Search publications of the members of the Royal Belgian institute of natural Sciences
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Van bloem naar bloem, van bloemen tot zaadjes, er waren eens bijen
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Verslag van de KBVE-excursie in het Madegemveld op 4 juni 2011
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Origin of the high frequency variability of bio-optical properties in complex coastal environments (OO121246).
- This study describes physical processes (mainly the turbulence and re-suspension of particles due to turbulence) which control the micro scale variability of the bio-optical properties in highly turbid coastal waters. Time series analyses of different bio-optical and physical properties (temperature, salinity) have been performed from a boat in coastal waters. The data base gathers high frequency (1 Hz) simultaneous measurements performed during about 12 hours at four different days and locations in the highly turbid coastal environments of North Sea. We mainly focus on the concentrations of Chlorophyll and coloured detrital matter, back-scattering, and attenuation. For each parameter we consider the statistics (mean values, coefficients of variance and probability density functions) and the dynamics (Fourier power spectra). We found that these optical parameters (bbp, bpslope g, Refractive index-n and cp) are influenced by turbulence and inherit some of turbulence characteristics; high frequency noise, scales of variability at lower frequencies.
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Seasonal and inter-annual variability of air-sea CO2 fluxes and seawater carbonate chemistry in the Southern North Sea.
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Seasonal and inter-annual variability of air-sea CO2 fluxes and seawater carbonate chemistry in the Southern Bight of the North Sea.
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Operational oceanographic products for the Belgian scientific community.
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Reconstruction of complete space-time surface chlorophyll a (chl), total suspended matter (TSM) and sea temperature (SST) over the North Sea with monovariate and multivariate exploitations of the data interpolating empirical orthogonal functions method.
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Reconstruction of missing satellite total suspended matter data over the Southern North Sea and English Channel using Empirical Orthogonal Function decomposition of satellite imagery and hydrodynamical modelling.
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Uses of DINEOF algorithm (Data interpolation with Empirical Orthogonal Functions) for reconstruction and analysis of incomplete satellite databases over the North Sea and the Mediterranean, synthesis from the RECOLOUR project.
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REconstruction of COLOUR scenes: project summary, North Sea preliminary results, perspectives.
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Physical controls on biogeochemical dynamics along the land-ocean continuum: implications for coastal ocean modelling.
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Spectral relationships for atmospheric correction. II. Improving NASA's standard and MUMM near infra-red modeling schemes.
- Spectral relationships, reflecting the spectral dependence of water-leaving reflectance, rho(w)(lambda), can be easily implemented in current AC algorithms with the aim to improve rho(w)(lambda) retrievals where the algorithms fail. The present study evaluates the potential of spectral relationships to improve the MUMM (Ruddick et al., 2006, Limnol. Oceanogr. 51, 1167-1179) and standard NASA (Bailey et al., 2010, Opt. Express 18, 7521-7527) near infra-red (NIR) modeling schemes included in the AC algorithm to account for non-zero rho(w)(lambda(NIR)), based on in situ coastal rho(w)(lambda) and simulated Rayleigh corrected reflectance data. Two modified NIR-modeling schemes are investigated: (1) the standard NASA NIR-modeling scheme is forced with bounding relationships in the red spectral domain and with a NIR polynomial relationship and, (2) the constant NIR rho(w)(lambda) ratio used in the MUMM NIR-modeling scheme is replaced by a NIR polynomial spectral relationship. Results suggest that the standard NASA NIR-modeling scheme performs better for all turbidity ranges and in particular in the blue spectral domain (percentage bias decreased by approximately 50\%) when it is forced with the red and NIR spectral relationships. However, with these new constrains, more reflectance spectra are flagged due to non-physical Chlorophyll-a concentration estimations. The new polynomial-based MUMM NIR-modeling scheme yielded lower rho(w)(lambda) retrieval errors and particularly in extremely turbid waters. However, including the polynomial NIR relationship significantly increased the sensitivity of the algorithm to errors on the selected aerosol model from nearby clear water pixels. (C) 2013 Optical Society of America
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Spectral relationships for atmospheric correction. I. Validation of red and near infra-red marine reflectance relationships
