currents, and waves influence the suspended particulate matter (SPM) dynamics in the TS. In winter, northeast (NE) winds drive the China Coastal Current southward. Cold water with a high SPM concentration is transported southward into the Strait. After the highest SPM concentration reaches its peak in December and January, the winds weaken and the SPM concentration decreases. During summer, winds are less strong and SPM concentration is lower. Although typhoons typically occur in summer, they generate only a weak signal in the surface SPM concentration data from MODIS because of the low number of cloud-free images during these periods. Typhoons result in a short-term increase in the SPM concentration but do not strongly influence the seasonal values in the satellite-derived SPM concentration maps.

Interannual variability of surface and bottom sediment transport on the Laptev Sea shelf during summer

Abstract. Sediment transport dynamics were studied during ice-free conditions under different atmospheric circulation regimes on the Laptev Sea shelf (Siberian Arctic). To study the interannual variability of suspended particulate matter (SPM) dynamics and their coupling with the variability in surface river water distribution on the Laptev Sea shelf, detailed oceanographic, optical (turbidity and Ocean Color satellite data), and hydrochemical (nutrients, SPM, stable oxygen isotopes) process studies were carried out continuously during the summers of 2007 and 2008. Thus, for the first time SPM and nutrient variations on the Laptev Sea shelf under different atmospheric forcing and the implications for the turbidity and transparency of the water column can be presented. The data indicate a clear link between different surface distributions of riverine waters and the SPM transport dynamics within the entire water column. The summer of 2007 was dominated by shoreward winds and an eastward transport of riverine surface waters. The surface SPM concentration on the southeastern inner shelf was elevated, which led to decreased transmissivity and increased light absorption. Surface SPM concentrations in the central and northern Laptev Sea were comparatively low. However, the SPM transport and concentration within the bottom nepheloid layer increased considerably on the entire eastern shelf. The summer of 2008 was dominated by offshore winds and northward transport of the river plume. The surface SPM transport was enhanced and extended onto the mid-shelf, whereas the bottom SPM transport and concentration was diminished. This study suggests that the SPM concentration and transport, in both the surface and bottom nepheloid layers, are associated with the distribution of riverine surface waters which are linked to the atmospheric circulation patterns over the Laptev Sea and the adjacent Arctic Ocean during the open water season. A continuing trend toward shoreward winds, weaker stratification and higher SPM concentration throughout the water column might have severe consequences for the ecosystem on the Laptev Sea shelf.

Weather and climate induced spatial variability of surface suspended particulate matter concentration in the North Sea and the English Channel

Images from the Moderate Resolution Imaging Spectroradiometer (MODIS) satellite have been used to investigate the meteorological and climate induced variability of suspended particulate matter (SPM) concentration in the North Sea. The meteorology has been characterized by the 11 weather types deduced from a refined system of Lamb’s classification of synoptic weather charts. Climatological effects have been related to the North Atlantic Oscillation index. The surface SPM concentration maps from MODIS have been ensemble averaged according to these weather types or climatological conditions. The data show that each type has a distinct distribution of surface SPM concentration in the North Sea. The differences are explained by different hydrodynamic and wave conditions. The occurrence of storms will impact the shallow regions by increasing the resuspension of bottom material. Prevailing winds will, on the other hand, change the residual transport of SPM in the North Sea. The more protected Southern Bight exhibits relatively stronger influences of advection, whereas in the central North Sea and the German Bight resuspension is more pronounced. This patterns result in an alternation of relatively high SPM concentration in the Southern Bight and in the rest of the southern North Sea during certain weather conditions. Limitations in satellite images have been assigned to stratification effects due to the occurrence of highly concentrated mud suspensions during certain weather types. The approach provides a tool to improve our understanding of coastal and shelf sea processes, especially with respect to variations of SPM concentration distribution according to weather, climate and climate change.

