Effects of kelp canopies on understorey species due to shade and scour

Mechanisms were investigated by which kelp canopies influence abundances of understorey species.First, the amount of shade afforded by kelp canopies and the frequency of scouring by kelp plants of various sizes were quantified.Next, the effects on understorey assemblages due to the reduction of light levels by kelp canopies were determined by manipulating the degree of shading in certain areas and comparing these with unmanipulated controls.Effects due to scouring by kelp laminae were determined by stopping scour in certain areas and comparing these with unmanipuiated controls.Several treatn~ents were included in these experiments to assess artifacts caused by the manipulations.Most species whose abundances changed after removing kelp (i.e.increases in filamentous and turfing algae, decreases in encrusting algae and ascidians), did not change in clearings where artificial shade was supplied.Scour by kelp plants showed no effects on most species, although the amount of sediment and microscopic silt were enhanced by stopping scour, as perhaps were the ahundances of an anemone and an entoprocl.The usefulness of field manipulations to test alternative hypotheses accounting for patterns detected in kelp communities is noted.