Seasonal variation in movement, aggregation and destructive grazing of the green sea urchin (Strongylocentrotus droebachiensis) in relation to wave action and sea temperature

Hydrodynamic forces are an important determinant of subtidal community structure, particularly when they limit the distribution and foraging ability of mobile consumers. We examined the effect of wave action on the rate of movement and destructive grazing of a kelp bed by the green sea urchin (Strongylocentrotus droebachiensis) under field conditions. We measured density and rate of advance at fixed intervals along ~100 m of a grazing front over 1 year, and quantified individual movement rates in the barrens 5-10 m behind the urchin front using a time- lapse videography. Seasonal variation in the mean rate of advance of the front (range: 0-4 m month^¡1) was explained by changes in urchin density at the front (120-360 individuals m^¡2), which in turn varied inversely with significant wave height (0.5-2 m). Water temperature (0.8-17.6°C) had no effect on the rate of advance or on urchin density (aggregation) at the front, except when temperature exceeded 17°C. Movement of individual urchins also was affected by wave action: we observed a significant decrease in speed and displacement of urchins with increasing significant wave height. Wave action had no effect on the proportion of urchins moving or the degree of linearity of their movements. We propose that the decrease in urchin density at the front associated with increased wave action, results from de-aggregation, which reduces the risk of dislodgement, combined with a reduction in urchin movement in barrens, which supplies new urchins to the front.