Effects of temperature and location on the photosynthesis-irradiance relationship of the seagrass Halophila ovalis

Date of Award


Degree Type


Degree Name

Master of Science (Biological Sciences)


School of Science

First Advisor

Professor Paul Lavery

Second Advisor

Doctor Kathryn McMahon

Field of Research Code

0 6 0 7 0 1, 0 5 0 2 1 1


Seagrasses are among the most threatened ecosystems globally, despite their ecological importance. The global area is declining at a median rate of 5-7% y-1 which can be attributed to two major anthropogenic causes: direct impacts from coastal development and dredging, often associated with ports; and indirect impacts from reduced water quality (Waycott et al., 2009). Both pose a threat due to reduced light quantity. Knowledge of seagrass photosynthetic characteristics can assist in predicting and managing impacts of light reducing activities, such as dredging. However, information on photo-physiology is quite limited. Most studies on seagrass photosynthetic characteristics generally focus only on one location or time of sampling. Yet, most seagrass species have broad distributions and grow over a range of temperatures. For effective management it is important to know if these characteristics vary in space and time and under different temperature conditions, so that the appropriate information can be applied.

The aim of this study was to assess whether the photosynthesisirradiance (P-I) relationship of seagrass varied in different locations and under different temperatures. The model species used was Halophila ovalis, an ecologically important seagrass, which is globally distributed, but little is known about the variation in its P-I relationship. Four locations were selected across a latitudinal gradient along the Western Australian coast (3 temperate & 1 tropical) and H. ovalis was collected from two sites within each location. Plants were acclimated to three different temperatures, 17°C, 23°C and 28°C, which relatively covered each locations average minimum and maximum temperature. P-I relationships of whole plants were determined in sealed chambers, with oxygen concentrations measured in the dark (respiration), and light (30-300 μmol photons m-2 s-1). At all locations, temperature affected all photosynthetic parameters, but there were significant Temp x Location/Site interactions (p

This study showed that there was a significance difference on the P-I relationship when using different plant segments, with a vast difference in using leaves alone compered to whole or reconstructed plants. In conclusion the P-I response of H. ovalis is affected by temperature, but the effect is spatially and temporally variable, with the methodology employed showing an effect on the P-I relationship. Therefore the management of H. ovalis, and likely other seagrasses, based on their photosynthetic light requirements will require location-specific knowledge and, in most cases, site-specific knowledge of P-I relationships.

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