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

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

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

A SWIR based algorithm to retrieve total suspended matter in extremely turbid waters

In ocean colour remote sensing, the use of Near Infra Red (NIR) spectral bands for the retrieval of Total Suspended Matter (TSM) concentration in turbid and highly turbid waters has proven to be successful. In extremely turbid waters (TSMN 100 mgL−1) however, these bands are less sensitive to increases in TSM. Here it is proposed to use Short Wave Infra Red (SWIR) spectral bands between 1000 and 1300 nm for these extreme cases. This SWIR spectral region is subdivided into two regions, SWIR-I (1000 nm to 1200 nm) and SWIR-II (1200 nm to 1300 nm) which correspond to local minima in the pure water absorption spectrum. For both spectral regions the water reflectance signal was measured in situ with an ASD spectrometer in three different extremely turbid estuarine sites: Scheldt (Belgium), Gironde (France), and Río de la Plata (Argentina), along with the TSMconcentration.A measurable water reflectance was observed for all sites in SWIR-I, while in the SWIR-II region the signal was not significant compared to the Signal-to-Noise Ratio (SNR) of current Ocean Colour (OC) sensors. For the spectral band at 1020 nm (present in Ocean and Land Colour Instrument ? OLCI, onboard Sentinel-3) and at 1071 nm, an empirical single band TSM algorithm is defined which is valid for both the Gironde and Scheldt estuarine sites. This means that a single algorithm can be applied for both sites without expensive recalibration.The relationship between TSM and SWIR reflectance at 1020 and 1071 nm is linear and did not show any saturation for the concentrations measured here (up to 1400 mg L−1), while saturation was observed for the NIR wavelengths, as expected. Hence, for extremely turbid waters it is advised to switch from NIR to SWIR-I wavelengths to estimate TSM concentration. This was demonstrated for an airborne hyperspectral dataset (Airborne Prism Experiment, APEX) from the Gironde estuary having several spectral bands in the SWIR-I. The empirical single band SWIR TSM algorithm was applied to the atmospherically corrected scene providing a TSM concentration map of the Gironde from mouth to more upstream with concentrations expected in this region ranging from a few to several hundreds mg L−1. These results, i.e. the existence of a single relationship for the Scheldt and Gironde, not showing any decrease of sensitivity, highlights the importance of having SWIR bands in future ocean colour sensors for studying extremely turbid rivers, coastal areas and estuaries in the world. A further implication of these results is that there is a TSMlimit for application of atmospheric correction algorithms which assume zero SWIR marine reflectance. That limit is defined here as function of wavelength and sensor noise level.Fil: Knaeps, E.. Flemish Institute for Technological Research (VITO); BélgicaFil: Ruddick, K. G.. Flemish Institute for Technological Research ; BélgicaFil: Doxaran, D.. Laboratoire d; FranciaFil: Dogliotti, Ana Inés. Consejo Nacional de Investigaciónes Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Astronomía y Física del Espacio. - Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Astronomía y Física del Espacio; ArgentinaFil: Nechad, B.. Royal Belgian Institute for Natural Sciences (RBINS); BélgicaFil: Raymaekers, D.. Flemish Institute for Technological Research; BélgicaFil: Sterckx, S.. Flemish Institute for Technological Research; Bélgic