Date of Award

2007

Degree Type

Thesis

Degree Name

Bachelor of Science Honours

School

School of Natural Sciences

Faculty

Faculty of Computing, Health and Science

First Advisor

Dr Glenn Hyndes

Second Advisor

Paul Lavery

Third Advisor

Dr Fernando Tuya

Abstract

The movement of nutrients and organisms between habitats provide important spatial subsidies on local and regional scales, resulting in increased primary and secondary production, especially where inputs supplement habitats of relatively low levels of comparable resources. In coastal south-western Australia, the brown kelp, Ecklonia radiata, is produced in large quantities on offshore reefs from where it detaches and passes through neighbouring habitats. This allochthonous resource is present in large quantities in seagrass meadows and thereby potentially influences the trophic dynamics of this habitat, providing an additional food source for grazers to those produced in situ. This study investigated the effects of the large detached kelp on mesograzer trophic dynamics in Posidonia and Amphibolis seagrass meadows. Laboratory choice and no-choice feeding experiments tested whether preferential consumption of the kelp occurred in comparison to autochthonous resources by two locally abundant gastropods, Pyrene bidentata and Cimtharidus lepidus. Results from the feeding experiments demonstrated that both species of gastropod did not preferentially consume fresh or aged kelp, but the rates of consumption were generally similar to locally abundant periphyton and red algae that are epiphytes on seagrass leaves. In comparison, the gastropods consistently avoided consumption of seagrass leaves. Field experiments were conducted at four sites during winter to measure the effects of mesograzer consumption of kelp in different dominant seagrass habitats and at different proximities to reef to encompass a range of landscape-scale effects on seagrass meadows. There were no significant effects of any main factor in the field experiment, due to an apparent lack of grazing of the large detached kelp by the mesograzers in the seagrass meadows. High variability influenced the consistency of the results, which may have been a result of cage artefacts, bacterial decomposition, or physical abrasion. Overall, the results suggest that, although seagrass-inhabiting mesograzers are capable of consuming detached E. radiata, consumption was either absent or not detected, possibly due to the high availability of autochthonous resources in seagrass meadows. Thus, it is possible that this allochthonous food source provides a negligible spatial subsidy to mesograzers in a habitat where comparable food resources are relatively unlimited, matching empirical thought. However, additional studies during different seasons and at different locations are necessary to further investigate these conclusions, to assess if allochthonous resources influence seagrass meadow trophic dynamics when in situ food limitation occurs.

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