MEPS 394:21-33 (2009)  -  DOI: https://doi.org/10.3354/meps08242

ABSTRACT: The responses of the seagrass Amphibolis griffithii to different experimental shading conditions were examined by characterising biomass, morphological and physiological features. In an in situ experiment, the intensity (ambient, moderate shading [13 to 19% of ambient] and high shading [5 to 11% of ambient]), duration (3, 6, 9 mo) and timing (post-summer, post-winter) of light reductions were manipulated. We observed interactive effects of all 3 factors, the most notable being with timing. When moderate shading was imposed at the end of summer there was a 57% loss of leaf biomass and 67% loss of rhizome carbohydrates within 3 mo. The same shading imposed at the end of winter caused no loss of leaf biomass and only a 25% decline in rhizome carbohydrates. This contrasting effect of time reflects the plant’s photo-physiological characteristics under the water temperature and light conditions. More prolonged or higher intensity shading produced more consistent responses at both times of year: moderate shading resulted in more than 93% loss of leaf biomass after 9 mo and high intensity shading resulted in more than 99% loss after 9 mo. The results highlight the importance of time of year when attempting to predict seagrass responses to shading. The study identified 14 potential early indicators of light reduction; these included leaf δ¹⁵N, which may reflect changes in the allocation of nitrogen in the photosynthetic apparatus. There is no evidence that A. griffithii is more susceptible to shading than larger seagrasses such as Posidonia spp.

KEY WORDS: Light reduction · Dredging · Seagrass disturbance · Australia