Belgian Coastal Zone Research Articles

Mercury distribution, mobilization and bioavailability in polluted sediments of Scheldt Estuary and Belgian Coastal Zone

In this study, the vertical distribution of mercury (Hg) in estuarine and marine sediment porewaters and solid phases was assessed by conventional and passive sampling techniques in the historically polluted Scheldt Estuary and Belgian Coastal Zone (BCZ). The Diffusive Gradients in Thin-films (DGT) measured labile Hg concentrations (HgLA) were mostly lower than the porewater Hg concentrations (HgPW), and they also presented different vertical distribution patterns. Still high Hg concentrations in the sediment solid phases, comparable to the historical ones, were observed. Even though pH, redox potential and dissolved sulfide concentration could influence the Hg biogeochemical behaviour, organic matter (OM) played a key role in governing Hg mobilization from sediment solid phase to porewater and in its partitioning between porewater and solid phase over depth. In the marine sediments, where OM had a marine signature, higher labile Hg concentrations in the porewater and faster resupply from the solid phase were observed. The DGT technique showed significant potential not only for the measurement of bioavailable Hg fractions in porewater, but also for the assessment of kinetic parameters governing the release of labile Hg species from the solid phase with the assistance of the DGT Induced Fluxes in Sediments (DIFS) model.

Integrating Ecosystem Services into Impact Assessments: A Process-Based Approach Applied to the Belgian Coastal Zone

Policy makers increasingly acknowledge the importance of considering ecosystem services (ESs) and biodiversity in impact assessment (IA) to reduce ecosystem degradation and halt ongoing losses of biodiversity. Recent research demonstrates how ESs can add value to IA, i.e., by shifting the focus from avoiding negative impacts to creating opportunities, by linking effects on ecological functioning to benefits for society, and by providing a multi-disciplinary framework that allows to consider cross-sectoral effects. However, challenges exist to its implementation in practice. The most commonly used ES models do not consider interactions among ESs. This restricts their capacity to account for cross-sectoral effects. Integrating ESs into IA also increases time investments as they cover a wide variety of disciplines and need detailed information. This paper presents a pragmatic approach that tackles these challenges and may facilitate the inclusion of ESs into IA. The approach focuses on ecosystem processes as the driver of ESs and biodiversity and the basis to evaluate effects of a project. Using the Belgian coastal ecosystem, we illustrate how the approach restricts data needs by identifying the priority ESs, how it improves the coverage of cross-sectoral effects in IA, and how it contributes to a more objective selection of impacts.

Footprint of fresh submarine groundwater discharge in the Belgian coastal zone: An overview study

The presence of fresh groundwater is not limited to land; it also extends offshore and discharges as submarine groundwater discharge (SGD). The freshwater component of SGD (fresh submarine groundwater discharge [FSGD]) can be detected using geophysical techniques that are sensitive to salinity such as resistivity measurements. However, these measurements are often limited to either the land or marine realm, neglecting the land-marine interface. In this study, we focus on this gap by combining onshore and offshore techniques to assess variability in the FSGD footprint near the Belgian coastline through electrical resistivity tomography and continuous resistivity profiling. The difficult working conditions of the highly dynamic North Sea make this offshore survey one of the first of its kind. The footprint varies from limited outflow on the upper beach (e.g., Wenduine) to discharge around and below the low water line (e.g., De Panne, Oostduinkerke, and Knokke-Heist) in the studied areas. The occurrence, footprint, and quantity of SGD seem to be controlled by the presence and size of dune formations that constitute freshwater resources along the shore. Heterogeneity can also play a determining factor in FSGD location.

Evaluation of eight band SuperDove imagery for aquatic applications.