Daily and seasonal dynamics of suspended particles in the Rhône River plume based on remote sensing and field optical measurements

Satellite ocean colour remote sensing can serve as a powerful tool to assess river plume characteristics because it provides daily mapping of surface suspended particulate matter (SPM) concentration at high spatial resolution. This study’s ultimate objective was to better understand daily and seasonal particle dynamics in a coastal area strongly influenced by freshwater discharge and wind—the Rhone River (France), this being the major source of terrestrial input to the Mediterranean Sea. SPM concentrations and biogenic composition (chlorophyll a, organic carbon) were investigated during several bio-optical field campaigns conducted in spring–autumn of 2010 both from aboard a research vessel and by means of an autonomous profiling float. Freshwater discharge and wind velocities varied significantly during the year, associated with marked fluctuations in surface SPM (upper 1 m), even within hours and not restricted to any specific season. Thus, the range was ca. 12–25 g m–3 (dry mass basis) on 9 April (spring), and ca. 3–39 g m–3 on 4–5 November (late autumn). Short-term variations were observed also in SPM composition in terms of POC (albeit not chl a), with POC/SPM ratios ranging between ca. 3 and 11% over ca. 3 weeks in spring. Nevertheless, the particulate backscattering coefficient (b bp) proved to be a robust proxy of SPM concentration in the river plume (b bp(770) = 0.0076 × SPM, R2 = 0.80, N = 56). It has recently been demonstrated that 80% of the Rhone’s terrestrial discharge occurs during flood events. The results of the present study revealed that, under these conditions, SPM is constrained largely within surface waters (i.e. at depths <5 m), with only weak daily vertical variability. By implication, ocean colour satellite data are highly suitable in meaningfully estimating the overall SPM load exported by the Rhone River to the Mediterranean. These findings make a solid contribution to future improvements of three-dimensional sediment transport models for the region and similar settings.

Improving the description of the suspended particulate matter concentrations in the southern North Sea through assimilating remotely sensed data

The integration of remote sensing data of suspended particulate matter (SPM) into numerical models is useful to improve the understanding of the temporal and spatial behaviour of SPM in dynamic shelf seas. In this paper a generic method based on the Ensemble Kalman Filtering (EnKF) for assimilating remote sensing SPM data into a transport model is presented. The EnKF technique is used to assimilate SPM data of the North Sea retrieved from the MERIS sensor, into the computational water quality and sediment transport model, Delft3D-WAQ. The satellite data were processed with the HYDROPT algorithm that provides SPM concentrations and error information per pixel, which enables their use in data assimilation. The uncertainty of the transport model, expressed in the system noise covariance matrix, was quantified by means of a Monte Carlo approach. From a case study covering the first half of 2003, it is demonstrated that the MERIS observations and transport model application are sufficiently robust for a successful generic assimilation. The assimilation results provide a consistent description of the spatial-temporal variability of SPM in the southern North Sea and show a clear decrease of the model bias with respect to independent in-situ observations. This study also identifies some shortcomings in the assimilated results, such as over prediction of surface SPM concentrations in regions experiencing periods of rapid stratification/de-stratification. Overall this feasibility study leads to a range of suggestions for improving and enhancing the model, the observations and the assimilation scheme.

Turbulent Mixing due to Surface Waves Indicated by Remote Sensing of Suspended Particulate Matter and Its Implementation into Coupled Modeling of Waves, Turbulence, and Circulation

