Sediments are sinks for contaminants in the world's oceans. At the same time, commercial bottom trawling is estimated to affect around 15 million km(2) of the world's seafloor every year. However, few studies have investigated whether this disturbance remobilises sediment-associated contaminants and, if so, whether these are bioavailable to aquatic organisms. This field study in a trawled contaminated Norwegian fjord showed that a single 1.8 km long trawl pass created a 3-5 million m(3) sediment plume containing around 9 t contaminated sediment; ie. 200 g dw m(-2) trawled, equivalent to c. 10% of the annual gross sedimentation rate. Substantial amounts of PCDD/Fs and non-ortho PCBs were released from the sediments, likely causing a semi-permanent contaminated sediment suspension in the bottom waters. PCDD/Fs from the sediments were also taken up by mussels which, during one month, accumulated them to levels above the EU maximum advised concentration for human consumption.

Tributyltin is a toxic compound that has been used in antifouling paint for boats andships. Due to the immediate toxic effects at very low concentrations it was banned in many countries in 1986. Due to its slow degradation it is expected to be stored in the sediment for many decades. This experiment simulated a small boat harbour with frequent resuspension events of TBT-contaminated sediment to measure the impacts onthe release, bioavailability and effects of tributyltin. Physiological stress responses were measured in two coastal species, the blue mussel, Mytilus edulis, and the red algaCeramium tenuicorne. Response variables measured were respiration, ammonia excretion, clearance rate and survival rate (for mussels) and growth inhibition (for thealga). Resuspension released both dissolved and particle-bound TBT to the surrounding water and made it bioavailable for both organisms. There was a clear toxic effect of the highest concentrations and it was evident that both mussels and algae showed a fasterand more negative response when the sediment was suspended. Repeated or continuous exposure to suspended TBT-contaminated sediment can exert a risk to the organisms living in environments such as harbours.

Sponges often dominate deep-water benthic faunal communities and can comprise up to 90% of the benthic biomass. Due to the large amount of water that they filter daily, sponges are an important link between benthic and pelagic ecosystems. Across the Tromso-flaket, Barents Sea, Norway, there are high biomasses of deep-water sponges. This area is also an important fishing ground, with fishing activity in some areas >27000 trawl hours yr(-1). Bottom trawling suspends large quantities of sediment into the water column, with measured concentrations up to 500 mg l(-1). This is the first study on the effects of suspended sediment exposure on deep-water sponges. In a laboratory experiment, Geodia barretti (Bowerbank 1858) (Class: Demospongiae) was exposed to 5 different sediment concentrations (0, 10, 50, 100 and 500 mg l(-1)). Respiration rates were measured before, during and after the exposure period. The results demonstrate that G. barretti physiologically shuts down when exposed to concentrations of 100 mg l(-1) (86% reduction in respiration), with thresholds of responses occurring between 10 to 50 mg l(-1). However, rapid recovery to initial respiration levels directly after the exposure indicates that G. barretti can cope with a single short exposure to elevated sediment concentrations. Given the high bottom-trawling frequency in Tromso-flaket, sponges may be frequently exposed to suspended sediments. Therefore, it is important that further investigations on the effects of suspended sediments on filter feeding organisms focus on the effects of repeated and long-term suspended sediment exposures to evaluate the overall ecological impacts.

This study compares the extent and frequency of seafloor disturbances due to anthropogenic processes (bottom trawling and dredging) with those due to wind-driven waves in the Baltic Proper. There is little overlap between areas affected by bottom trawlers and naturally from waves, and bottom trawling is the main cause of sediment resuspension at greater (>73m) depths. Swedish bottom trawlers annually suspend between 3.8 and 13.6 M tonnes in this area, while from the reported dredging projects (reported between 2007 and 2009) the estimated maximum annual spillage was 0.14 M tonnes. The dominance of bottom trawling as a factor suspending sediment is clear, but management approaches for bottom trawling and dredging are different, despite similar expected effects on organisms and the environment.