Planet's SuperDove constellation is evaluated for remote sensing of water targets. SuperDoves are small satellites with on board eight band PlanetScope imagers that add four new bands compared to the previous generations of Doves. The Yellow (612 nm) and Red Edge (707 nm) bands are of particular interest to aquatic applications, for example in aiding the retrieval of pigment absorption. The dark spectrum fitting (DSF) algorithm is implemented in ACOLITE for processing of SuperDove data, and its outputs are compared to matchup data collected using an autonomous pan-and-tilt hyperspectral radiometer (PANTHYR) installed in the turbid waters of the Belgian Coastal Zone (BCZ). Results for 35 matchups from 32 unique SuperDove satellites indicate on average low differences with PANTHYR observations for the first seven bands (443-707 nm), with mean absolute relative differences (MARD) 15-20%. The mean average differences (MAD) are between -0.01 and 0 for the 492-666 nm bands, i.e. DSF results show a negative bias, while the Coastal Blue (444 nm) and Red Edge (707 nm) show a small positive bias (MAD 0.004 and 0.002). The NIR band (866 nm) shows a larger positive bias (MAD 0.01), and larger relative differences (MARD 60%). Root mean squared differences (RMSD) are rather flat at around 0.01 with peaks in the bands with highest water reflectance of around 0.015. The surface reflectance products as provided by Planet (PSR) show a similar average performance to DSF, with slightly larger and mostly positive biases, except in both Green bands, where the MAD is close to 0. MARD in the two Green bands is a bit lower for PSR (9.5-10.6%) compared to DSF (9.9-13.0%). Higher scatter is found for the PSR (RMSD 0.015-0.020), with some matchups showing large, spectrally mostly flat differences, likely due to the external aerosol optical depth (τa) inputs not being representative for these particular images. Chlorophyll a absorption (aChl) is retrieved from PANTHYR measurements, and the PANTHYR data are used to calibrate aChl retrieval algorithms for SuperDove in the BCZ. Various Red band indices (RBI) and two neural networks are evaluated for aChl estimation. The best performing RBI algorithm, i.e. the Red band difference (RBD), showed a MARD of 34% for DSF and 25% for PSR with positive biases of 0.11 and 0.03 m-1 respectively for 24 PANTHYR aChl matchups. The difference in RBD performance between DSF and PSR can be largely explained by their respective average biases in the Red and Red Edge bands, which are opposite signs for DSF (negative bias in the red), and positive for both bands for PSR. Mapping of turbid water aChl and hence chlorophyll a concentration (C) using SuperDove is demonstrated for coastal bloom imagery, showing how SuperDove data can supplement monitoring programmes.

Towards quantitative metabarcoding of eukaryotic plankton: an approach to improve 18S rRNA gene copy number bias

Plankton metabarcoding is increasingly implemented in marine ecosystem assessments and is more cost-efficient and less time-consuming than monitoring based on microscopy (morphological). 18S rRNA gene is the most widely used marker for groups’ and species’ detection and classification within marine eukaryotic microorganisms. These datasets have commonly relied on the acquisition of organismal abundances directly from the number of DNA sequences (i.e. reads). Besides the inherent technical biases in metabarcoding, the largely varying 18S rRNA gene copy numbers (GCN) among marine protists (ranging from tens to thousands) is one of the most important biological biases for species quantification. In this work, we present a gene copy number correction factor (CF) for four marine planktonic groups: Bacillariophyta, Dinoflagellata, Ciliophora miscellaneous and flagellated cells. On the basis of the theoretical assumption that ‘1 read’ is equivalent to ‘1 GCN’, we used the GCN median values per plankton group to calculate the corrected cell number and biomass relative abundances. The species-specific absolute GCN per cell were obtained from various studies published in the literature. We contributed to the development of a species-specific 18S rRNA GCN database proposed by previous authors. To assess the efficiency of the correction factor we compared the metabarcoding, morphological and corrected relative abundances (in cell number and biomass) of 15 surface water samples collected in the Belgian Coastal Zone. Results showed that the application of the correction factor over metabarcoding results enables us to significantly improve the estimates of cell abundances for Dinoflagellata, Ciliophora and flagellated cells, but not for Bacillariophyta. This is likely to due to large biovolume plasticity in diatoms not corresponding to genome size and gene copy numbers. C-biomass relative abundance estimations directly from amplicon reads were only improved for Dinoflagellata and Ciliophora. The method is still facing biases related to the low number of species GCN assessed. Nevertheless, the increase of species in the GCN database may lead to the refinement of the proposed correction factor.

