Annual variations in suspended particulate matter within the Dover Strait (original) (raw)
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Oceanologica Acta, 2000
Suspended Particulate Matter (SPM) concentrations at various levels within the water column, together with salinity and temperature, were measured using water samples collected from six stations across the Straits of Dover. The sampling programme covered a 16-month period, undertaken during 23 cruises. On the basis of the spatial variability in the concentrations, the water bodies are divided by several boundaries, controlled by tidal and wind conditions. Within the water column, SPM concentrations were higher near the sea bed than in the surface waters. Throughout the cross-section, maximum concentrations occurred adjacent to the coastlines. Temporal variability in the SPM concentration exists on daily and seasonal scales within the coastal waters (4.2 to 74.5 mg·L − 1 ): resuspension processes, in response to semi-diurnal tidal cycles (with a period of around 12.4 h) and spring-neap cycles (with a period of 15 days) make significant contributions. Distinctive seasonal/annual concentration changes have also been observed. In the offshore waters, such variability is much less significant (0.9 to 6.0 mg·L − 1 ). In the summer the English Coastal Zone is associated with relatively high SPM concentrations: the Central Zone has a low and stable SPM concentration between these zones, there is a Transitional Zone, where there is a rapid response of SPM concentration to wind forcing. Finally, the French Coastal Zone is characterized by variable (sometimes high) SPM concentrations. Because of the zonation, SPM fluxes within the Dover Strait are controlled by different transport mechanisms. Within the Central Zone, the flux can be represented by the product of mean water discharges and SPM concentrations. However, within the coastal zones fluctuations in SPM concentrations on various time-scales must be considered. In order to calculate the maximum and minimum SPM fluxes, 10 cells were divided in the strait. A simple modelling calculation has been proposed for this complex area. The effect of spring-neap tidal cycles and seasonal changes can contribute significantly to the overall flux, which is of the order of 20× 10 6 t·yr − 1 (through the Dover Strait, towards the North Sea). Such an estimate is higher than most obtained previously. © 2000 Ifremer/ CNRS/IRD/É ditions scientifiques et médicales Elsevier SAS suspended particulate matter / flux / transport mechanisms / straits / English Channel Résumé -Flux de matière particulaire en suspension dans le détroit du Pas-de-Calais : observations et modélisation. Les concentrations pondérales de matières en suspension (MES) ont été mesurées, conjointement à la salinité et la température, sur six stations dans le détroit du Pas-de-Calais, au cours de 23 campagnes en mer programmées sur une période de 16 mois. Les variabilités spatiales des concentrations de MES permettent d'identifier des masses d'eaux dont les limites sont contrôlées par les conditions de marée et de vent. Dans la colonne d'eau, les concentrations de MES sont plus fortes près du fond qu'en surface. Dans le détroit du Pas-de-Calais, les concentrations maximales sont observées le long des cô tes. Des variations temporelles des concentrations de MES sont observées à l'échelle journalière et saisonnière dans les eaux cô tières (de 4,2 à 74,5 mg·L -1 ), variations liées principalement à des phénomènes de remise en suspension au cours des cycles semi-diurnes de marée (période moyenne de 12,4 h) et au cours de cycles vives-eaux / mortes-eaux (période de 15 j). Des modifications de concentrations de MES à l'échelle saisonnière ou annuelle ont été également observées. Dans les eaux du large, ces variations sont nettement moins significatives (de 0,9 à 6,0 mg·L -1 ). Pendant la période estivale, la zone cô tière anglaise est caractérisée par des concentrations relativement fortes de MES, la zone centrale du Pas-de-Calais conservant de faibles valeurs de concentrations. Entre ces deux zones, se trouve une zone de transition, où l'élévation des