Environmental impacts resulting from the disposal of dredged material at the S1 dumping site, Belgian Continental Shelf

To guarantee a safe access to harbours, dredging of the maritime access channels is needed. Moreover, shipping channels have to be adapted to the continuously increasing size of vessels. In some cases, the dredged material is reused for beach nourishments or infrastructure projects, but mostly, it is dumped on offshore dumping sites. The efficiency of a dumping place is determined by economic, physical and ecological criteria: (1) dumping places need to be situated close to the navigation channels; (2) a low recirculation of the dumped material towards the dredging places is aimed at; and (3) a negligible effect on the ecosystem should be ensured. The goal of this research is providing baseline information on the impact of dumping on the morphology, sedimentology and biology and the adaptation after cessation of dumping. Both the dumping site as its environment have been considered. The investigated dumping site S1, the largest on the Belgian Continental Shelf, and its surroundings were surveyed with a multibeam echosounder (RV Belgica) providing bathymetrical and backscatter data. Additionally, seabed samples were taken for sedimentological/biological analyses with a box corer, a Reineck corer or Van Veen grab. Finally, chrono-sequential single-beam echosounding was used to investigate the morphodynamic evolution from 1995 until 2002. Generally, the results reveal that on a short- and medium-term, the impact of the dumping of dredged material on the morphology, sedimentology and ecology is drastic, but localised. After the cessation of the disposal of dredged sediments, it seems that the site has restored a morphodynamic equilibrium, both from a morphological, sedimentological point of view, however this is hard to say for the biology (cf. nine stations are devoid of macrobenthos)

Acoustic habitat modelling for the mapping of biological communities

In the framework of the MAREBASSE project (‘‘Management, Research and Budgeting of Aggregates in Shelf Seas related to End-users’’, EV/02/18, Van Lancker et al. 2005) and the MESH project (“Mapping European Seabed Habitats, Interreg IIIb; www.searchmesh.net), biologically relevant habitat maps are produced, based on multibeam acoustic datasets, ground truthed with physical and biological samples. The habitat map production comprised of the following four key steps: (1) getting the best out of the ground truth data; (2) selecting and deriving the best available input and most appropriate data coverages; (3) using the most appropriate techniques for interpreting the data through integration and modelling, and; (4) designing the map layout to create a map fit for purpose. Results are presented where classes derived from acoustic seabed classification are translated into habitat maps. Cross tabulation is used to correlate biological ground truthing data with the acoustic classes. Finally, habitat maps are produced, representing the likely occurrences of the different macrobenthic communities occurring on the Belgian part of the North Sea (BPNS). For the study areas of Oostende, Hinder Banken, Sierra Ventana, habitat maps of macrobenthic communities have been created. The habitat maps are based on a cross tabulation of acoustic backscatter classification of the multibeam images, overlaid with biological samples worked out on a macrobenthic community level

Ecosystem engineers stabilize sand bank systems: <i> Owenia fusiformis</i> aggregations as ecologically important microhabitat

Ecosystem engineers modify the physical environment and have profound effects on ecosystem functioning and on local biodiversity. Yet, in soft bottom marine environments, they have rarely been included in hydrodynamic studies or in management strategies. The lack of quantified stabilization potential and ecological impact are, respectively, the main reasons for not including ecosystem engineers. The present study evaluates the ecosystem engineering capacity of the tube dwelling polychaete Owenia fusiformis. The ecological implications of aggregations of this species are investigated using a long term dataset (1994- 2006). Results show that its presence has significant implications for species richness and species density. These aggregations are further investigated with different remote sensing tools. Owenia fusiformis is able to stabilize sand dunes that normally migrate 12 m a year. Specific biological characteristics explain the stabilizing effects in a highly dynamic environment. Our results confirm the need to pay attention to biota in sediment transport modelling. We conclude that O. fusiformis creates an important marine system which can be investigated with sensing techniques. These techniques can therefore be used to visualize hotspots of biodiversity, to quantify their biogeomorphological impacts and to underpin ecosystems based management in the marine environment