Optical and biogeochemical properties of diverse Belgian inland and coastal waters

Abstract. From 2017 to 2019, an extensive sampling campaign was conducted in Belgian inland and coastal waters, aimed at providing paired data of optical and biogeochemical properties to support research into optical monitoring of aquatic systems. The campaign was focused on inland waters, with sampling of four lakes and a coastal lagoon during the growth season, in addition to samples of opportunity from other four lakes. Campaigns also included the Scheldt estuary over a tidal cycle and two sampling campaigns in the Belgian coastal zone. Measured parameters include inherent optical properties (absorption, scattering and beam attenuation coefficients, near-forward volume scattering function, turbidity), apparent optical properties (Secchi disc depth, substrate and water-leaving Lambert-equivalent bi-hemispherical reflectance), and biogeochemical properties (suspended particulate matter, mineral fraction of particle mass, particle size distribution, pigment concentration, DNA metabarcoding, flow microscopy counts, and bottom type classification). The diversity of water bodies and environmental conditions covered a wide range of system states. The chlorophyll a concentration varied from 0.63 to 382.72 mg m−3, while the suspended particulate matter concentration varied from 1.02 to 791.19 g m−3, with mineral fraction varying from 0 to 0.95. Depending on system and season, phytoplankton assemblages were dominated by cyanobacteria, green algae (Mamiellophyceae, Pyramimonadophyceae), or diatoms. The dataset is available from https://doi.org/10.1594/PANGAEA.940240 (Castagna et al., 2022).

Anthropogenic activities influence the mobilization of trace metals and oxyanions in coastal sediment porewaters

The biogeochemical cycle of trace metals and oxyanions in marine sediments is mainly controlled by early diagenesis of organic material, but anthropogenic activities such as dumping of dredged sludge or shipyard activities can strongly perturb these natural processes. Therefore, the impact of dumping of dredged sludge, shipyard activities such as blasting of boat hulls and the mixing of polluted Scheldt estuary water with less polluted coastal water on the mobilization of trace metals and oxyanions in the sediments was studied in the pre- and post-bloom period (March and October) in Belgian Coastal Zone (BCZ). Vertical profiles of total and labile dissolved trace element concentrations, dissolved sulfide, pH and redox potential were assessed in sediment porewaters. Generally, total and labile dissolved element profiles in the porewaters are well correlated, showing higher concentrations in March when the redox potential is lower and sulfide concentrations higher. Low pH levels were documented together with high amounts of Fe and associated elements in March at the dumping station. At the coastal station in the vicinity of the shipyards, sediment porewaters are enriched in dissolved Zn while at the mouth of the Scheldt estuary the highest dissolved Cu levels were found. Theoretical calculations with Visual MINTEQ confirm the increased Fe and associated element concentrations in March (lower pH) and the lower metal concentrations in October (lower Eh). R values, ratio of labile dissolved to total dissolved concentrations, reflect the potential of the solid sediment phase to supply labile elements to the porewater. They were lowest for Cu and highest for V. This study showed that anthropogenic activities in the BCZ such as dumping of dredged sludge and blasting of boat hulls have a serious impact on the levels, distribution and bioavailability of contaminants in the sediments, which may form a real threat to the benthic ecosystem.

Fantastic Beasts: Unfolding Mixoplankton Temporal Variability in the Belgian Coastal Zone Through DNA-Metabarcoding

