Surface-active Macrofauna Associated with Wrack Deposits

Wrack deposits provide potential food, shelter and breeding sites for marine and terrestrial invertebrate macrofauna and can support communities with high diversity, abundance and biomass. Macrofauna may also play important roles in the fragmentation, decomposition and incorporation of wrack into the beach and nearshore ecosystem. In this chapter I sampled surface-active macrofauna using pitfall traps set overnight and assessed abundance, species richness and community structure of sandy-beach macrofaunal communities. Two studies were carried out. In Study 1, I sampled the macrofauna that occurred at various positions on the beach face, specifically investigating occurrence with respect to wrack deposits. Sampling was conducted in 6 positions on the beach; three positions within the driftline (DL, in bare sand, among wrack and under wrack), above the DL (in bare sand and among wrack) and below the DL (in bare sand but not among wrack since none was present). A total of 6607 individuals from 20 species were captured, with the beach pill bug Actaecia pallida comprising 76% of the total abundance. The abundance and species richness of fauna differed significantly among positions, with communities in the DL having the most abundant and diverse fauna. Multivariate analyses indicated that positions within the DL (in bare sand, among wrack and under wrack) had similar macrofaunal communities that shared the same species and/or had similar abundances. The aim of Study 2 was to determine whether macrofaunal communities utilising wrack deposits differed temporally (i.e. seasonally) and/or spatially (i.e. between beaches). I sampled the macrofaunal communities that occurred on 4 local South Australian sandy beaches (Robinson Point, Moana, Aldinga and Normanville) in each of four seasons over a one-year period. The sampling effort was designed to capture the widest representation of macrofaunal communities (i.e. as many of the Chapter 4: Macrofaunal communities 127 species occurring on the beach as possible), based on the results of Study 1. Pit-fall trapping was thus carried out within the DL in bare sand and under wrack and yielded 5961 individuals, comprising 2 marine and 60 terrestrial species. On average, 47 (± 4) individuals and 5.9 (± 0.2) species were captured in each quadrat (i.e. in two pit-fall traps within a 1m 2 area). Five species each accounted for over 5% of the total individuals: a seaweed fly Coelopidae sp. 1; the sandhopper amphipod Talorchestia quadrimana; a mite Acarina sp. 1; the beach pill-bug A. pallida; and the weevil Aphela phalenoides. Macrofaunal communities (abundance, species richness and overall community structure) were variable in time (visits) and space (beaches and positions), as indicated by 3-way Analysis of Variance and multivariate analyses. The macrofauna encountered in this study were diverse, with detritivores and predators present. The presence of predators and the dominance of terrestrial species in these wrack deposits suggest that the wrack had been present for a reasonably long time. The wrack deposits sampled in this study thus support multiple trophic levels and provide a basis for a food web spanning both marine and terrestrial habitats. Within the DL, macrofaunal abundances were higher and a different macrofaunal community was present, compared with outside the DL. Within the driftline itself, there were few differences between bare sand and wrack-covered areas, suggesting that the entire driftline area is equally important as a habitat and food resource. The driftline thus provides an area of beach with concentrated resources, which in turn concentrates a distinct macrofaunal community.