Shoots of the macroalga Chara tomentosa L. were exposed to different levels of turbidity, sedimentation or turbulence in aquarium experiments. Plants in turbid water or with a sediment cover became green, tall and weak. Some of them collapsed towards the end of the treatment period and were partially decomposed at the bases. The plants in turbulent water became orange, short and robust and some of them broke during the treatment. The shading by sediment in the water or on the plants presumably induced growth towards the light. As the plants in all treatments had about the same weight, the length was obtained at the expense of strength. A combination of the treatments could be disastrous. Plants weakened by shading would break even more easily if exposed to turbulence. It seems very plausible that these factors would be the causes for the drastic decrease in the occurrence of C. tomentosa at many localities in Southwestern Finland. Shallow bays are often subject to dredgings and motorboat traffic, factors that both increase the water movement and set sediment afloat.