An estimate of the suspended particulate matter (SPM) transport in the southern North Sea using SeaWiFS images, in situ measurements and numerical model results

A study is presented where satellite images (SeaWiFS), in situ measurements (tidal cycle and snapshot) and a 2D hydrodynamic numerical model have been combined to calculate the long term SPM (Suspended Particulate Matter) transport through the Dover Strait and in the southern North Sea. The total amount of SPM supplied to the North Sea through the Dover Strait is estimated to be 31.74 x 106 t. The satellite images provide synoptic views of SPM concentration distribution but do not take away the uncertainty of SPM transport calculation. This is due to the fact that SPM concentration varies as a function of tide, wind, spring-neap tidal cycles and seasons. The short term variations (tidal, spring-neap tidal cycle) have not been found in the satellite images, however seasonal variations are clearly visible. Furthermore the SPM concentration in the satellite images is generally lower than in the in situ measurements. The representativness of SPM concentration maps derived from satellites for calculating long term transports has therefore been investigated by comparing the SPM concentration variability from the in situ measurements with those of the remote sensing data. The most important constraints of satellite images are related to the fact that satellite data is evidence of clear sky conditions, whereas in situ measurements from a vessel can be carried out also during rougher meteorological conditions and that due to the too low time resolution of the satellite images the SPM concentration peaks are often missed. It is underlined that SPM concentration measurements should be carried out during at least one tidal cycle in high turbidity areas to obtain representative values of SPM concentration

Chlorophyll-a, total suspended matter and sea surface temperature maps of the North Sea available through the BELCOLOUR project

Since the launch of the first Ocean Colour sensors at the end of the previous century, much research has been devoted to transform Top of Atmosphere radiance measurements into reliable concentration maps of oceanographic parameters at the sea surface like e.g. chlorophyll content, total amount of suspended matter, sea surface temperature. While algorithms to determine chlorophyll in clear open water (so-called case 1 waters) are well established because this is the only parameter changing the spectral signal, they fail in coastal and turbid waters where the spectral signal is the result of the optical properties of a variety of constituents. The BELCOLOUR project improved the theoretical base for establishing concentration maps in coastal waters and developed quality control algorithms. Non reliable or unrealistic data are masked out in the final products to avoid misinterpretation of the data. The BELCOLOUR project worked mainly on satellite imagery from the Ocean Colour sensors SeaWiFS, MERIS and MODIS. All satellite data of the North Sea of these sensors (if not completely clouded) have been processed and transformed into quasi-true colour (RGB), chlorophyll (CHL), total suspended matter (TSM) and, for MODIS, sea surface temperature (SST) maps and made available for public through an easy browsing system on http://www.mumm.ac.be/BELCOLOUR. The satellite data of MERIS and MODIS are processed in near real time in an automated way and the products are presented one day after the acquisition in the Near Real Time Database on the BELCOLOUR website (http://www.mumm.ac.be/BELCOLOUR/EN/Products/NRT/index.php), where they stay for 14 days. Later the data are reprocessed and archived in the BELCOLOUR Image Database (http://www.mumm.ac.be/BELCOLOUR/EN/OCDB/browse.php), also accessible through the BELCOLOUR website. The images are available for different standard geographical areas (North Sea, Southern North Sea and the Channel, Southern North Sea) with both linear and logarithmical scales and are presented as jpeg-files. Different areas and file-formats can be processed by the Remote Sensing and Ecosystem Modelling team of MUMM on request

Spatio-temporal variation of surface suspended particulate matter concentration in the Belgian-Dutch coastal zone

sensing (MODIS-Aqua) data, were evaluated for their use in the assessment of coastal turbidity maximum (CTM) dynamics in Belgian coastal waters. The CTM is a dynamic coastal feature of which the geographic position and extent varies under different meteorological, astronomical and climatological conditions. Analyses were based on grouping-averaging of SPM concentration maps, using different classification schemes. To better spatially depict the CTM, entropy grouping was introduced. This technique analyses, per pixel, the total information contained within the probability distribution of SPM concentration. Results revealed wind-induced variations in position and extent of the CTM, with southwesterly winds inducing a largest CTM extent, in contrast to a strong reduction under northeasterly winds. Climate-induced variations were assessed contrasting 2 winters with opposing indices of the North Atlantic Oscillation (NAO). In a winter with a positive NAO index, hence stronger-than-average southwesterly winds, the CTM was extended to the Dutch waters, whereas the opposite occurred in winters with a negative NAO index, hence less-than-average southwesterly winds. To evaluate astronomical forcing (tides) grouping-averaging was performed of SPM concentration maps over a tidal cycle, and spring-neap conditions. Although, only part of the tidal cycle can be analysed, due to the sun-synchronicity of the MODIS-Aqua satellite, comparison of the results with in-situ data from a single observatory station showed good resemblance. It is concluded that MODIS-Aqua satellite data can be used to assess SPM concentration variability related to tides, neap-spring cycles, meteorological and climatological events

