Impacts of aggregate dredging on sediment composition and associated benthic fauna at an offshore dredge site in the southern North Sea.

Dredging and associated screening at a dredge site in the southern North Sea (Area 408) is associated with areas of well-sorted fine sand that extend for up to 3 km to the south-east of the dredged area and overlay sediments with a more variable particle size composition. This well-sorted fine sand may reflect deposition and transport of material mobilised by the dredging and screening processes at the dredge site. Multivariate analysis of the benthic community structure suggests that marine aggregate dredging, at the level of intensity employed in the study area prior to sample collection, has had a limited impact on benthic community composition compared with that reported from studies elsewhere. This is ascribed to the likely rapid rates of recolonisation by the mobile opportunistic polychaetes and crustaceans that dominate the macrofauna of the sandy gravel deposits at this particular dredge site. Analysis of variance showed, however, that significant differences existed between the sample treatments in terms of species evenness (Pielou's J). Dredged samples were found to have the lowest mean species evenness (0.71) when compared to controls (0.77). The present study highlights the inherent difficulties in the application of general impact/recovery predictions to dredged sites with varying environmental characteristics.

Adjustment of the components of energy balance in response to temperature change in Ostrea edulis.

1. Routine oxygen consumption (VO2µl 285 mg-1 h-1) by specimens of Ostrea acclimated to temperatures between 5° and 25°C increases with exposure temperature throughout much of the range 5 to 30°C. 2. There is little evidence of a relative suppression of the acutely-measured rate: temperature curves for routine oxygen consumption following warm acclimation. Energetic costs in Ostrea thus rise sharply with increase of environmental temperature. 3. The clearance rate (V w ,ml 285 mg-1 h-1) of Phaeodactylum measured synchronously shows a marked thermal optimum within the range 15-28°C, followed by a decline towards high exposure temperatures. 4. Maximal clearance rates of 600-750 ml water h-1 occur approximately 5°C above the temperature to which the animals have been acclimated and are adjusted according to environmental temperature. 5. The changes in filtration efficiency (V w /VO2, ml/µl) in Ostrea following thermmal acclimation are controlled by compensatory responses of the irrigation rate rather than by a relative reduction of energy losses from metabolism. 6. Because the clearance rate declines in animals acclimated to 25°C, whereas the metabolic losses increase throughout the temperature range 5 to 30°C, maximum filtration efficiency is achieved at 20°C. 7. Calculation of the energetic cost of filtration suggests that the improvement of filtration following warm acclimation offsets metabolic energy losses even at low food concentrations in Ostrea. Even if energy losses not accounted for in this study were substantial, a positive index of energy balance could be maintained during the summer months at ration levels which commonly occur in inshore waters.

Factors affecting oxygen consumption in the woodlouse Porcellio scaber latr.

1. There is a well-defined time sequence in the oxygen consumption of Porcellio scaber. A maximal rate (the 'active rate') is obtained in woodlice subjected to the maximal disturbance of food presentation, handling and light. After initial exploratory activity has ceased, the oxygen consumption declines through a series of 'excited rates' towards the 'standard rate' which is approached after 2-3 hr equilibration in a respirometer. 2. The rate following starvation for 1-3 days is similar to the standard rate. 3. Thermal acclimation has no significant effect on the slope or the level of the rate: temperature curves for the excited or the standard rate of oxygen consumption of Porcellio scaber. 4. The rate: temperature curves for active, excited and standard oxygen consumption of Porcellio scaber have a double sigmoid form with high temperature coefficients at the extremes of the experimental temperature range. 5. The respiration rate of a homogenate of Porcellio scaber in the presence of 9.0 mM succinate is intermediate between the active and standard rate of the intact animal. 6. The maximum scope for activity is highest at 10-15°C and falls towards both higher and lower experimental temperatures. 7. The overall oxygen consumption of intact Porcellio scaber may reflect the summation of 'cellular' and 'organismic' metabolic components which account for approximately equal proportions of the total metabolism at the normal environmental temperature range of 10-15°C. Below this temperature the cellular component is predominant whilst at the upper limits of the experimental temperature range oxidative metabolism is represented solely by the organismic component.