Protists engaging in photo- and phago- mixotrophy (mixoplankton) are common members of the global plankton community. They are involved in primary production and contribute to the carbon and nutrient cycling. Two major mixoplankton functional types (MFTs) are considered based upon the origin of their photosynthetic abilities: innate for constitutive-mixoplankton (CM) and obtained from prey for non-constitutive mixoplankton (NCM). Regardless of their significance, little attention has been paid to their diversity and temporal succession. We performed a metabarcoding survey of the V4-18S rRNA gene in 92 surface water samples collected during 2018–2019 in five fixed stations of the Belgian Coastal Zone. Environmental data such as nutrients, sea surface temperature, salinity, Chl-a and light were collected to understand their influences over mixoplankton community changes. The temporal diversity of mixotrophs, autotrophs, and heterotrophs was analyzed and the distinct seasonal patterns were evidenced. Results showed that dinoflagellates and ciliates were the major mixoplankton contributors. There were no significant differences among protist communities between the stations sampled. The time-series showed high proportional abundances of CM, accounting in average for 24.4% of the reads, against the low contribution of NCM, 4.8%. CM dinoflagellates belonging to Heterocapsa, Alexandrium, Karlodinium, and Tripos genus were the most abundant, and co-occurred with strict autotrophic plankton. Strombidium genus ciliates were the most representative organisms for NCM. Mixoplankton showed lower diversity than autotrophs and heterotrophs throughout the time series, however, the environmental factors controlling the seasonal community shifts (β-diversity) were similar. Overall, the metabarcoding approach allowed to depict with high resolution the composition of mixoplankton and its diversity among auto- and heterotrophs in the Belgian Coastal Zone.

Optical Detection of Harmful Algal Blooms in the Belgian Coastal Zone: A Cautionary Tale of Chlorophyll c3

Phaeocystis globosa is a nuisance haptophyte species that forms annual blooms in the southern North Sea and other coastal waters. At high biomass concentration, these are considered harmful algal blooms due to their deleterious impact on the local ecosystems and economy, and are considered an indicator for eutrophication. In the last two decades, methods have been developed for the optical detection and quantification of these blooms, with potential applications for autonomous in situ or remote observations. However, recent experimental evidence suggests that the interpretation of the optical signal and its exclusive association with P. globosa may not be accurate. In the North Sea, blooms of P. globosa are synchronous with those of the diatom Pseudo-nitzschia delicatissima, another harmful bloom-forming species with similar pigmentation and optical signature. Here we combine new and published measurements of pigmentation composition and inherent optical properties from pure cultures of several algal and cyanobacterial groups, together with environmental spectroscopy data, to identify the pigments generating the optical signals captured by two established algorithms. We further evaluate the association of those pigments and optical signals with P. globosa. We found that the interpretation of the pigment(s) generating the optical signals were incorrect and that previous methods are not specific to P. globosa, even in the context of the phytoplankton assemblage of the southern North Sea. Additionally, we found that the optical and pigment signatures of Phaeocystis species are part of a broad pigmentation trend across unrelated taxonomic groups related to chlorophyll c3 presence, with important consequences for the interpretation of pigment and optical data. We then develop and evaluate an algorithm to detect this pigmentation pattern with minimal influence of co-occurring species and elaborate general recommendations for the future development of algorithms.

Offshore Wind Farm Footprint on Organic and Mineral Particle Flux to the Bottom

Offshore wind farms (OWFs) are an important source of renewable energy accounting for 2.3% of the European Union's electricity demand. Yet their impact on the environment needs to be assessed. Here, we couple a hydrodynamic (including tides and waves) and sediment transport model with a description of the organic carbon and mineral particle dynamics in the water column and sediments. The model is applied to the Belgian Coastal Zone (BCZ) where OWFs currently occupy 7% of its surface area which is estimated to double in the next 5 years. The impact of OWFs on the environment is represented through the filtration of the water column and fecal pellets production by the blue mussel, the dominant fouling organism. Our model simulations show that the impact of biodeposition on the mud particle sedimentation and on sediment composition is small compared to the fluxes associated with tidal deposition and resuspension and the lateral inputs. In contrast, the total organic carbon (TOC) flux to the sediment is significantly altered inside the OWF perimeters and TOC deposition is increased up to 50% in an area 5 km around the monopiles. Further away, the TOC flux to the bottom decreases with a notable effect up to 30 km away. The major changes are found along the direction of the main residual current and tidal ellipse's major axis. In addition, sub-mesoscale gyres act as retention areas with increased carbon deposition. A future OWF in the BCZ will be located close to gravel beds in a Natura 2000 area, considered as vulnerable habitats and biodiversity hotspots. The different scenarios for this OWF, varying in turbine number and positioning, are compared in terms of impact on the carbon and mineral particle deposition flux in the BCZ and, particularly, to these gravel beds. The scenarios show that the number of turbines has only a slight impact on the TOC deposition flux, unlike their positioning that significantly alters the TOC flux to the gravel beds. The TOC deposition flux exceeds 50%, when the turbines are placed next to the gravel beds; while a limited increase is simulated, when the turbines are located the farthest possible from them.