Optical detection of a <i>Noctiluca scintillans</i> bloom

Noctiluca scintillans blooms are often observed as reddish patches in Belgian waters in June-July in calm weather. The possibility of mapping these blooms is investigated here. In June 2005 a dataset of in situ measured reflectance spectra, airborne hyperspectral images, experimental reflectance and absorption spectra of Noctiluca scintillans was collected. The strong optical signature of dense Noctiluca scintillans blooms suggests that mapping these blooms should be feasible. A detection algorithm is proposed based on a combination of a high reflectance threshold with a condition of sharp increase in reflectance in the range 520-580 nm. This algorithm will detect only intense blooms but should distinguish between Noctiluca scintillans and both intense phytoplankton blooms and very turbid water. Noctiluca scintillans detection by optical sensors mounted on ships and airplanes has been confirmed for the June 2005 bloom in Belgian waters. Detection from satellites should also be feasible but only if suitable wavelengths are available and only if the spatial resolution is sufficiently high. The optical properties of this species are thought to be related to gut content. The applicability of this algorithm to other regions and situations therefore remains to be tested

Optical remote sensing in support of eutrophication monitoring in the southern North Sea

Spring mean and maximum chlorophyll a (chla) concentrations are main factors to determine the eutrophication status of the Belgian waters as agreed within OSPAR in 2002. Other important assessment parameters to measure the degree of nutrient enrichment - the amounts of inorganic phosphate and nitrogen in winter - appeared to be above thresholds for most measurements performed in the period 1974-2002. As the standard in situ monitoring programme does not give a clear picture of the temporal and spatial distribution of chl a, it is logical to complement these measurements with optical remote sensing. However, chlorophyll concentrations derived from sensors such as SeaWiFS are unreliable in the Case 2 waters of this region because of high particulate and dissolved yellow substance absorption. Another important limitation of ocean colour sensors is the amount of useful images due to cloud cover. The combination of data from different ocean colour sensors in order to enable a better temporal coverage might be hampered by the different chlorophyll retrieval algorithms used. This study compares different global chl a algorithms (MODIS, SeaWiFS, MERIS) as well as a turbid water algorithm for the Southern North Sea. This is done by running the different algorithms on in situ reflectance spectra collected at 107 stations in the period 2001-2002 over the Southern North Sea and comparing them with in situ chl a concentrations, as well as by running the algorithms on a MERIS image of the 29th of July 2002. Based on this validation the accuracy of these products and their suitability for eutrophication monitoring in the Southern North Sea are assessed

MOnitoring en MOdellering van het cohesieve sedimenttransport en evaluatie van de effecten op het mariene ecosysteem ten gevolge van bagger- en stortoperatie (MOMO): activiteitsrapport 4 (1 oktober 2007 - 31 maart 2008)

The "MOMO" project is part of the general and permanent duties of monitoring and evaluation of the effects of all human activities on the marine ecosystem to which Belgium is committed following the OSPAR convention (1992). The goal of the project is to study the cohesive sediments on the BCP using numerical models as well as by carrying out of measurements. Through this data will be provided on the transport processes, which are essential in order to answer questions on the composition, origin and residence of these sediments on the BCP, the alterations of sediment characteristics due to dredging and dumping operations, the effects of the natural variability, the impact on the marine ecosystem, the estimation of the net input of hazardous substances and the possibilities to decrease this impact as well as this in-put

Calibration and validation of an algorithm for remote sensing of turbidity over La Plata river estuary, Argentina

The La Plata River, located at 35°S on the Atlantic coast, is one of the largest waterways of South America. It carries a large amount of suspended particulate and dissolved organic matter, and is considered among the most turbid waters of the world. Very high values of total suspended matter have been reported in this region, with mean values ranging from 100 to 300 g m-3 and extreme concentrations up to 400 g m-3. Satellite sensors have shown to be the best tools available to map river plumes and to study their influence on the adjacent ocean. However, global algorithms for remotely estimating sediment concentration are not currently available. Moreover, such high sediment loads represent a challenge to atmospheric correction algorithms which usually rely on the assumption of zero water-leaving reflectance in the near infrared or short wave infrared part of the spectrum (black pixel assumption). In the extremely turbid waters of La Plata Estuary such assumptions are not valid. A two band algorithm to estimate turbidity using near infrared and the short wave infrared bands (858 nm and 1240 nm) of the MODIS-Aqua sensor is presented. The model is calibrated using in situ reflectance and turbidity measurements from turbid waters of the Southern North Sea and Scheldt River (Belgium) and then applied to MODIS imagery of La Plata River estuary (Argentina). A good correlation was found between modelled and in situ turbidity values when the algorithm was applied to concurrent MODIS imagery. Moreover, satellite-derived turbidity maps show a spatial distribution of sediment consistent with patterns and characteristic features of the estuary