Recognizing the seafloor’s characteristics using habitat signatures

Seafloor images become increasingly available, both derived from video or photographs and from acoustic remote sensing. Very-high resolution acoustic imagery has indeed the potential of depicting a recognisable sign on an image that relates to a physical and biological nature, i.e. its habitat signature. Still, most of this information is stored at institutes or universities and no up-to-date comprehensive compilation is yet available. Moreover, the acoustic imagery often remains hard to interpret; this is mainly because of the multitude of factors influencing the image and the lack of reference material. When an interpreter studies a remote sensing image, he indeed needs to refer to particular textures and patterns that are recognisable on the image and relate that to reality. Ground truthing remains crucial; still comparison with a large number of similar cases is a necessity. In the framework of marine environmental issues this becomes increasingly important and the need for sound interpretations is real. To anticipate on this need, a web-based catalogue of seabed habitat signatures is being built in the framework of the MESH project (Mapping European Seabed Habitats), for both scientists and non-scientists. The catalogue contains a collection of images produced by different remote sensing techniques (acoustic and optically derived images, photographs and video). As such, the results of the different techniques can be compared and can strengthen interpretations in view of seabed assessments. The catalogue has a comprehensive list of metadata per habitat signature, both in terms of its physical and biological environment and the conditions under which the signatures were generated. The web catalogue is easy manageable. Habitats can be searched using their own name or by typing a key word or choosing a EUNIS code or making a query on physical factors. For every habitat one or more significant locations in the Mesh area are chosen and every location displays all the signatures available. Every location is identified by its coordinates (lat., long.) to be easily positioned on the MESH webGIS (http://www.searchmesh.net/webGIS). The signatures are presented as little thumbnails to let the web user have an easy overview. These link to a page where a description of the image, an enlarged image and all technical data referred to it can be found. The catalogue will largely increase the visibility of how the seafloor looks like, but above all it is hoped that it will assist in the interpretation of newly acquired data in view of - 48 - environmental assessments. Any potential contributor to this catalogue is invited to share their images to a wider European community. The web-catalogue is developed at Ifremer (http://www.ifremer.fr/meshmalo/ essai_signatures). RCMG is responsible for the input of imagery related to the Belgian part of the North Sea

