Trophic relationships among animals associated with drifting wrack

Abstract

Wrack accumulates commonly in surf zones of sandy beaches and can be a semipermanent feature. Very few studies have investigated the trophic pathways associated with wrack accumulations in sandy beach surf zones, despite their potential importance to nearshore food webs. In the present study, we were specifically interested in determining the fish–wrack trophic associations in the nearshore. Macrophytes, macroinvertebrates and fish were sampled from drifting wrack at two sites with different macrophyte compositions (i.e. algae v. an algae–seagrass mix) in South Australia. The gut contents of fish were sampled, and the δ13C and δ15N stable isotope signatures of fish, macroinvertebrates and macrophytes were analysed. Using both the stable isotope and diet data, we identified that fish are feeding among wrack accumulations, but some unexplained trophic pathways suggest that fish are also likely to be foraging over multiple habitats elsewhere for food. In contrast, there was more evidence that grazing macroinvertebrates may be feeding on and around macrophytes within the accumulations, as well as using them as habitat. Thus, the present study established some baseline trophic pathways associated with wrack accumulations in sandy beach surf zones. Given the modest evidence for use of wrack as a food source, the lower trophic levels of the food webs identified remain unknown and should be an area for future research.