concentrations de MES peut être rapide, en raison de la houle. Enfin, la zone cô tière française est caractérisée par des concentrations de MES variables dans le temps. Les flux de MES dans le détroit du Pas-de-Calais sont contrôlés par des mécanismes de transport différents selon la zone considérée. Dans les eaux centrales, le flux peut être représenté par le produit des flux moyens d'eau par les concentrations de MES. Dans les eaux cô tières, les variations des concentrations de MES doivent être appréhendées à plusieurs échelles de temps. Pour calculer les flux particulaires minimum et maximum, le détroit a été subdivisé en dix cellules. Un calcul simple de modélisation est proposé pour cette zone très complexe. Les cycles vives-eaux / mortes-eaux et les variations saisonnières peuvent modifier significativement le flux total, qui est de l'ordre de 20 × 10 6 t par an à travers le Pas-de-Calais, de la Manche vers la Mer du Nord. Cette estimation est plus élevée que celles obtenues antérieurement. © 2000 Ifremer/CNRS/IRD/É ditions scientifiques et médicales Elsevier SAS matières en suspension / flux / mécanismes de transport / détroit / Manche
Continental Shelf Research, 2015
The objective of the study described in this paper is to localize the transport path of suspended particulate matter (SPM) in the Dutch coastal zone in the southern North Sea. It is known that a large mass of SPM is transported northward from the Strait of Dover, which is however mostly hidden from satellite and other surface measurements. The study area is located at 80 km north of the Rhine-Meuse estuary mouth in the far-field plume of the region of freshwater influence (ROFI). We investigate the occurrence and persistence of a turbidity maximum zone (TMZ) in an area closer to the coast than studied in previous observational programs. Shipboard measurements of vertical profiles of SPM concentrations, density and current velocities with a high cross-shore spatial resolution are presented. A turbidity maximum zone is found at a distance between 0.5 and 3 km from the coast along 30 km of the coastline. Observed concentrations are shown to vary strongly within a tidal cycle, and also between contrasting meteorological conditions in terms of the spring-neap tidal cycle, the significant wave height and the wind force. Temporary stratification is observed during spring tides, and occurs on the ebb phase of the tidal cycle. Cross-shore transports at a transect perpendicular to the coast show an accumulation of SPM in the TMZ within one tidal cycle. Possible mechanisms for this accumulation close to the coast are discussed.
Estuaries, 1999
The effects of fortnightly, semidiurnal, and quaterdiurnal lunar tidal cycles on suspended particle concentrations in the tidal freshwater zone of the Seine macrotidal estuary were studied during periods of medium to low freshwater flow. Long-term records of turbidity show semidiurnal and spring-neap erosion-sedimentation cycles. During spring tide, the rise in low tide levels in the upper estuary leads to storage of water in the upper estuary. This increases residence time of water and suspended particulate matter (SPM). During spring tide periods, significant tidal pumping, measured by flux calculations, prevents SPM transit to the middle estuary which is characterized by the turbidity maximum zone. On a long-term basis, this tidal pumping allows marine particles to move upstream for several tens of kilometers into the upper estuary. At the end of the spring tide period, when the concentrations of suspended particulate matter are at their peak values and the low-tide level drops, the transport of suspended particulate matter to the middle estuary reaches its highest point. This period of maximum turbidity is of short duration because a significant amount of the SPM settles during neap tide. The particles, which settle under these conditions, are trapped in the upper estuary and cannot be moved to the zone of maximum turbidity until the next spring tide. From the upper estuary to the zone of maximum turbidity, particulate transport is generated by pulses at the start of the spring-neap tide transition period.
