Date of Award

2017

Document Type

Thesis

Publisher

Edith Cowan University

Degree Name

Doctor of Philosophy

School

School of Science

First Supervisor

Kathryn McMahon

Second Supervisor

Paul Lavery

Third Supervisor

Gary Kendrick

Fourth Supervisor

John Statton

Abstract

Seagrass meadows provide crucial ecosystem services to the coastal zone but globally are threatened. Seagrass loss to date has mainly been attributed to anthropogenic activities that reduce light quantity, such as dredging, declining water quality from urban and agricultural run-off and eutrophication. However, light quality (wavelengths of light) is also altered by these anthropogenic stressors as well as natural events. This study consisted of three main components: (1) characterising light quality to which seagrasses are exposed across a local natural estuarine-ocean gradient and with a human impact pressure; (2) the influence of monochromatic light quality (blue λ=451 nm; green λ=522 nm; yellow λ=596 nm and red λ=673 nm wavelengths and full-spectrum light λ=400 – 700 nm, at 200 μmol photons m-2 s-1) on Halophila ovalis and Posidonia australis at different life-history stages; and (3) the effects of light quality and quantity representative of a commercial dredging operation (15 mg L-1 TSS, 50 and 200 μmol photons m-2 s-1) on H. ovalis adult plants. The field work demonstrated that the quality of light to which seagrasses are exposed varies along a natural gradient but the nature of the shift is also dependent on time of year. Additionally, human impact such as dredging can expose seagrasses to spectra outside of the natural range detected in this study, and the magnitude of this shift is dependent on depth and TSS concentrations. Results from the monochromatic light quality experiments demonstrated, for the first time, the seagrass responses to light quality across several plant scales as well as different life-history stages. Halophila ovalis and P. australis showed different responses, likely due to their respective growth strategies. Adult H. ovalis (a colonising species) plants were negatively impacted by monochromatic blue, green and yellow light treatments, while seeds and seedlings performed better under red and full-spectrum light. Conversely, P. australis (a persistent species) adults showed no significant responses to any of the monochromatic light quality treatments, while seedlings demonstrated a physiological acclimation to blue light. The simulated dredging spectrum experiment demonstrated a significant impact of reduced light quantity on H. ovalis photo-physiology and growth, but the only significant effect of light quality was on the concentration of the pigment antheraxanthin. The lack of effect of light quality on growth indicates that: a) while seagrass are sensitive to changes in light quality, not all shifts induce negative biomass responses; b) the effects of altered light quality are less severe when a mixture of wavelengths are present; and c) in this species, reduced light quantity was more important than changes in light quality. Therefore, when considering the practical management of seagrasses in relation to short-term dredging activities, using light quantity thresholds is sufficient for management triggers. Overall, while there were some (positive and negative) responses to blue, green, yellow and red light, lethal effects were not detected for either the colonising or persistent species, suggesting that seagrasses have the VI capacity to acclimate to and/or tolerate extreme changes in light quality and maintain short-term growth at sufficiently high irradiances. However, sub-lethal responses were determined which may affect the ability of seagrasses to maintain resilience against other stressors. Therefore, environmental conditions that alter light quality have the potential to indirectly influence the overall resilience seagrasses.

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