Latitudinal, seasonal, and small-scale spatial differences of the giant kelp, Macrocystis pyrifera, and an herbivore at their southern range limit in the northern hemisphere

Although several studies have described kelp-urchin interactions, little is known about these interactions toward the distributional range limits of these species in areas that may experience different physical conditions than at the center of the distributional range. We explored seasonal and latitudinal changes in the population structure of the giant kelp and the purple sea urchin at their southern limits in the northern hemisphere. Densities of both organisms were positively correlated, showing a significant effect of season and latitude, and were highest in the temperate kelp populations and in the spring. The near subtropical kelp populations, however, contained significantly larger urchins. Along the boundary between the kelp beds and the urchin barrens studied, no significant spatial difference was found in sea urchin size or density. However, a general pattern of increasing probability of urchin presence with distance outside of the bed was found in spring at all sites. Spring bottom temperatures were generally colder and more variable in the temperate kelp populations, potentially explaining the larger seasonal variability found there. Data suggest seasonal differences may be modulated by the environmental variability in the temperate kelp populations and support local coupling of these two organisms at their southern range limit, in spite of the particularly unique physical conditions which occur in these populations at times.