Structural and functional biological assessment of aggregate dredging intensity on the Belgian part of the North Sea

Marine aggregate dredging in the Belgian part of the North Sea (BPNS) is restricted to four dedicated concession zones. Within these zones, there are areas under different dredging pressure, but with the advantage that these are situated within a similar habitat (cfr. similar sediment characteristics) . As such, this study assessed how different degrees of dredging pressure executed on a similar sandy habitat affect the benthic ecosystem. Possible responses of the macrobenthos on the dredging pressure were evaluated based on both structural (species number, species composition, abundance and biomass) and functional (e.g. bioturbation potential, BTA) characteristics of the benthic ecosystem. The structural changes in benthic characteristics were summarised by the benthic indicator BEQI.The most obvious impact of dredging on the benthic community was observed in the most intensely used area (high dredging intensity and frequency) with significant changes in the structural benthic characteristics, and a moderate to poor score for the benthic indicator BEQI. For the benthic functional characteristics, no impact of dredging was measured in any of the areas. Furthermore, the hearturchin (Echinocardium cordatum) was observed to be the most sensitive species to dredging, because it reduced substantially in numbers or even disappeared in all impacted areas.Our results suggest that the current benthic sandy ecosystem of the BPNS is resilient enough to buffer aggregate dredging when performed at low or at high, but infrequent intensities. However, when dredging focuses on a small surface area, and when it is performed at high and frequent intensities, changes in sediments result in clear biological changes

Macrofaunal community structure and zonation of an Ecuadorian sandy beach (Bay of Valdivia)

The sandy beach macrofauna of the Bay of Valdivia (Ecuador) was sampled in August-September 1999 along six replicate transects between the high and low water line. The sediment consisted of well-sorted, fine to medium sand. Taking into account the dimensionless fall velocity (O) and the relative tidal range, the beach was characterized as an exposed, low tide terrace -rip beach. The distribution of the macrofauna was mainly determined by the elevation on the beach. Thirty-one taxa were found throughout the study, varying between 10 and 22 taxa per transect. Molluscs were the most dominant taxon (overall average: 285 ind/m2, max. : 2135 ind/m2), followed by crustaceans. The gastropod Olivella semistriata (overall average: 243 ind/m2, max. : 2131 ind/m2) was the most abundant species. The crustaceans were the most diverse taxon (10 spp.); Haustorius sp., Excirolana braziliensis and Emerita rathbunae were the most abundant species. Densities of polychaetes were low in comparison with the previous two taxa mentioned (overall average: 13 ind/m2). The total average density over the entire bay was 370 ind/m2. Three zones of macrofaunal distribution along the tidal gradient could be distinguished: an upper beach, a middle beach and a lower beach assemblage. The upper beach assemblage consisted mainly of Excirolana braziliensis and ghost crabs. The middle beach assemblage had highest macrofaunal densities and was dominated by Olivella semistriata andHaustorius sp. The most diverse assemblage was found on the low beach, with representatives of different taxa, but with lower densities. These results are in conformity with other studies along the Pacific coast of South America and fit into the zonation scheme of DAHL (1952). Given that many other studies from South America were done at temperate latitudes, we conclude that, although differences in species composition were found, the general pattern of macrobenthos zonation on sandy beaches is similar in tropical regions