Geo-Marine Letters, 2012
The study of water clarity is essential to understand the variability in biological production,particularly in coastal seas. The spatial and temporal variability of non-algal suspended particulate matter (SPM) in surface waters of the English Channel was investigated and related to local forcing by means of a large satellite dataset covering the study area with a spatial resolution of 1.2 km and a daily temporal resolution. This analysed dataset is a time series of non-algal SPM images derived from MODIS and MERIS remote-sensing reflectance by application of an IFREMER semi-analytical algorithm over the period [2003][2004][2005][2006][2007][2008][2009]. In a first step, the variability of time series of MODIS images was analysed through temporal autocorrelation functions. Then, nonalgal SPM concentrations were assessed in terms of site-specific explanatory variables such as tides, wind-generated surface-gravity wave amplitudes and chlorophyll-a (Chl-a), based on three statistical models with fitting parameters calibrated on a dataset of merged MERIS/MODIS images gathered from 2007 to 2009 over the whole English Channel. Correlogram analysis and the first model highlight the local patterns of the influence of the tide, especially the neap-spring cycle, on non-algal surface SPM. Its effect is particularly strong in the central and eastern English Channel and in the western coastal areas. The second model shows that waves prevail as driver at the entrance of the English Channel. The most sophisticated of the three statistical models, although involving only three explanatory variables-the tide, waves and Chl-a -is able to estimate nonalgal surface SPM with a coefficient of determination reaching 70% at many locations.
Continental Shelf Research, 2000
A comparison based on the bulk composition of sur"cial sediment particulate organic matter (POM) has been carried out on mud#ats representative of two large European macrotidal estuarine zones: the Marennes-OleH ron Bay and the Humber Estuary. Samples were collected in early summer, at low tide, during medium spring tide. Six sites were investigated along a 2500 m cross-shore transect with 12 cores collected at each site. Elemental analyses (C, N) and proximate analyses (chloropigments, carbohydrates and lipids) were performed on the fraction (125 m at 4 levels: 0}1, 1}3, 3}6 and 6}10 cm. The POM was largely detrital, especially in the Humber mud#at. No clear decrease with depth in sediment was recorded for particulate organic carbon, total nitrogen and lipids, which emphasized the importance of sediment reworking by bioturbation and resuspension processes. However, decreases were observed for chloropigments and carbohydrates. Spatially, the general seaward decrease in the concentration of most compounds suggested a preferential accumulation of POM in the sheltered sites close to shore. Multivariate analysis showed that the POM collected at the sediment surface is similar between Marennes-OleH ron and the Humber: it is mainly fresh and labile POM, probably largely derived from microphytobenthos. By contrast, deeper layers develop a signature which depends on the wider environment of the area studied: the signi"cance of nitrogen and carbohydrates in Marennes-OleH ron samples may re#ect the in#uence of bivalve biodeposits rich in polysaccharides, whereas the importance of carbon and lipids in Humber samples may be partly related to pollution. This study demonstrated that it was possible to discriminate
2000
The geochemistry, as de®ned by amounts of easily reducible Mn (ERMn; Mn oxides), reducible Fe (RFe; Fe oxides), organic matter (% loss on ignition), total metal (Cu, Pb and Zn) and metals associated with the ERMn, RFe and organic matter components of deposited sediments (DS) and suspended particulate matter (SPM) were contrasted over a 1-year period (two-way ANOVA with sediment type and month as the two factors) within the Fraser River Estuary, BC, Canada. The geochemistry of SPM as compared to DS was distinctly dierent. The geochemistry of SPM displayed a marked seasonality. By contrast, seasonal dierences in the geochemistry of DS were much less pronounced over the 12-month sampling period. Concentrations of organic matter and RFe in SPM were signi®cantly greater (two-way ANOVA; P < 0.05) in winter months (maximums of 23% and 53 g kg À1 , respectively) as compared to the rest of the year (maximums of 9.3% and 11 g kg À1 , respectively). Concentrations of organic matter in DS did not change over the 12-month period; however, RFe in DS was signi®cantly greater in winter months (7.3 g kg À1) as compared to summer months (2.3 g kg À1). Easily reducible Mn in both SPM and DS was highly variable throughout the year with no apparent seasonal dependence. Total concentrations of Cu, Pb and Zn and their partitioning among the 3 sediment components (i.e. ERMn, RFe and organic matter) were also month-dependent (two-way ANOVA, P < 0.05); metal concentrations in SPM were up to 17 times greater than DS with a higher proportion of these metals associated with the easily reducible component (oxides of Mn and amorphous forms of Fe oxides) during winter as compared to summer months. Trace metal concentrations and partitioning in DS showed the same seasonal trends, although not to the same degree as occurred for SPM, throughout the 4 seasons of study. Seasonal changes in the partitioning of metals in addition to greater proportions of the metal occurring in an easily reducible form in SPM relative to DS has potentially important implications for sediment ingesting organisms capable of ®lter-feeding on both SPM and DS. Speci®cally, metal bioavailability to sediment ingesting organisms from SPM may be seasonally dependent with periods of greatest exposure occurring during winter months, as compared to DS where no seasonal dependence occurs. To identify main vectors of metal exposure to sediment ingesting organisms, both the type of sediment and when they are feeding on the particular type of sediment need to be determined.