Mud Origin, Characterisation and Human Activities (MOCHA): Final report

The cohesive sediments, which are frequently found in the Belgian nearshore zone (southern North Sea), are of different age such as tertiary clays and Holocene, modern and recently deposited muds. The area is characterised by a turbidity maximum. The source areas of the recently deposited muds and the effect of human impact vs. natural processes on the distribution and/or erosion of these sediments have been investigated using historic and recent bottom samples, in situ and remote sensing (satellite images) SPM concentration measurements, numerical modelling, GIS and clay mineral and microfossil analysis. The Schelde estuary, the potential erosion areas of cohesive sediments on the BCS and adjacent areas and the SPM transport through the Dover Strait have been considered as possible source areas.The historic bottom samples have been collected in the beginning of the 20th century, the quality of these samples and the meta-information is very high and they have proven to be a major reference to understand the evolution of the cohesive sediment distribution. The recent bottom samples consist of box core, Reineck core and Van Veen grab samples collected during the last 10 years. The processing of the historic and recent data on cohesive sediments was mainly based on field descriptions of the samples (consolidation, thickness) and morphological evolution. On some of the recent samples radioactive and gamma densitometric measurements have been carried out. During the processing the emphasis was put on the occurrence of thick layers (>30 cm) of freshly deposited to very soft consolidated mud and of clay and mud pebbles, because these sediments are witnesses of changes.Satellite images, in situ measurements and a 2D hydrodynamic numerical model have been combined to calculate the long term SPM transport through the Dover Strait and in the southern North Sea. The satellite images (SeaWiFS) provide synoptic views of SPM concentration. The representativness of SPM concentration maps derived from satellites for calculating long term transports has been investigated by comparing the SPM concentration variability from the in situ measurements with those of the satellite data. 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.Areas where the thickness of the Quaternary cover is less than 2.5 m were defined as potential erosion areas of Palaeogene clay containing deposits. In the framework of this project, the geological data related to the BCS have been reviewed and the relevant information was compiled into a GIS. This also included a small part of the French continental shelf. Additionally information was added from vibrocores analysis and Dutch geological data. Approximately 20 % of the BCS, 6 % of the small part of the French area and only 3% of the Dutch study area could possibly serve as a source for fine suspended sediments. Quaternary muds are mostly presented in the 2 eastern nearshore area; on the Dutch part they occur more offshore. Their occurrence represents 11% of the BCS and approximately 35% of the Rabsbank area Cretaceous microfossils are present in all samples and have been transported into the area with the residual water transport. Material from the east, in particular from the Eocene-Oligocene transitional strata, has been found in the eastern nearshore area up to about Oostende. This zone coincides with the extension of the Holocene mud and could indicate an erosion of these sediments and/or a transport of clay minerals from the Schelde estuary.Clay mineral analysis has been carried by two approaches in order to determine source areas. The results of the second approach show that no systematic differences in the clay mineralogy depending on geographic location could have been found within the samples. The results clearly prove the necessity of using more elaborate sample preparation procedures in examining the provenance of the mud deposits.Thick layers of fresh mud were deposited in the beginning of the 20th century mainly in a narrow band along the coast from about Nieuwpoort up to the mouth of the Westerschelde. These deposits were mainly the result of natural morphological changes. Today, most of the depositions of thick layers of fresh mud have been induced by anthropogenic operations, such as dumping, deepening of the navigation channels and construction and extension of the port of Zeebrugge. Comparing the actual situation with the situation 100 years ago reveals that the area around Zeebrugge where fresh mud is deposited extends more offshore today

Changes in the marine environment: the Belgian part of the North Sea revisited

Sustainable development requires the quantification of human impacts, against the seafloor’s ecological value. Recent impact studies have shown localised effects only, though indications of a longer-term and broader-scale degradation of the seafloor exist. This is due possibly to cumulative anthropogenically-induced effects, but the natural evolution and the response of the seafloor due to sea-level rise are poorly known. Such evolution needs to be disentangled against the impact of dredging, aggregate extraction, fisheries and beach replenishment.Naturally-, as well as anthropogenically-induced sediment dynamics are studied in detail along the Belgian part of the North Sea [Van Lancker et al., 2008]. State-of-the-art observations/sampling, advanced modelling, as well as analyses of long-term datasets on sediment nature and dynamics, geomorphology and macrobenthos are carried out. Additionally, integrated sand/mud models and dynamically coupled current/wave models are being developed, with boundary conditions generated from models focusing on the Scheldt estuary and on the coast. A historic reference framework has been set-up, based on a sediment and macrobenthos dataset of 100 yrs ago [Gilson dataset, van Loen & Houziaux, 2002]. In representative areas, erosion/sedimentation patterns and rates are quantified and balanced against the occurrence and intensity of human activities.Results are integrated in the view of developing criteria, monitoring strategies and recommendations for a more sustainable exploitation/management of the EEZ. Particularly, the allocation of efficient dumping grounds, large-scale aggregate extraction and sustainable coastal protection schemes are being considered, also in the perspective of future sea-level rise scenarios.REFERENCESVan Lancker, V., Du Four, I., Fettweis, M., Van den Eynde, D., Devolder, M., Francken, F., Monbaliu, J., Verwaest, T., Janssens, J., Degraer, S., Houziaux, J.-S., Vandenberghe, N., Goffin, A., 2008. QUantification of Erosion/Sedimentation patterns to Trace the natural versus anthropogenic sediment dynamics (QUEST4D). Annual Scientific Report Year 1. Science for Sustainable Development. Belgian Science Policy: Brussels, Belgium. 27 pp