Date of Award

2000

Degree Type

Thesis

Degree Name

Bachelor of Science Honours

Faculty

Faculty of Computing, Health and Science

First Advisor

Dr Paul Lavery

Abstract

Artificial seagrass units were used to determine whether seagrass leaf movement influences the biomass, species richness and composition of epiphytic macroalgae on the leaves of seagrasses, and whether the influence of leaf movement is altered by the degree of exposure to water movement and to depth. The influence of leaf movement on epiphytic biomass is important from an environmental management perspective, as there is the potential for epiphytic productivity to be underestimated if leaf movement is reducing the standing crop of epiphytes on seagrass leaves. Two forms of artificial seagrass units were used in three experiments to achieve these objectives; untethered units mimicked natural seagrass leaves, that were free to move in response to water movement, and tethered units mimicked seagrass leaves that were unable to move. The first experiment examined the effect of seagrass movement on the biomass, species richness and species composition over time. The second and third experiments examined the influence of exposure and depth on the effect of seagrass leaf movement on epiphytic biomass. In all three experiments the epiphytic biomass, measured in terms of dry weight (DW) and ash free dry weight (AFDW), was far greater in tethered than untethered units, where the epiphytic biomass was on average eight times higher on tethered leaves. Similarly species richness was shown to be greater in tethered vs untethered units. These results provide clear evidence that the movement of seagrass leaves has a profound effect on the accumulation of epiphytic algae. In addition, ordination revealed clear differences in epiphytic species composition and species richness between the tethered and untethered units, and over time. Algae of the genus Hypnea were characteristic of tethered leaves, while Griffithsia australe and Antithamnion spp. were characteristic of untethered units. Untethered leaves at sheltered and exposed sites showed no difference in epiphytic biomass. The results also showed that there was no difference in epiphytic biomass on the untethered leaves between deep and shallow sites but that tethered leaves had a significant higher biomass at shallow depths. The results of this study provide strong evidence that the movement of seagrass leaves strongly influences the biomass, species richness and species composition of epiphytic algae. However, it is not as clear whether the process of leaf movement is reducing the standing crop of epiphytes through abrasion, or if leaf movement is in fact inhibiting the settlement of propagules onto the seagrass leaves. If leaf movement results in an abrasional loss of epiphytic algae, previous studies may have underestimated epiphytic production in our seagrass meadows, thus the production of seagrass ecosystems and their inherent value. However, if leaf movement is instead limiting the settlement of propagules then the underestimation of epiphytic productivity is not as likely.

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