Seagrass-epifauna relationships in a temperate South African estuary: Interplay between patch-size, within-patch location and algal fouling

Understanding the primary and interactive effects of patch-size, within-patch location and degree of algal fouling on epifaunal assemblages is important for seagrass ecology and management, especially in fragmented habitats. In this paper we examine the influence of these factors on epifaunal assemblages in the Heuningnes Estuary, which is one of South Africa's oldest protected systems. Multivariate analyses indicated that all seagrass properties significantly affected epifaunal assemblages and that two gastropods viz. Assiminea globulus and Haminoea alfredensis contributed most to the dissimilarities between factors. Abundance of epifauna and A. globulus were significantly greater at the edges of seagrass beds, whereas species diversity was higher in the interior. Differences in assemblage structure between inner cores and outer edges of seagrass beds are most likely due to the physical properties of each of these zones as well as competitive interactions and habitat preferences of species that utilize each section. Epifaunal richness and A. globulus abundance were significantly greater in large beds compared to small ones and the degree of algal fouling on seagrass beds inversely affected epifaunal abundance, species richness and A. globulus abundance. Importantly, there was a strong interaction between within-patch location and patch-size, indicating that the effects of within-patch location can be altered by patch-size. When explored further, ordinations indicated that epifaunal assemblages in cores and edges became more dissimilar as bed size increased. Similarly, differences in epifaunal abundance, and diversity as well as abundance of A. globulus and H. alfredensis between cores and edges of seagrass beds became greater with increasing bed size. Our results indicate the importance of large beds in enhancing spatial heterogeneity, richness and diversity and the need for adequate protection to prevent their fragmentation in future. This is especially pertinent given the small number of large beds currently found in the system. We discuss the importance of current management strategies in the Heuningness Estuary, specifically the artificial maintenance of an open mouth state, in the persistence of large seagrass beds in the system.