INFLUENCE OF BENTHIC MACROFAUNA ON THE GEOTECHNICAL AND GEOPHYSICAL PROPERTIES OF SURFICIAL SEDIMENT, NORTH-SEA

Spatial and temporal Variations in the structure of an Amphiura-Echino cardium macrobenthic community were studied in relation to geotechnica l and geophysical properties of the seabed at a muddy-sand site in the seasonally stratified region of the southern North Sea. Vertical prof iles of geotechnical properties were recorded in sediments collected b y box corer. Maxima in water, organic matter, and fine particle conten ts coincided with the presence of the burrowing brittle star Amphiura filiformis and the mud shrimp Callianassa subterranea in the upper and lower parts, respectively, of the cores. A significant relationship e xisted between the abundance of A. filiformis and the water content of the upper 0.05 m of the bed; There were important temporal variations in rigidity modulus, derived from acoustic shear wave propagation in freshly recovered cores, of the upper 0.06 m of the sediment. The rigi dity modulus was 45% greater in January than in May and this has been related to the burrowing/feeding activity of the macrobenthic communit y; there was an inverse relationship between A. filiformis abundance a nd bed rigidity. Thus the bed had a lower bulk density and lower rigid ity in summer due to biological modification of the sediment fabric. T his implies that the bed was less resistant to erosion in the summer. Such an effect may be important during summer storms in the shallowest parts of the seasonally stratified zone (ie close to shelf fronts). ( C) 1998 Elsevier Science Ltd. All rights reserved.