Date of Award
Bachelor of Science Honours
School of Natural Sciences
Faculty of Computing, Health and Science
Dr Glenn Hyndes
Dr Mat Vanderklift
The transport of detached macrophytes from seagrass meadows and reefs, otherwise known as wrack, has been identified in some parts of the world as a marine subsidy which can increase the productivity of coastal ecosystems. The aim of this study was to determine whether wrack deposits act as a marine subsidy on sandy beaches in Perth, by determining whether it influences the structure of, and is a source of nutrient to, the macroinvertebrate communities in this ecosystem. This was achieved by examining the abundance, composition and nutrient source of macrofauna, including epifauna, infauna and flying fauna, on three sandy beaches with high wrack coverage and three beaches with low or no wrack coverage in the Perth metropolitan area. Carbon and nitrogen stable isotope analysis was conducted on macroinvertebrates and potential sources of production to determine whether they were gaining nutrients from marine or terrestrial primary production. There was a clear difference in the volume of wrack between beaches with high and low volumes of wrack, while sediment moisture and organic matter showed no difference between the two categories of beaches. High wrack beaches all had similar volumes of wrack present, while wrack was either absent or in very low volumes on low wrack beaches. There were strong differences between high wrack beaches and low wrack beaches in the abundance and taxa richness of macroinvenebrates caught using quadrats and cores. Epibenthic and infaunal macroinvertebrates, predominantly beetles and flies, were consistently present on high wrack beaches, whereas few, if any, individuals were found on low wrack beaches. No differences occurred for adult flies caught using sticky traps. Stable isotope analyses indicated that marine-derived wrack was the main source of nutrients for macroinvertebratcs on beaches containing wrack material. δ13C values of wrack ranged from -5.79 to -17.47%o, while terrestrial vegetation had much lower values, around -27.01%o. All macroinvertebrates δ13C values were similar to those of wrack, ranging from -8.56 to -20.77%o. The δ15N values indicated that spiders, staphylinid beetles and beetle larvae occupied a higher trophic level than other consumers; they are likely to be predators of other fauna within wrack accumulations. It therefore appears that macro invertebrates are utilizing wrack as a source of both shelter and food. There was a lack of differences in macrofauna abundance among wrack beaches and between zones for high wrack beaches. This is most easily explained by the absence of any significant differences in the volume and distribution of wrack among beaches and between zones. Family composition did differ among beaches, a result that is likely to be related to the variability in wrack characteristics such as composition and age. The results indicate that wrack is acting as a marine subsidy for macroinvertebrates on Perth beaches. The removal of wrack is likely to result in the loss of macrofauna assemblages from sandy beaches in Perth, or changes in the abundance and composition of those assemblages. This is likely to influence the food web in the sandy beach ecosystem, the magnitude of which is not yet fully understood.
Ince, R. (2004). Beach-Cast Wrack as a Determinant of Macroinvertebrate Assemblages and Trophic Subsidy to Sandy Beach Ecosystems. https://ro.ecu.edu.au/theses_hons/1009