I investigated aspects of the natural history of the common Dictyotalean brown alga Zonaria farlowii Setchell & Gardner 1924 across several temperate, rocky reefs surrounding Santa Catalina Island, California USA. Depth strongly affected every Zonaria measure. Zonaria biomass, coverage, and reproductive output were maximal at shallow depths. Laboratory experiments confirmed maximum growth under high light conditions corresponding to shallow depths. Nutrients and sessile grazers were not correlated with adult Zonaria variation across depths. Recruitment was highly variable in space and time, but was positively correlated with depth. Recruitment peaked at the deepest depth and was negatively related to adult abundance and reproductive output. This appears to be the first documented example of this phenomenon. I estimate that at least1 billion Zonaria propagules settle upon the 47 kilometer eastern coastline of Santa Catalina Island each year, with more than half of those surviving 6 months or less deeper than 15 m.

Zonaria farlowii is more abundant upon the rugose shells of the large mollusk Lithopoma undosa than upon surrounding rocky reef substrates at Santa Catalina Island. I conducted a yearlong, multi-factorial recruitment experiment utilizing models of these Lithopoma shells and flat, concrete tiles as recruitment substrates to test the hypothesis that substrate rugosity, grazing, and irradiance interact to influence the recruitment of Zonaria and other organisms. Shading did not significantly affect subsequent recruits, but rugosity and grazing interacted to influence recruitment. Algal density, cover, and morphology never differed significantly between experimental surfaces when large grazers were excluded; significant and consistent trends emerged only when surfaces were exposed to all ambient grazers. Rugosity and grazing independently affected algal and invertebrate community diversity. Subsequent surveys across Catalina reefs showed Zonaria abundance and algal diversity were significantly correlated with substrate rugosity only when grazers were abundant. Such meso-scale rugosity appears to strongly influence algal abundance and biodiversity in aquatic habitats, with the nature and magnitude of this effect dependent upon algal natural history and grazer abundance. This substratum-attachment hypothesis is consistent with observed recruitment patterns from a wide variety of communities across the globe.

Lastly, I evaluated the impact of variable light and grazing upon sessile organisms recruiting to settlement tiles deployed upon temperate rocky reefs surrounding Santa Catalina Island. Two separate experiments utilized cages to influence grazer access to tile surfaces and canopies to reduce incident light upon tile surfaces. The range of attenuated light in the first experiment was relatively small and motivated a substantially modified second experiment conducted on a nearby reef the following year in which light treatments spanned much of the natural range of light incident upon shallow reef areas. In this experiment, light x grazer interactions dominated recruitment patterns. Here, increased grazing pressure depressed algal cover and recruit density under high (< 15 m equivalent depth) and intermediate (20 m equivalent depth) light conditions, but did not significantly affect recruitment under low (45 m equivalent depth) light levels. Invertebrates (excluding spirorbid worms) showed the inverse of the algal pattern. Invertebrate cover and density were not affected by grazing within higher light treatments but were typically depressed by grazers in low light treatments. This light x grazer interaction was qualitatively different for algal and invertebrate diversity. Grazers generally depressed algal and invertebrate diversity under high and intermediate light levels but elevated diversity under low light. The response of diversity was independent of the particular metric (richness, evenness, heterogeneity) employed when examining algal and invertebrate communities individually. Overall fouling community diversity response, however, was metric-dependent. The response of the common brown alga Zonaria farlowii Setchell & Gardner 1924 to experimental treatments was of particular interest, as previous work showed Zonaria recruitment peaks at deeper depths while coverage and performance are maximal in shallow depths. High recruitment variation masked any light effect upon settlement and/or immediate post-settlement survival of Zonaria, but light clearly affected dictyotalean algae overall. Zonaria and overall dictyotalean recruits were common only under low light conditions, suggesting grazing alone cannot adequately explain variation across Catalina reefs. Detecting these light and light interaction effects required capturing a sizeable portion of the inherent range of irradiance incident upon local reefs, a rarely-emphasized component of most field investigations into the effects of variable irradiance.

pongo@stanford.edu