Atmospheric correction of Sentinel-3/OLCI data for mapping of suspended particulate matter and chlorophyll-a concentration in Belgian turbid coastal waters

The performance of different atmospheric correction algorithms for the Ocean and Land Colour Instrument (OLCI) on board of Sentinel-3 (S3) is evaluated for retrieval of water-leaving radiance reflectance, and derived parameters chlorophyll-a concentration and turbidity in turbid coastal waters in the Belgian Coastal Zone (BCZ). This is performed using in situ measurements from an autonomous pan-and-tilt hyperspectral radiometer system (PANTHYR). The PANTHYR provides validation data for any satellite band between 400 and 900 nm, with the deployment in the BCZ of particular interest due to the wide range of observed Near-InfraRed (NIR) reflectance. The Dark Spectrum Fitting (DSF) atmospheric correction algorithm is adapted for S3/OLCI processing in ACOLITE, and its performance and that of 5 other processing algorithms (L2-WFR, POLYMER, C2RCC, SeaDAS, and SeaDAS-ALT) is compared to the in situ measured reflectances. Water turbidities across the matchups in the Belgian Coastal Zone are about 20–100 FNU, and the overall performance is best for ACOLITE and L2-WFR, with the former providing lowest relative (Mean Absolute Relative Difference, MARD 7–27%) and absolute errors (Mean Average Difference, MAD -0.002, Root Mean Squared Difference, RMSD 0.01–0.016) in the bands between 442 and 681 nm. L2-WFR provides the lowest errors at longer NIR wavelengths (754–885 nm). The algorithms that assume a water reflectance model, i.e. POLYMER and C2RCC, are at present not very suitable for processing imagery over the turbid Belgian coastal waters, with especially the latter introducing problems in the 665 and 709 nm bands, and hence the chlorophyll-a and turbidity retrievals. This may be caused by their internal model and/or training dataset not being well adapted to the waters encountered in the BCZ. The 1020 nm band is used most frequently by ACOLITE/DSF for the estimation of the atmospheric path reflectance (67% of matchups), indicating its usefulness for turbid water atmospheric correction. Turbidity retrieval using a single band algorithm showed good performance for L2-WFR and ACOLITE compared to PANTHYR for e.g. the 709 nm band (MARD 15 and 17%), where their reflectances were also very close to the in situ observations (MARD 11%). For the retrieval of chlorophyll-a, all methods except C2RCC gave similar performance, due to the RedEdge band-ratio algorithm being robust to typical spectrally flat atmospheric correction errors. C2RCC does not retain the spectral relationship in the Red and RedEdge bands, and hence its chlorophyll-a concentration retrieval is not at all reliable in Belgian coastal waters. L2-WFR and ACOLITE show similar performance compared to in situ radiometry, but due to the assumption of spatially consistent aerosols, ACOLITE provides less noisy products. With the superior performance of ACOLITE in the 490–681 nm wavelength range, and smoother output products, it can be recommended for processing of S3/OLCI data in turbid waters similar to those encountered in the BCZ. The ACOLITE processor for OLCI and the in situ matchup dataset used here are made available under an open source license.

Drivers of the variability of dimethylsulfonioproprionate (DMSP) and dimethylsulfoxide (DMSO) in the Southern North Sea

