Factors influencing recruitment of the dictyotalean brown alga Zonaria farlowii and other sessile marine organisms at Santa Catalina Island, California

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. In a large light manipulation experiment, Zonaria and overall dictyotalean recruits were common only under low light conditions, suggesting grazing alone cannot adequately explain variation across Catalina reefs. At least 1billion 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 is more abundant upon the rugose shells of the large mollusk Lithopoma undosa than upon surrounding rocky reef substrates at Catalina. A yearlong, multi-factorial recruitment experiment utilizing models of 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.