Suspended matter in the Scheldt estuary
Hydrobiologia, 2005
The Scheldt estuary is characterised by a specific energy pattern resulting from the interaction of wave energy, tidal energy and river energy. It divides the estuary into three parts and governs suspended matter transport and distribution pattern. Observation of suspended matter transport shows the existence of three estuarine turbidity maxima (ETM), a marine-dominated ETM in the lower estuary at the river mouth, a river-dominated ETM in the upper estuary with suspended matter concentration reaching up to 300 mg/l, and the most important tide-dominated ETM in the middle estuary with suspended matter concentrations from several hundred milligrams per litre up to a few grams per litre. Resuspension is the dominant phenomenon in this last ETM due to the tidal related bottom scour, which is initiated when a critical erosion velocity of 0.56 m/s is exceeded. An assessment of residual current along the axis of the estuary shows distinctive pattern between the surface water flow and the near bottom water flow. Also the local morphology of the river, natural or man-made, has a prominent effect on the orientation and strength of the residual currents flowing along either side of the river or river bend. Evaluation of suspended matter concentration in relation to the current flow shows no systematic correlation either because of phenomena as scour lag and settling lag mainly in the middle estuary, or because of the current independency character of uniform-suspension mainly in the upper and lower estuary. Quantification of suspended matter load exhibits a net downstream transport from the upper estuary, a near-equilibrium sustainable status in the middle estuary and a net upstream transport of suspended matter from the lower estuary. The characteristic of suspended matter is induced by and is a function of e.g. tidal phase, spring-neap tide, longitudinal and vertical distribution mechanisms, seasons, short and long terms of anthropogenic influence and/or estuarine maintenance. Suspended matter is dominated by complex and cohesive organo-mineral aggregates. It consists of a variable amount of an inorganic fraction (average of 89%) and an organic fraction and occurs largely as flocs, the size of which is remarkably larger in the upper estuary and smallest within the ETM in the middle estuary. Independent time series measurements (1990–2000) of suspended matter property show an increasing sand fraction, a decreasing organic matter content, a rise in δ13C as well as a decrease in water transparency. These independent measurements exhibit coherent consequences of estuarine maintenance operations. Maintenance dredging of the shipping channel and harbours and dumping operation in the Scheldt strengthen marine influence further landward, resulting in a sustained tidal range increment and upstream flow and transport of suspended matter.
Continental Shelf Research, 1999
We simulate the distribution of suspended particulate matter in the southern North Sea using a three-dimensional baroclinic shelf-sea model. Process studies with individual coastal sources show the transport of material across the southern North Sea and out into the Norwegian trench is sensitive to specific wind conditions and settling velocities, so that material which settles during the summer can be transported faster than a passive tracer if its flux is unidirectional. Results from model runs with a number of coastal sources and size classes are compared with CTD data from the North Sea Project, using multiple linear regression to estimate the source strengths. The agreement is best during the winter, with correlation coefficients greater than 0.7; this falls below 0.4 in the summer. We estimate the input from Holderness and the Wash to be 5.6 M tonnes/year, and from the East Anglian coast to be 2.1 M tonnes/year. Due to the limited resolution of the model and the observations, we find we are unable to estimate the input from the English Channel or the Rhine with any degree of certainty.