Abstract This paper studies the impact of the surface waves on the turbulent mixing. The satellite observations of suspended particulate matter (SPM) at the ocean surface as an indicator of turbulent quantities of the flow are used. In a water column, SPM builds a vertical profile depending on settling velocities of the particles and on vertical mixing processes; thus, SPM is a perfect marker to study the turbulent quantities of the flow. Satellite observations in the North Sea show that surface SPM concentrations, in locations of its deposition, grow rapidly and build plume-shaped, long (many kilometers) uninterrupted and consistent structures during a storm. Also, satellites reveal that SPM rapidly sinks to the seabed after the storm peak has passed and wave height decreases (i.e., in the absence of strong turbulence). The nonbreaking wave-induced turbulence has been discussed, parameterized, and implemented into an equation of evolution of turbulent kinetic energy (TKE) in the frame of mean-flow concept, which can be used in existing circulation models. The ratio between dissipated and total wave energy is used to describe the influence of wave damping on the mean flow. The numerical tests reproduce experiments in a wave tank very well and are supported by observations of SPM in the North Sea. Their results show that the motion of an individual nonbreaking wave includes turbulent fluctuations if the critical Reynolds number for wave motion is exceeded, independent of the presence of currents due to wind or tides. These fluctuations can produce high diffusivity and strongly influence mixing in the upper water layer of the ocean.

Evaluation of in situ and remote sensing sampling methods for SPM concentrations, Belgian continental shelf (southern North Sea)

Large sets of suspended particulate matter (SPM) concentration data from in situ and remote sensing (moderate resolution imaging spectroradiometer, MODIS) samplings in the Belgian nearshore area (southern North Sea) are combined in order to evaluate their heterogeneity and the sampling techniques. In situ SPM concentration measurements are from a vessel (tidal cycle) and from a tripod. During the tidal cycle measurements, vertical profiles of SPM concentration have been collected; these profiles have been used as a link between satellite surface and near-bed tripod SPM concentrations. In situ time series at fixed locations using a tripod are excellent witnesses of SPM concentrations under all weather conditions and may catch SPM concentration variability with a much finer scale. The heterogeneity has been statistically assessed by comparing the SPM concentration frequency distributions. Tidal cycle, tripod and MODIS datasets have different distributions and represent a different subpopulation of the whole SPM concentrations population. The differences between the datasets are related to meteorological conditions during the measurements; to near-bed SPM concentration dynamics, which are partially uncoupled from processes higher up in the water column; to the sampling methods or schemes and to measurement uncertainties. In order to explain the differences between the datasets, the tripod data have been subsampled using wave height conditions and satellite and tidal cycle sampling schemes. It was found that satellites and low-frequent tidal cycle measurements are biased towards good weather condition or spring–summer seasons (satellite). The data show that the mean surface SPM concentration derived from satellite data is slightly lower than from in situ tidal cycle measurements, whereas it is significantly lower than the mean SPM concentration interpolated to the water surface from the tripod measurements. This is explained by the errors arising from the interpolation along the vertical profiles, but also by the fact that satellite-measured signal saturates in the visible band used to retrieve SPM concentration in very turbid waters.

Elemental characteristics of surface suspended particulates off the Changjiang estuary during the 1998 flood

Surface suspended particulate matter (SPM) samples were collected on two latitudinal transects over the Changjiang estuary, covering a wide salinity regime from 8 to 30‰, instantaneously after the largest Changjiang flood peak in August 1998 that caused severe disaster in China. They have been analyzed for 20 major and trace elements and 7 rare earth elements (REEs) using ICP-MS. The SPM concentrations varied a very wide range of up to two orders of magnitude, sharply decreasing southward from 18 mg/l in the Changjiang river mouth to less than 1 mg/l at around 28°N, which may be representative of a background in the East China Sea (ECS) shelf. All selected particulate elements exhibit large variability in space. SPM collected around the Changjiang river mouth was dominated by the lithogenic components approximately accounting for 80% or more of the bulk, and when going south the proportions decreased to less than half of the total. Based on the results of enrichment factor (EF) calculation, elements Fe, Ti, Mn, Ba, Sr, Co, Be, Tl, Nb, V, Ni, and Mo with mean EF values of less than 3 were categorized into the unpolluted group, and elements Zn, Cu, Pb, As, Ag, and Sb with EF values of higher than 5 into the polluted group. This investigation demonstrated that anthropogenic metals could be dispersed over a large extent along the Chinese coast and on the ECS shelf although a majority of terrigenous sediments would be deposited within the Changjiang estuary. Additionally, our study reveals that the Qiantangjiang might contribute non-negligible pollutants to Hangzhou Bay. We therefore argue that the study area has been in moderate pollution, different from most of previous suggestions. Overall, most samples have similar chondrite- and especially UCC-normalized distribution patterns (UCC: upper continental crust); the sample/UCC ratios of REEs generally vary within a factor of 3 and display a flat (or slightly convex) pattern, essentially revealing little fractionation for particulate REEs. We estimated the import fluxes of particulate metals to the Changjiang estuary during the flood period; our results highlight the significance when compared to the annual fluxes. The study provides an informative data set with respect to SPM compositions in the Changjiang estuary during a catastrophic flood for assessing environmental changes in the future after the Three-Gorges Dam's operation and when the Yangtze Valley is going to be more industrialized and urbanized.