The influence of abiotic and biotic factors on the concentration of dimethylsulfoniopropionate (DMSP) and dimethylsulfoxide (DMSO) was investigated and compared during two annual cycles in 2016 and 2018 in the Belgian coastal zone (BCZ) in the southern North Sea at five fixed stations. These stations covered a near-offshore gradient from stations close to the mouth of the Scheldt estuary to most offshore stations. Significant differences of Chlorophyll-a (Chl-a) concentrations were observed between the two years with higher values in early spring 2018 (due to better light and nutrient conditions coupled to colder temperatures) and in summer 2018 (due to warmer conditions) compared to 2016. Nevertheless, the seasonal and spatial DMSP and DMSO (DMS(P,O)) patterns, as well as the yearly average were nearly identical in 2016 and 2018. This can be explained by the fact that the phytoplankton groups responsible for the large differences in Chl-a in 2018 and 2016 were low DMSP-producers characterized by several diatom and dinoflagellate species, occurring in early spring and summer. Further, the Prymnesiophyceae Phaeocystis globosa, occurring in late spring and responsible of most of DMS(P,O) measured in the area, reached similar biomass both years. The DMSP:Chl-a ratio obtained from the field measurements were similar to those previously published for the main observed phytoplankton groups, but more differences were observed for the DMSO:Chl-a ratio. DMS(P,O) estimations based on Chl-a linear regressions for the whole dataset need to account on two relationships discriminating the low and high-DMSP producing species.

ROMS Based Hydrodynamic Modelling Focusing on the Belgian Part of the Southern North Sea

The North Sea is a shallow sea that forms a complex physical system. The nonlinear interaction of the astronomical tides, varying wind fields and varying pressure systems requires appropriate approaches to be described accurately. An application based on the advanced numerical model Regional Ocean Modeling System (ROMS) was newly developed by the authors, tailored to simulate these hydrodynamic processes in the North Sea and the Belgian Continental Shelf, which is the area of particular interest in the present study. The purpose of this work is to develop and validate a state-of-the-art three-dimensional numerical model to form the basis of a compound operational and forecasting tool for the Belgian coastal zone. The model was validated with respect to water levels and temperature. Validation for astronomical tides was accomplished through the comparison of the principal constituents between the model results and observations at a number of tidal gauges in Belgium and other countries. A statistical analysis of the results showed that the model behaves as expected throughout the North Sea. The model response to the varying meteorological conditions was also validated using hindcast data for 2011 as input. In this case, the comparison between observed and modelled water levels showed a good agreement with average RMSE in Belgium 9.5 cm. Overall, the added value of this work is the development of an independent model for validation and comparison with other models and which can be used as an efficient tool for operational and forecasting purposes.

Cysteine-modified silica resin in DGT samplers for mercury and trace metals assessment

Diffusive gradients in thin-films (DGT) is an in situ passive sampling technique to assess labile trace metal concentrations in different environmental matrix. The technique is consisting of a diffusive domain backed up by a resin gel that binds free metals and metal complexes that dissociate in the diffusive domain. This technique requires specific resin for special metals, for example mercury (Hg), since the classic resin (Chelex-100) gel is not applicable for Hg measurement. A simultaneous determination of Hg with other metals by the DGT was not yet reported. Two biomolecule-based resins were prepared by glutaraldehyde immobilisation of cysteine onto 3-amino-functionalised silica and 3-aminopropyl-functionalised silica, respectively. The load of functional groups on modified resins was qualitatively and quantitatively characterised. The modified resins were applied in the DGT technique and the uptake efficiency, elution efficiency, and linear accumulation of analytes of the DGT were tested. This novel DGT technique, using two cysteine-modified resins, can accumulate Hg and other metals in a broad range of pH and ionic strength in solutions. In the Belgian coastal zone (BCZ), the concentrations of Hg and other trace metals sampled by cysteine-modified resin-DGTs were similar as those by the other two DGT assemblies for Hg and other trace metals, respectively. The cysteine-modified silica resin combined the features of Chelex-100 resin and 3-mercaptopropyl silica resin and allowed simultaneous determination of labile Hg and other trace metals. The resin with a higher load of functional groups also showed higher performance in the further application in the DGT technique.

