Characterising responses of the seagrass Posidonia Sinuosa to changes in light availability

Date of Award


Degree Type


Degree Name

Doctor of Philosophy


Faculty of Computing, Health and Science

First Advisor

Paul Lavery

Second Advisor

Ray Masini

Third Advisor

Peter Ralph


The unrelenting threat of seagrass loss resulting from reduced light availability has motivated this characterisation of responses of Posidonia sinuosa Cambridge et Kuo to light availability and their application as monitoring tools. The study comprised of three major components: an assessment of P. sinuosa characteristics across a depthrelated gradient in long-term light availability; an in situ experiment to test for responses to short-term light reduction; and experimental investigations into the role of translocation to cope with reduced light availability. Across depth-related gradients of light, in two locations, near Perth in Western Australia, the minimum light requirement (MLR) of P. sinuosa at the depth limit was 8.5% of sub-surface irradiance. Shoot density and biomass consistently differed across the gradient while other morphological and growth differences between depths were inconsistent between location and season or did not follow the gradient of light reduction. Physiological characteristics including RLC-derived characteristics, light harvesting and photoprotective pigments and nutrient and carbohydrate concentration demonstrated few depth-related differences but showed limited adjustment between seasons. The light dependency of these observations was validated experimentally by in situ shading at a shallow (3- 4 m) and deep (7- 8 m) site with light (LS; 87% of ambient light), moderate (MS; 27%) and heavy shading (HS; 9% at shallow only). After 106 d, significant shoot loss had occurred but complete loss did not occur after 206 d. Shoot loss substantially reduced the light attenuation coefficient of the canopy (and self-shading) from 2.8 (control) to 0.5 m-1 (HS). Carbohydrate reduction occurred in most shade treatments probably supporting respiration and growth (which was not affected by shading except in MS at the deep site). Few other responses were observed in treatments near or above MLR, but in those below MLR (HS at the shallow and MS at the deep site) photosynthetic and morphological responses were detected including photosynthetic characteristics, length/weight ratio of leaf growth and δ13Cvalues. Following removal of shading, recovery of shoot density was slow, remaining significantly lower than the control after 384 d, and depended on shoot density; rate of shoot production was faster when higher shoot densities remained following shading. Nutrient translocation of 15N and 13C within and between shoots was investigated in the early phase of imposed shading. Following incubation of a mature leaf (ML), the 13C and 15N accumulated in the young growing leaf(YL) and rhizome, with up 32% and 44% of the 15N and 13C, respectively, appearing in the YL after 29 d. Resorption of structural nitrogen (N) from the ML may also contribute to YL N requirements but not for carbon (C). For 13C and 15N exported from the shoot, there was a trend for greater 13C accumulation away from, rather than towards, the rhizome apex but not for 15N. Most of the C and N was recovered in the rhizome, and not shoots, after 8 and 15 d. There was no evidence that the translocation of C or N within or between shoots is altered as an early response to shading. For P. sinuosa, shoot density reductions dominate the response to reduced light availability at light intensities above MLR and can be considered a meadow-scale response with benefits for reduced self-shading. Growth rate of shoots is usually unaffected by depth (during summer) or shading. Below MLR, other morphological and physiological responses can also occur. Clonal integration via the rhizome is important for carbohydrate storage and remobilisation during periods of low light, but changes to translocation between or within shoots are not important shade responses. These findings should also be relevant to other meadow-forming and persistent seagrass species. Shoot density was recommended as an appropriate monitoring indicator because it is consistently responsive and crucial in P. sinuosa's response mechanisms, while other physiological responses including carbohydrates in the rhizome have potential as monitoring tools.

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