Simulated eelgrass Zostera marina structural complexity: effects of shoot length, shoot density, and surface area on the epifaunal community of San Diego Bay, California, USA

Abstract

In marine ecology, a long-standing issue has been determining which components of habitat structure predominantly influence ecological processes structuring communities. In sea-grasses, faunal abundance and species diversity commonly are positively correlated with structural complexity (often quantified as shoot biomass or leaf surface area). These measures of complexity often covary with other attributes of seagrass structure, including shoot length and shoot density, which independently may influence ecological processes. We used artificial seagrass units (ASUs) to determine the relative influences of shoot length, shoot density, and surface area on epifaunal community structure in San Diego Bay, California, USA. Our first experiment tested how habitat attributes and exposure time influenced epifaunal density, diversity, and recruitment. In a second experiment we determined the interactive effects of habitat treatment and site on epifaunal density, diversity, biomass and community composition. In Expt 1, exposure time had a stronger effect on community measures than habitat treatment. When habitat treatment had an effect, surface area accounted for more of the variability in the epifaunal community than did shoot length and shoot density, but patterns of epifaunal density and recruitment were species-specific. In Expt 2, site explained proportionally more of the variation in epifaunal density and diversity than did eelgrass habitat characteristics. In >50 % of the tests, surface area accounted for more variability in epifaunal density and diversity than did shoot length or shoot density, but responses to habitat treatments again were species-specific. Our results demonstrate that the effects of habitat characteristics are highly variable with site and exposure time, and that variability in epifaunal density and diversity in seagrass habitat cannot always be ascribed to simple species-area relationships.