Hydrodynamic variability in the Southern Bight of the North Sea in response to typical atmospheric and tidal regimes. Benefit of using a high resolution model

In this paper, the hydrodynamics of the Southern Bight of the North Sea (SBNS) and in particular, the Belgian Coastal Zone (BCZ) is investigated on daily to seasonal time scales using a high resolution hydrodynamical model. The Regional Ocean Modeling System (ROMS) is implemented over the SBNS with 5 km resolution and downscaled at 1 km resolution over the BCZ in a two-way nesting configuration run over a three years period (i.e. 2006–2008). The benefit of using a high resolution model over the BCZ is assessed through an extensive comparison of model results with data from satellite and in-situ fixed platforms as well as reference products available for the region. The validation exercise and the results analysis are conducted with a particular focus on hydrodynamic features that are expected to impact the sediment transport. We find that despite the validation procedure does not allow to clearly demonstrate better performance of the high resolution model compared to the coarse resolution model in terms of overtidal circulation, sea surface temperature (SST) and salinity (SSS), the high resolution model resolves additional details in the variability of residual circulation and Scheldt salinity plume dynamics. The analysis of the response of the simulated hydrodynamics to atmospheric regimes for neap and spring tide highlights the major role played by the wind direction on the averaged currents and plume extension. The strongest currents and minimum plume extension are obtained under southwestern winds and neap tide while when northeastern winds prevail, the plume extension is at its maximum and the circulation is the weakest. We show that while neap tides allow the establishment of streamlined circulation, the spring tides induce more turbulent circulation which can favor the retention of transported elements. This latter property could not be resolved with the 5 km resolution model.

Digital image analysis of flatfish bleeding injury

To improve accuracy of post-release mortality predictions and facilitate the routine collection of information about physical condition of catches after commercial fishing capture, traditional visual assessment by potentially subjective human observers or raters may be automated by digital image analysis. The purpose of this study was to develop a method and device that can eliminate subjectivity in scoring external injury of commercially beam-trawled flatfish by taking standardized, high resolution images to allow for automated calculation of the % surface area of visible bleeding injury relative to the whole fish based on digital image analysis. A reference library was compiled by photographing ventral sides of 67 fish of six flatfish species of different sizes and freshness (fresh vs defrosted). All fish were sourced from the R/V Simon Stevin while beam-trawling in the Belgian coastal zone of the Southern North Sea. All images that were neither over- nor under-exposed were compiled (n = 51) and scored for the extent (%) of multifocal cutaneous petechial ('point bleeding'), and suffusion or haemorrhaging ('bruising') of the ventral head and body region, respectively, by three experienced raters using a continuous scale (between 0 and 100 %). Then, several state-of-the-art computer vision algorithms were tested on the dataset to develop a protocol that can 1) align each image; 2) identify fin, body and head regions; and 3) quantify the surface area of bleeding injury of each region by using appropriate thresholding techniques. For validation of the computer-derived % surface coverage estimates of bleeding injury, these were compared to the average rater’s score. For bruising injury, a significant difference between human- vs computer-derived scores persisted. For point bleeding of the head region, computer-based estimates of % coverage were not different from those of the human raters. Overall, species, size and their freshness did not have a significant effect. By consistently recording the coverage of externally visible bleeding injury, this image analysis protocol may find its application in measuring the effect of different capture techniques on whole fish quality, and in improving vitality assessments as part of the transition towards a more sustainable fishery and the implementation of the European Landing Obligation.

Automated water surface temperature retrieval from Landsat 8/TIRS

Satellite remote sensing of Land and Water Surface Temperature (L/WST) has many applications in studies of terrestrial and aquatic ecology. Retrieval of L/WST requires a well calibrated radiometer and an accurate atmospheric correction. In the present study, the performance of the Thermal InfraRed Sensor (TIRS) on board Landsat 8 is evaluated for the retrieval of L/WST. libRadtran is used to retrieve atmospheric correction parameters based on atmospheric profiles of relative humidity and temperature from three global atmospheric models. Performance of single band retrievals is compared to typical MODTRAN results from the Atmospheric Correction Parameter Calculator (ACPC) and a split-window approach. A multi-temporal land masking method using imagery from the Operational Land Imager (OLI) on board Landsat 8 is demonstrated, and is used to automatically classify imagery in the matchup dataset in three classes of cloud cover. Two sources of in situ data covering the Belgian Coastal Zone (BCZ) are used for validation of the L/WST product: (1) fixed locations in the Flemish Banks measurement network and (2) underway data from regular RV Belgica campaigns. In the present study the single band methods outperformed the split-window approach, and consistent retrievals are found for the MODTRAN and libRadtran simulations. Typical single band surface temperature retrievals in quasi cloud-free conditions have Root Mean Squared Differences (RMSD) of 0.7 K and 1 K for Bands 10 and 11 with low bias, depending on the method and atmospheric profile source. For imagery with scattered clouds, RMSD values increase to 1 K and 2 K respectively with an approximately 0.5 K cold bias, likely caused by cloud proximity. The calibration efforts combined into Collection 1 allows for accurate absolute surface temperature retrievals from B10 on Landsat 8/TIRS for homogeneous targets with known emissivity, such as liquid water. The method is adapted to global processing and can be used for Land Surface Temperature retrieval with a suitable source of emissivity data.