THE CORRELATION BETWEEN THE AVERAGE REFLECTANCE OF CASE II WATER IN CAOFEIDIAN OFFING AND THE CONCENTRATIONS OF SURFACE SUSPENDED PARTICULATE MATTER

Numerous spectroradiometric measurements were carried out in the Caofeidian offing and,on such a basis,correlation was made between the average reflectance and the concentrations of surface suspended particulate matter(SPM).The empirical relationships were established between remote-sensing reflectance(Rrs) in TM-HRV bands and SPM concentrations.The results show that the remote-sensing reflectance increases with SPM concentrations.When the SPM concentrations are lower than 20 mg/L,the amplitudes of remote-sensing are wider in the 570 nm than in the 670 nm,but when the SPM concentrations are higher than 20 mg/L,things are just the opposite.The best correlations are obtained for Rrs(TM3) and SPM concentrations,which show remarkable exponential correlations.As a result,SPM concentrations within the surface waters in Caofeidian offing are estimated with an average accuracy better than ±28%.

Biogeochemistry of suspended and sedimentary material in the Ob and Yenisei rivers and Kara Sea: amino acids and amino sugars

Suspended particulate matter (SPM) and sediments were sampled in the Ob and the Yenisei rivers and the adjacent Kara Sea up to 77°N during the ice-free period in August and September 1997 (only sediments), 1999 and 2000. The material was analyzed for organic carbon (OC) and nitrogen (N) as well as for the content and monomeric composition of the labile organic constituents amino acids (AA) and amino sugars (AS). Despite interannual variation, surface SPM concentration generally revealed decreasing values from 8 to 4 mg/L for the Ob and Yenisei to values <0.5 mg/L in the offshore region reflecting the importance of fluvial input for SPM distribution. The quality of surface SPM was mainly determined by planktonic productivity as revealed by C/N, the content of AA+AS and their contribution to OC and N. Sedimentation caused a qualitative change of surface SPM at ∼ 73.5°N with POC-% averaging 6.7% south and 21% north of this latitude. This was accompanied by a change from 12–90 to 105–202 mg AA+AS/g SPM, respectively. Together with the monomeric composition of AA this indicates a higher reactivity of OM and a reduced dilution by detrital material in the offshore region. Whereas SPM was homogenously distributed in the rivers, the estuaries were characterized by a salt-wedge at depth. Deep water revealed highest SPM concentrations due to sediment resuspension and/or the presence of a nepheloid layer, which is mirrored in distinctively reduced OM content and reactivity relative to upper water column SPM. Compared to SPM, sediments are characterized by elevated contents of terrestrial OM in the southern study area especially off the Yenisei. OM content and reactivity decrease in offshore direction due to lower sedimentation rates and terrestrial OM input towards the north. The AA-derived reactivity index (RI) and degradation index (DI) together sensitively reflect the degradational stage of OM in SPM and sediments allowing a precise classification from fresh plankton-dominated SPM to degraded sediments. The information on organic carbon formation, recycling and preservation obtained during this study is a valuable contribution towards the assessment of an organic carbon budget for the investigated area.

