Algal zonation in tidepools: experimental evaluation of the roles of physical disturbance, herbivory and competition

Abstract

Tidepool algal community structure and organization is poorly understood relative to our understanding of communities on emergent intertidal substrata. The patterns and causes of algal zonation in small, steeply sided, mid-intertidal pools (potholes) were investigated on the Oregon coast. From top to bottom, pools were characterized by a foliose algal zone (pool edges), a crustose coralline algal zone (pool sides), and a bare zone (pool bottoms). These zonation patterns were extremely persistent. Erect fleshy algae fluctuated seasonally, with peaks in abundance during the summer. Seasonal variation was consistent between years, not only among all pools but also in individual pools. The relative importances of physical disturbance and biotic interactions in determining tidepool community structure were evaluated along a gradient of physical harshness. The pools studied here offered the opportunity to experimentally manipulate physical disturbance which is often difficult, rarely done, and essential to our understanding of community organization. Using a factorial experimental design, cobbles, limpets and coralline algae, a major space occupier in tidepools, were manipulated. Identically designed experiments were performed at both wave-exposed and wave-protected sites using identical replicate pools chiseled out of rock benches. Scouring by cobbles generally reduced the abundance of organisms in pools, limited them to higher areas in pools, or both. The effects of cobbles were especially apparent on pool bottoms or on the depth distribution of algae, suggesting that there was a strong vertical gradient of scouring where cobbles were present. This scouring gradient was confirmed in natural pools by measuring scouring rates with painted plates, and scouring evidently led to the zonation patterns observed in natural pools. Herbivores had no effect on algal distributions within tidepools, but evidently influenced the relative abundance of fleshy algae at different wave exposures. Abundant herbivorous snails at protected sites may have resulted in a high proportion of herbivore-resistant coralline algae and more bare space, while at exposed sites, herbivorous snails were scarce and fleshy algae were more abundant. Removal of all corallines resulted in an increase in the abundance of fleshy crustose algae at both sites, suggesting that corallines pre-empt space from fleshy crusts. Competition among fronds of erect algal groups was evaluated by reciprocally removing the fronds of erect fleshy and erect coralline algae. After the removal of erect fleshy or erect coralline algal fronds, only slight changes were observed over 18 months, suggesting that competition between algal fronds is unimportant in determining algal distribution and abundance. For these intertidal potholes, algal distributions within pools are largely determined by the physical process of scouring, but herbivores may cause between site variations in pool communities.