Response of marine methane dissolved concentrations and emissions in the Southern North Sea to the European 2018 heatwave

During the European heatwave of 2018 that led to record-breaking temperatures in many countries across northern and central Europe, average seawater temperature in July was 2.5 °C higher than the mean from 2004 to 2017 for same month in the Belgian coastal zone (BCZ) (Southern Bight of the North Sea). The mean dissolved CH4 concentration in surface waters in July 2018 (338 nmol L−1) was three times higher than in July 2016 (110 nmol L−1), and an extremely high dissolved CH4 concentration in surface waters (1607 nmol L−1) was observed at one near-shore station. The high dissolved CH4 concentrations in surface waters in the BCZ in July 2018 seemed to be due to a combination of enhancement of methanogenesis and of release of CH4 from gassy sediments, both most likely related to warmer conditions. The emission of CH4 from the BCZ to the atmosphere was higher in 2018 compared to 2016 by 57% in July (599 versus 382 μmol m−2 d−1) and by 37% at annual scale (221 versus 161 μmol m−2 d−1). The European heatwave of 2018 seems to have led to a major increase of CH4 concentrations in surface waters and CH4 emissions to the atmosphere in the BCZ.

Trace metal speciation in North Sea coastal waters.

Most trace metals exhibit a dual role in marine waters, acting as nutrients at low concentration and being toxic at high concentration. But besides concentration range, speciation is also an important factor. They both show both seasonal and spatial variations. A thorough comparison between total dissolved and particulate concentrations estimated from manual sampling and an assessment of the bioavailability using Diffusive Gradients in Thin Films (DGT) has been performed in this work for Cd, Co, Cu, Ni and Pb, at several sampling points of the Belgian Coastal Zone (BCZ). Additional information to trace back the origin and identify the anthropogenic fingerprint of Suspended Particulate Matter (SPM) was measured using stable carbon isotope measurements in particulate organic matter. Our results show that: (i) particulate and total dissolved metal concentrations are higher at two stations, one in the harbor of Oostende and one offshore; (ii) dissolved and particulate trace metal concentrations do not correlate with the dissolved labile fractions; and (iii) SPM in the harbor zone is likely from allochthonous sources, while in the offshore station marine origin has been evidenced. Our results indicate that, even though contamination is higher in the harbor zones, the trace metal toxicity, which is linked to the metal bioavailability, is most likely not higher than in the open sea. However, with increasing acidification of the ocean, a shift from particulate to dissolved phase might lead to increasing adverse effects on the coastal environment.

IMPACT OF SEA LEVEL RISE ON THE ACCESSIBILITY OF COASTAL PORTS: A CASE STUDY OF THE PORT OF ZEEBRUGGE (BELGIUM)

The effect of sea level rise on the hydrodynamic flow in the Belgian coastal zone is investigated in the light of the nautical accessibility of the port of Zeebrugge in Belgium. To this end, numerical simulations are performed considering three different scenarios of sea level rise along with a reference scenario (current situation). Specifically, a moderate, a warm and a worst case scenario of sea level rise (SLR) equal to 60 cm, 90 cm and 200 cm by the year 2100, are considered. The main objective is to find out how the strong tidal currents, which are mainly directed transversely to the access channel and limit the access to the port to a certain tidal window, will be affected by the considered SLR scenarios.