Remotely Sensed Observations of the Turbidity Maximum in the Highly Turbid Humber Estuary, UK

The Humber Estuary, UK, divides into the Ouse and Trent estuaries at the so-called Apex within its upper reaches. Remotely sensed Compact Airborne Spectrographic Imager (CASI) images and boat measurements were used to observe a strong turbidity maximum in the upper Humber and Ouse during a spring tide in November 1995. Surface suspended particulate matter (SPM) concentrations during the late ebb, as estimated from the CASI data, increased from approximately 6 to 13 g I−1 moving up-estuary into the Ouse. Greater SPM concentrations (∼10 g I−1) were evident in the deeper channels of the Ouse, compared with shallower areas, possibly due to faster ebb currents there and differential down-estuary advection of the turbidity maximum. Ribbons, or streaks, of lower SPM and slightly cooler waters were observed. It appears that slightly cooler and lower turbidity waters from the confluent Trent estuary remained fairly distinct for distances of approximately 2 km down-stream of its confluence with the upper Humber and Ouse. These waters eventually broke into ribbon-like or streak-like structures within the higher SPM-laden and slightly warmer waters of the Humber. They were discernible for more than 5 km down-estuary of the confluence of the Humber, Ouse, and Trent. Boat measurements showed that the turbidity maximum occurred over a fairly restricted region of the upper Humber, between about 20 to 50 km from the tidal limit at high water. The turbidity maximum’s sediment load was largely suspended in the water column during stronger currents. SPM rapidly settled close to the bed during high water and low water slack periods. At these times, SPM concentrations in a thin, near-bed layer were >60 g I−1 in the turbidity maximum region of the Ouse and >30 g 1−1 in the upper Humber (where channel volumes were much greater). SPM within the turbidity maximum comprised very fine-grained material and its low organic content demonstrated that the SPM was essentially mineral, clastic sediment derived originally from erosion and decay of crustal rocks.

Suspended sediment modelling in a shelf sea (North Sea)

This paper extends the modelling of suspended particulate matter (SPM) on the local coastal scale (described in preceding papers) to SPM modelling on the scale of the North Sea, focusing on representing SPM patterns and their seasonal distribution. The modelling includes a sensitivity study, in which model results are assessed using surface SPM concentration patterns extracted from NOAA reflectance imagery, as well as North Sea Project in situ data. Over the past decade or so, first-order estimates of the net suspended load and its associated sources and sinks have been available and are generally substantiated. However, developments in the simulation of large-scale SPM behaviour are still severely restricted by the available descriptions of available sediment sources and sediment erosion and deposition processes. This paper indicates how remotely sensed reflectance images can provide additional information on the spatial distribution of (sea surface) suspended sediments. A primary objective of this paper is to examine sensitivities of SPM simulations in 2D (vertically averaged) and 3D models. A boundary-fitted coordinate modelling approach with intra-tidal resolution and synoptic meteorology is applied, as well as more schematic approaches. A related objective is to examine how both limited in situ observational data and reflectance imagery can be used to assess and improve such simulations. An integrated modelling-monitoring approach, using inverse and ‘Goodness-of-Fit’ (GoF) approaches applied to remotely sensed reflectance imagery, is used to derive a structured sensitivity analysis providing a quantified assessment of the strengths and weaknesses of modelling and input data. It is shown that, especially in the coastal zone where salinity stratification may occur, 3D modelling is required while much of the sensitivity analysis can be based on a 2D modelling approach. This quantification of the effects of uncertainties of inputs and erosion/deposition parameters improves understanding of the sediment distribution and budgets on the North Sea scale. It is concluded that whilst process studies are likely to contribute to improving erosion/deposition algorithms, and model developments will provide enhanced dynamical descriptions, accurate overall simulation will remain dependent on some (inverse) processes to reduce the uncertainty in sediment sources.