Impact of bottom trawling on water turbidity and muddy sediment of an unfished continental shelf

Two experiments were carried out to study the effects of trawling in the mu ddy prodeltaic deposit of the Llobregat River in the northwestern Mediterra nean. Trawling was conducted in two experimental lines, and bottom morpholo gy, sediment texture, and water turbidity were analyzed before trawling and at different time intervals afterward. The tracks of the trawl gears were still observed in sonographs of the bottom 1 yr after the first experiment. The vertical grain size distribution of bottom deposits indicated that the thickness of the sediment removed by the net between the gears was about 2 -3 cm on average, though the erosion produced by the gears was deeper. Resu spended aggregates with a high silt content settled during the first hour a fter trawling, generating a temporary increase in the silt content of the s urface sediment. One day after trawling, the surface sediment was mixed and already had a similar grain size distribution to that before trawling. Aft er the beginning of trawling, water turbidity increased first near the bott om for a few hours and later also at shallower levels of the water column w ithin a period of 2-5 d after trawling. At the end of the experiment, about 10% of the sediment affected by trawling was diffused in the water column and the remaining 90% had settled on the bottom. Average turbidity in the w ater column increased by a factor of up to three for 4-5 d after trawling. This experiment shows that intense and continued trawling on continental sh elves has a noticeable effect on water turbidity, which must be considered in addition to natural processes.