Date of Award

2006

Degree Type

Thesis

Degree Name

Doctor of Philosophy

School

School of Natural Sciences

Faculty

Faculty of Computing, Health and Science

First Advisor

Glenn Hyndes

Abstract

Detached macrophytes (sea grass and macroalgae) are transported from more offshore areas and accumulate in large volumes in surf zones, where they are commonly called wrack. In coastal regions in other parts of the world, wrack transported from one habitat to a second habitat can be considered as a "spatial subsidy" for the recipient habitat with significant consequences for community dynamics and food webs. The primary aim of this study was to determine the significance of the different components of wrack (i.e. sea grass and brown, red and green algae) as a direct and indirect food source and habitat for invertebrates and fish in surf zones of south-western Australia. The importance of different volumes of surf zone wrack to determining fish abundance and composition was also investigated. These aims were achieved by examining the food and habitat preference of invertebrates and the habitat preference of fish through laboratory trials and field experiments. Gut content analysis was used to examine the importance of wrack-associated invertebrates as a food source for fish, while stable isotope analysis (carbon, nitrogen and sulfur) and lipid analysis (lipid class and fatty acid composition) were conducted on macrophytes, amphipods and fish to determine the source of nutrients and energy. The composition of surf zone wrack in the region comprises large quantities of seagrass, then brown and red algae, with negligible quantities of green algae. Allorchestes compressa, the dominant macroinvertebrate in surf zone wrack, showed a preference for consuming brown algae over other macrophyte types. Similarly, stable isotope analysis from some locations and fatty acid analyses indicated that A. compressa assimilates nutrients predominantly from brown algae. The influence of brown algae on secondary production extends to second-order consumers. Allorchestes compressa was the major prey of juveniles of the cobbler Cnidoglanis macrocephalus and the sea trumpeter Pelsartia humeralis, the main fish species in surf zone wrack accumulations in the region. Detached brown algae therefore contributes most to the detached macrophyte - amphipod - fish trophic pathway in the surf zones, and thus drives secondary production in these regions and provides a crucial link between coastal ecosystems. Detached macrophytes also provide an important, but transient, habitat for invertebrates and fish in south-western Australia. Under laboratory conditions, Allorchestes compressa showed a strong preference for inhabiting seagrasses over macroalgae, iii however in situ caging experiments showed that A. compressa has a strong preference for brown algae, red algae or a mixture of macrophytes, but tended to avoid seagrass. Therefore, A. compressa showed a clear preference for different types of detached macrophytes as a habitat, with seagrass ranking below other types of macrophyte under field conditions. In contrast, neither Cnidoglanis macrocephalus or Pelsartia humeralis showed a preference for inhabiting different types of detached macrophytes as a habitat, but showed a strong positive influence by increasing volumes of wrack The species composition, densities and biomass of fish, which were dominated by juveniles, were strongly influenced by increasing volume of wrack in surf zones of south-western Australia. This study has shown that both the type and volume of detached macrophytes transported from more offshore regions subsidizes consumers and plays a crucial role in supporting secondary production in less productive surf-zone habitats of south-western Australia. The removal of large amounts of wrack from nearshore areas could have a detrimental impact on the biodiversity or abundance of macroinvertebrate and fish populations, which rely on wrack for food and shelter.

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