Epi- en hyperbenthische gemeenschappen van Belgische zandstranden

This is the first study on the epi- and hyperbenthic fauna of the surf zone of sandy beaches of the Belgian coast. Therefore, as a first objective of this thesis, a lot of attention has been paid to the inventarisation (species composition), and to the investigation of the temporal and spatial distribution patterns of both communities. A temporal and a spatial campaign were performed during which the hyper- and epibenthos was sampled.
During the temporal campaign monthly samples were taken at 4 selected stations and the spatial campaign involved sampling of 12 stations along the Belgian coast. All in all, the surf zone of the Belgian coast is a habitat for a considerable number of species. The surf zone harbours a rich hyperbenthic fauna and also the epibenthic assemblage is diverse. Total hyper- and epibenthic densities were comparable or even higher than those of the adjacent, subtidal regions. It seems that the Belgian surf zone can be seen as an extension of the rich shallow sand bank system in front of the coast. Differences in total hyperbenthic densities with other European sandy beaches might be linked to the degree of exposure. An attempt was made to elucidate the major environmental variables structuring the communities. Therefore several environmental variables were measured at the moments of sampling. Different uni- and multivariate statistical techniques were used for the analyses of the data. Although clear temperature-related seasonal distribution patterns were observed, the hyper- and epibenthic communities of the surf zone can also be influenced severely by hydrodynamic factors and this both by short-term events (e.g. storm) and long-term effects (influence on morpho- dynamics of the beach). Spatial differences were found both in species richness and in density. In general, highest hyper- and epibenthic densities were found on flat, wide beaches, but also intermediate sites and sites with a high turbidity were characterised with high densities and a high number of species. Although clear distribution patterns could be observed, the much lower densities of the epibenthos compared to the hyperbenthos, might require a higher sampling effort with replicate sampling in future research. Furthermore, this study indicates that both intertidal and subtidal characteristics should be considered if the (mobile) surf zone fauna is investigated. Stomach/gut content analyses were performed on five common flatfish fish species from epibenthic samples taken during the temporal campaign in order to unravel part of the surf zone food web. Additional fish were obtained from commercial fishermen. Prey items of all species included hyperbenthic (e.g. mysids), endobenthic (e.g. polychaetes) and epibenthic (e.g. shrimps) species. Little dietary overlap was observed. If diet overlap did occur, it mainly involved prey species that are dominant in the surf zone of the studied beaches, such as shrimps and mysids. These results confirm the opportunistic utilisation by flatfish of the available food resources in surf zone ecosystems. Such opportunistic utilisation of food resources can be important for teleosts frequenting physically stressed environments such as surf-exposed beaches. The diets of turbot Scophthalmus maximus and brill Scophthalmus rhombus reveal part of the main interactions between the hyper- and epibenthos, whereas those of plaice Pleuronectes platessa, dab Limanda limanda and sole Solea solea reveal important links between the epibenthos and the macro-endobenthos within the local surf zone food web. Three 24-hours sampling cycles were performed to study short term migration patterns. Again the hyper- and epibenthos was sampled alternatingly each hour. Stomach content analyses were performed on juvenile plaice Pleuronectes platessa of the first 24h-cycle (spring tide). Possible interactions between the epi- and hyperbenthic organisms are discussed. Despite the turbulent conditions of the studied sites, clear tidal (e.g. I-group plaice Pleuronectes platessa, the brown shrimp Crangon crangon) and diurnal (e.g. juvenile sole Solea solea) periodicities were observed in many macrocrustacean and demersal fish species. An opportunistic utilisation of available food resources is again suggested as I-group plaice clearly migrate high up the beach during flood to disperse in order to profit from the rich macrobenthic area (mainly the polychaete Scolelepis squamata and ends of Arenicola species), while the 0-group feeds mainly on the most abundant hyper- and macrobenthic organisms from somewhat deeper water (mainly palps of the polychaete Magelona papillicornis and cyprid larvae). Finally, it can be concluded that the Belgian surf zone functions as a nursery and/or transient are for hyper- as well as epibenthic stages of several species.