- The present study provides an extensive overview of red and near infra-red (NIR) spectral relationships found in the literature and used to constrain red or NIR-modeling schemes in current atmospheric correction (AC) algorithms with the aim to improve water-leaving reflectance retrievals, rho(w)(lambda), in turbid waters. However, most of these spectral relationships have been developed with restricted datasets and, subsequently, may not be globally valid, explaining the need of an accurate validation exercise. Spectral relationships are validated here with turbid in situ data for rho(w)(lambda). Functions estimating rho(w)(lambda) in the red were only valid for moderately turbid waters (rho(w)(lambda(NIR)) 3.10(-3)). In contrast, bounding equations used to limit rho(w)(667) retrievals according to the water signal at 555 nm, appeared to be valid for all turbidity ranges presented in the in situ dataset. In the NIR region of the spectrum, the constant NIR reflectance ratio suggested by Ruddick et al. (2006) (Limnol. Oceanogr. 51, 1167-1179), was valid for moderately to very turbid waters (rho(w)(lambda(NIR)) 10(-2)) while the polynomial function, initially developed by Wang et al. (2012) (Opt. Express 20, 741-753) with remote sensing reflectances over the Western Pacific, was also valid for extremely turbid waters (rho(w)(lambda(NIR)) 10(-2)). The results of this study suggest to use the red bounding equations and the polynomial NIR function to constrain red or NIR-modeling schemes in AC processes with the aim to improve rho(w)(lambda) retrievals where current AC algorithms fail. (C) 2013 Optical Society of America
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Comparison of three SeaWiFS atmospheric correction algorithms for turbid waters using AERONET-OC measurements
- The use of satellites to monitor the color of the ocean requires effective removal of the atmospheric signal. This can be performed by extrapolating the aerosol optical properties in the visible from the near-infrared (NIR) spectral region assuming that the seawater is totally absorbant in this latter part of the spectrum. However, the non-negligible water-leaving radiance in the NIR which is characteristic of turbid waters may lead to an overestimate of the atmospheric radiance in the whole visible spectrum with increasing severity at shorter wavelengths. This may result in significant errors, if not complete failure, of various algorithms for the retrieval of chlorophyll-a concentration, inherent optical properties and biogeochemical parameters of surface waters. This paper presents results of an inter-comparison study of three methods that compensate for NIR water-leaving radiances and that are based on very different hypothesis: 1) the standard Sea WiFS algorithm (Stumpfet al., 2003; Bailey et al., 2010) based on a bio-optical model and an iterative process; 2) the algorithm developed by Ruddick et al. (2000) based on the spatial homogeneity of the NIR ratios of the aerosol and water-leaving radiances; and 3) the algorithm of Kuchinke et al. (2009) based on a fully coupled atmosphere-ocean spectral optimization inversion. They are compared using normalized water-leaving radiance nL(w) in the visible. The reference source for comparison is ground-based measurements from three AERONET-Ocean Color sites, one in the Adriatic Sea and two in the East Coast of USA. Based on the matchup exercise, the best overall estimates of the nL(w) are obtained with the latest SeaWiFS standard algorithm version with relative error varying from 14.97\% to 35.27\% for lambda = 490 nm and lambda = 670 nm respectively. The least accurate estimates are given by the algorithm of Ruddick, the relative errors being between 16.36\% and 42.92\% for lambda = 490 nm and lambda = 412 nm, respectively. The algorithm of Kuchinke appears to be the most accurate algorithm at 412 nm (30.02\%), 510 (15.54\%) and 670 nm (32.32\%) using its default optimization and bio-optical model coefficient settings. Similar conclusions are obtained for the aerosol optical properties (aerosol optical thickness tau(865) and the Angstrom exponent, alpha(510, 865)). Those parameters are retrieved more accurately with the SeaWiFS standard algorithm (relative error of 33\% and 54.15\% for tau(865) and alpha(510, 865)). A detailed analysis of the hypotheses of the methods is given for explaining the differences between the algorithms. The determination of the aerosol parameters is critical for the algorithm of Ruddick et al. (2000) while the bio-optical model is critical for the algorithm of Stumpf et al. (2003) utilized in the standard SeaWiFS atmospheric correction and both aerosol and bio-optical model for the coupled atmospheric-ocean algorithm of Kuchinke. The Kuchinke algorithm presents model aerosol-size distributions that differ from real aerosol-size distribution pertaining to the measurements. In conclusion, the results show that for the given atmospheric and oceanic conditions of this study, the SeaWiFS atmospheric correction algorithm is most appropriate for estimating the marine and aerosol parameters in the given turbid waters regions. (C) 2011 Elsevier Inc. All rights reserved.
