Chapter Six: Assessing the Incorporation of Wrack into Beach and Nearshore Ecosystems

Wrack inputs can supply the bulk or sole source of primary production on some beaches and can provide an important potential food source and site for nutrient regeneration. In this chapter, the incorporation of wrack into beach and nearshore ecosystems was assessed via two pathways; decomposition and incorporation into trophic webs. Wrack decomposition was assessed using litterbags containing wrack, which were deployed onto a beach and left in situ for up to 85 days. Decomposition was measured as mass loss and two experiments were conducted. In the first experiment, two mesh sizes were used for the litterbags (coarse mesh with holes 1.5 x 1.5 cm vs. fine mesh with holes 0.5 x 0.25mm, the latter to exclude macrofauna) and one algal (Ecklonia radiata) and one seagrass (Posidonia sinuosa) species were used. There was no difference in mass loss between the coarse and fine mesh litterbags but algal wrack appeared to lose a greater mass than seagrass wrack. Thus, in a second experiment, coarse mesh litterbags were used and two algal (E. radiata and Sargassum spp.) and two seagrass (P. sinuosa and Posidonia coriacea) species were used. In addition a subset of samples also analysed for elemental content (%C and %N) and stable isotope signatures (δ 13 C and δ 15 N). In both studies, there was a rapid initial loss of mass followed by very slow or no further decomposition. The exception to this was the seagrass P. coriacea, which showed a slow but relatively steady loss of mass. The carbon content (%C) of the wrack did not differ over time, suggesting that most of the non-structural C had already been lost from the wrack prior to the start of the experiment. For %N the results were more variable among the species and over time, suggesting that the processes affecting %N differ among species and during decomposition (e.g. as microbial communities colonise and proliferate on the detritus). δ 13 C did not change over time but δ 15 N increased slightly suggesting that consumers may have colonised the wrack. Thus, rates of Chapter 6: Incorporation of wrack into ecosystems 199 decomposition and changes in elemental composition and isotopic signature may be taxon(algae vs. seagrass) and species-specific, and vary depending on the structure and chemical composition of the material. I used stable isotopes (δ 13 C and δ 15 N) to assess whether beach macrofauna or nearshore macro-invertebrates and fish might rely on wrack as a source of nutrition. I sampled a total of 15 beaches across 3 bio-geographical regions of South Australia (Metropolitan Adelaide, Fleurieu Peninsula and South East regions) in winter and summer of 2007. Wrack, beach macrofauna and nearshore invertebrates, fish and crabs were collected from each beach. Nearshore fish and macroinvertebrate communities differed between beaches, regions and visits, i.e. were variable in time and space. Seven species of fish were sampled using seine nets, which is similar to, or lower than, other studies. The amount of wrack on the beach and in the surf zone did not affect the abundance and species richness of fish and invertebrates. Stable isotopes indicated that seagrass wrack did not provide a food source for any of the consumers found in this study. Algae, particularly brown algae including kelps, appeared to be potential sources of nutrition for consumers such as amphipods and dipterans. Predation on these consumers by predators such as staphylinid beetles and nearshore fish and crabs may also facilitate the incorporation of organic matter into higher trophic levels. Wrack thus provides a pathway for the transfer of allochthonous organic matter and nutrients from offshore algal reefs into primaryand higher-level consumers in sandy beach and nearshore ecosystems.