Translation of biological and sedimentological point data towards habitat suitability maps of biological communities and EUNIS level 5 maps. Part 2: From habitat suitability maps of biological communities towards EUNIS level 5 maps

The full coverage habitat suitability maps of the macrobenthic communities serve as an input to apply the EUNIS classification on the Belgian Continental Shelf and to translate the maps into EUNIS habitat types (EUNIS level 5 maps). The whole analysis was performed within a GIS (Geographic Information System). The habitat suitability maps were classified by means of the natural breaks classification scheme. Two derivative maps were generated, respectively exceeding probabilities of 60% and 70%. Subsequently, the derived habitat suitability maps were translated into EUNIS habitat types. A large proportion of the Belgian shelf is covered and assigned to EUNIS classes. Each defined EUNIS habitat type has a habitat suitability percentage exceeding 60%. So far, only the Macoma balthica community matches within the current EUNIS classification. The other communities do not exactly match classes within the EUNIS classification. As such, only temporary codes are created and those need an expert review. The Habitat model (Degraer et al., in prep.) does not foresee transitional communities; as such they cannot be mapped. Once these are defined, a complete full coverage EUNIS map can be attained

Geo-environmental characterization of the Kwinte Bank

A detailed geomorphological and sedimentological study has been performed at a tidal sandbank, which has been dredged during 30 years. Localised intensive aggregate extraction created a depression in the central part of the sandbank, upon which the Government decided to close this section of the bank for further exploitation. Multibeam and side-scan sonar technology was used to survey the bank, in combination with extensive ground-truthing. Automated seabed classification was performed, but showed no direct correlation with the mean grain-size; the primary drivers influencing the classification being the sorting of the sediments, the presence of shells and of fine sediments. Very high resolution seismics revealed the internal architecture of the bank. In the central depression, the upper unit is locally severely dredged.The central depression is characterized by distinct morphosedimentary facies, compared to the western and eastern part of the bank and the Kwinte swale, adjacent of it. The difference between the western and the eastern part is essentially due to different tidal current characteristics, each having their particular sedimentation-erosion patterns. These processes seem to be rather stable, though the evolution of the sediments in the central depression shows similarities with the Kwinte swale sediment evolution.Since the depression is somewhat oblique to the normal crestline, it now forms an open transport pathway from the swale up to the crest of the sandbank. This led to a canalization of the flood current which is witnessed mostly by the northwards and faster progression of bedforms. Because of the difference in sediment characteristics between the dredged material and the present-day supply of sand, it is unlikely that natural processes will be able to counterbalance the severe dredging activities.Moreover, the presence of the central depression is located close to the kink of the sand bank, which is influenced by a high-energy hydrodynamic regime. Its presence could intensify the current action in this area and could enhance the evolution of the bank

Morphological evolution of the Kwinte Bank central depression before and after the cessation of aggregate extraction

Analyses of the records of ships registers and Electronic Monitoring Systems, of the trailer suction hopper dredgers, operating on the Belgian Continental Shelf, reveal that since the beginning of extraction in 1976, 75% of the total extracted volume originates from only one sandbank, the Kwinte Bank. At present, two morphologically-distinguished depressions are observed along the two most dredged areas of this sandbank: one in the central; and one in the northern part of the bank. In order to limit the impact of sand extraction on the bathymetry, the central depression of the Kwinte Bank was closed for exploitation, in February 2003. An understanding of the morphological evolution of this central depression is based upon data obtained: (a) from November 1999, until the closure for extraction in February 2003; and (b) on the subsequent post-dredging evolution, until June 2005. During this 5-year period, a total of 17 surveys were carried out with a multibeam echosounder over the area of the central depression (KBMA) and over a reference zone on an adjacent non-exploited sandbank. The resulting time-series of bathymetrical digital terrain models, together with backscatter strength maps, permit a detailed comparison of the bathy-morphological and sedimentary evolution of both of the monitored areas. Since the commencement of multibeam monitoring in 1999, an overall deepening (by 0.5m) of the entire KBMA monitoring zone is observed, until the cessation of dredging, in February 2003. Subsequently, the deepening slowed down and the variation in sediment volumes became similar to that of the adjacent non-exploited sandbank. From this, marine aggregate extraction appears to have only a local impact

Seafloor change detection using multibeam echosounder backscatter: case study on the Belgian part of the North Sea

To characterize seafloor substrate type, seabed mapping and particularly multibeam echosounding are increasingly used. Yet, the utilisation of repetitive MBES-borne backscatter surveys to monitor the environmental status of the seafloor remains limited. Often methodological frameworks are missing, and should comprise of a suite of change detection procedures, similarly to those developed in the terrestrial sciences. In this study, pre-, ensemble and post-classification approaches were tested on an eight km2 study site within a Habitat Directive Area in the Belgian part of the North Sea. In this area, gravel beds with epifaunal assemblages were observed. Flourishing of the fauna is constrained by overtopping with sand or increased turbidity levels, which could result from anthropogenic activities. Monitoring of the gravel to sand ratio was hence put forward as an indicator of good environmental status. Seven acoustic surveys were undertaken from 2004 to 2015. The methods allowed quantifying temporal trends and patterns of change of the main substrate classes identified in the study area; namely fine to medium homogenous sand, medium sand with bioclastic detritus and medium to coarse sand with gravel. Results indicated that by considering the entire study area and the entire time series, the gravel to sand ratio fluctuated, but was overall stable. Nonetheless, when only the biodiversity hotspots were considered, net losses and a gradual trend, indicative of potential smothering, was captured by ensemble and post-classification approaches respectively. Additionally, a two-dimensional morphological analysis, based on the bathymetric data, suggested a loss of profile complexity from 2004 to 2015. Causal relationships with natural and anthropogenic stressors are yet to be established. The methodologies presented and discussed are repeatable and can be applied to broad-scale geographical extents given that broad-scale time series datasets become available

Biogeomorphology in the field: bedforms and species, a mystic relationship

Fine-scale seabed mapping (Owenia fusiformis, and the razor clam Ensis directus) occur near bedload convergence zones resulting from a mutually evasive flood- and ebb-dominant channel system. Such zones are at the end of the channels, hence also fine grained sediments, food and larvae are trapped. The combination of the coarser-grained bedload with thedeposition of fines is indeed the optimum for a lot of suspension and detritus feeders. Still, highest abundances occur at the fringes of such a system where stress levels are intermediate. Hypotheses were successfully tested along the Dutch coastal zone. Those insights are important to assess changes in seafloor integrity and hydrographic conditions