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Optical remote sensing of turbidity and total suspended matter in the Gulf of Gabes
- Optical remote sensing was used to provide scientific information to support environmental management in the Gulf of Gabes that is located in the southeastern coast of Tunisia. This region is characterized by shallow continental shelf subjected to semi-diurnal tides. Industrial activities in this area since the early 1970s may have contributed to the degradation of the biodiversity of the ecosystem with eutrophication problems and disappearance of benthic and planktonic species. To assess the long-term effect of anthropogenic and natural discharges on the Gulf of Gabes, the optical environment of the coastal waters is assessed from in situ measurements of total suspended matter concentration (TSM), Secchi depth and turbidity (TU). This monitoring requires regular seaborne measurements (monthly), which is very expensive and difficult to obtain. The objective of the present study is the evaluation of the Moderate Resolution Imaging Spectrometer (MODIS) AQUA data compared with two sampling campaigns realized at the study area. To map turbidity data from MODIS images, a semi-empirical algorithm was applied at band 667 nm. This bio-optical algorithm has already been calibrated and validated on the Belgian coast. The validation of this algorithm on the Gulf of Gabes using in situ measurements of turbidity and remotely sensed turbidity obtained from MODIS imagery shows a correlation coefficient of 68.9\%. Seasonal and annual average maps for TSM and TU were then computed over the Gulf of Gabes using MODIS imagery. The obtained results of TSM and TU from remotely sensed data are conformable with those obtained through the analysis of in situ measurements. Therefore, remote sensing techniques offer a better and efficient tool for mapping and monitoring turbidity over the whole region.
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To see in different seas: spatial variation in the rhodopsin gene of the sand goby (Pomatoschistus minutus)
- Aquatic organisms living in a range of photic environments require specific mechanisms to tune their visual pigments. Maximum absorbance (lambda(max)) of retinal rods in populations of the marine demersal sand goby, (Pomatoschistus minutus; Gobiidae, Teleostei) correlates with the local optic environment. It has been shown that this is not regulated through a physiological response by exchanging the rhodopsin chromophore. To test for evolutionary adaptation, the sequence of the rhodopsin (RH1) gene was analysed in 165 Pomatoschistus minutus individuals from seven populations across its distribution range. Analysis showed a high level of intraspecific polymorphism at the RH1 gene, including nonsynonymous mutations on amino acids, known as spectral tuning sites. Population differentiation at these sites was in agreement with the observed differentiation in lambda(max) values. Analyses of d(N)/d(S) substitution rate ratios and likelihood ratio tests under site-specific models detected a significant signal of positive Darwinian selection on the RH1 gene. A strong discrepancy in differentiation was noticed between RH1 gene variation and the presumably neutral microsatellites and mitochondrial data. Samples did not cluster according to geographical or historical proximity with regards to RH1, but according to the general photic conditions of the habitat environment of the sand goby. This study highlights the usefulness of sensory genes, like rhodopsin, for studying the characteristics of local adaptation in marine nonmodel organisms.
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Calibration and validation of a generic multisensor algorithm for mapping of total suspended matter in turbid waters
- Mapping of total suspended matter concentration (TSM) can be achieved from space-based optical sensors and has growing applications related to sediment transport. A TSM algorithm is developed here for turbid waters, suitable for any ocean colour sensor including MERIS, MODIS and SeaWiFS. Theory shows that use of a single band provides a robust and TSM-sensitive algorithm provided the band is chosen appropriately. Hyperspectral calibration is made using seaborne TSM and reflectance spectra collected in the southern North Sea. Two versions of the algorithm are considered: one which gives directly TSM from reflectance, the other uses the reflectance model of Park and Ruddick (2005) to take account of bidirectional effects. Applying a non-linear regression analysis to the calibration data set gave relative errors in TSM estimation less than 30\% in the spectral range 670-750 nm. Validation of this algorithm for MODIS and MERIS retrieved reflectances with concurrent in situ measurements gave the lowest relative errors in TSM estimates, less than 40\%, for MODIS bands 667 nm and 678 nm and for MERIS bands 665 nm and 687 nm. Consistency of the approach in a multisensor context (SeaWiFS. MERIS, and MODIS) is demonstrated both for single point time series and for individual images. (C) 2009 Elsevier Inc. All rights reserved.
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Reconstruction of MODIS total suspended matter time series maps by DINEOF and validation with autonomous platform data
- In situ measurements of total suspended matter (TSM) over the period 2003-2006, collected with two autonomous platforms from the Centre for Environment, Fisheries and Aquatic Sciences (Cefas) measuring the optical backscatter (OBS) in the southern North Sea, are used to assess the accuracy of TSM time series extracted from satellite data. Since there are gaps in the remote sensing (RS) data, due mainly to cloud cover, the Data Interpolating Empirical Orthogonal Functions (DINEOF) is used to fill in the TSM time series and build a continuous daily “recoloured” dataset. The RS datasets consist of TSM maps derived from MODIS imagery using the bio-optical model of Nechad et al. (Rem Sens Environ 114: 854-866, 2010). In this study, the DINEOF time series are compared to the in situ OBS measured in moderately to very turbid waters respectively in West Gabbard and Warp Anchorage, in the southern North Sea. The discrepancies between instantaneous RS, DINEOF-filled RS data and Cefas data are analysed in terms of TSM algorithm uncertainties, space-time variability and DINEOF reconstruction uncertainty.
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A model of diffuse attenuation of downwelling irradiance for ecosystem models
- Estimation of the underwater attenuation of light is important to ecosystem modellers, who require information on Photosynthetically Available Radiation (PAR), and on the euphotic depth for calculation of primary production. Characterisation of these processes can be achieved by determining the diffuse attenuation coefficient of PAR, KPAR. A review of bio-optical models of the spectral diffuse attenuation coefficient for downwelling irradiance, K-d, is presented and stresses the necessity for a better knowledge and parameterization of these coefficients. In the second part of this work, radiative transfer simulations were carried out to model K-dZ1\% the spectral diffuse attenuation of downwelling irradiance averaged over the euphotic depth Z(1\%) (depth where the downwelling irradiance is 1\% of its surface value). This model takes into account the effects of varying sun zenith angle and cloud cover and needs absorption and backscattering coefficients (the inherent optical properties, IOPs) as input. It provides average and maximum relative errors of 1\% and 5\% respectively, for sun zenith angles [0 degrees-50 degrees] and of 1.7\% and 12\% respectively at higher sun zenith angles. A relationship was established between K-dZ1\% at a single wavelength (590nm) and KPAR at Z(PAR1\%) (where PAR is 1\% of its value at the surface) which allows for a direct expression of KPAR(ZPAR1\%) in terms of inherent optical properties, sun angle and cloudiness. This model provides estimates of KPAR within 25\% (respectively 40\%) relative errors respectively with a mean relative error less than 7\% (respectively 9\%) for sun zenith angles ranging from 0 degrees to 50 degrees (respectively higher than 50 degrees). A similar method is applied to derive a model for the diffuse attenuation of photosynthetically usable radiation, KPURZPUR1\%, with similar performance.
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Diurnal variability of turbidity and light attenuation in the southern North Sea from the SEVIRI geostationary sensor
- This study follows up on the successful feasibility study of Neukermans et al. (2009) for mapping suspended matter in turbid waters from the SEVIRI sensor on board the METEOSAT geostationary weather satellite platform. Previous methodology is extended to the mapping of turbidity. T, and vertical attenuation of photosynthetically active radiation (PAR), K-PAR. The spatial resolution of the SEVIRI products is improved from 3 km x 6.5 km to 1 km x 2 km using the broad high resolution visual band. The previous atmospheric correction is further improved and the uncertainties on marine reflectance due to digitization are considered. Based on a two year archive of SEVIRI imagery, available every 15 min, the diurnal variability of T and K-PAR is investigated during cloud free periods and validated using half-hourly T and K-PAR data obtained from a system of moored buoys (SmartBuoys) in the southern North Sea. Based on numerous match-ups, 80\% of SEVIRI derived T and K-PAR are within 53\% and 39\% of SmartBuoy T and K-PAR, respectively. Results further show that on cloud free days, the SEVIRI T and Km signals are in phase with the SmartBuoy data, with an average difference in the timing of the maximum T and K-PAR of 11 min and 23 min, respectively. It is concluded that diurnal variability of T and K-PAR can now be mapped by remote sensing offering new opportunities for improving ecosystem models and monitoring of turbidity. Limitations of the current SEVIRI sensor and perspectives for design of future geostationary sensors and synergy with polar orbiting satellites are discussed. (C) 2012 Elsevier Inc. All rights reserved.
