The photosynthetic light response of Halophila stipulacea growing along a depth gradient in the Gulf of Aqaba, the Red Sea

Abstract

Photosynthetic responses to irradiance were measured on the seagrass Halophila stipulacea growing along an extensive depth gradient (7–30 m) in the Gulf of Aqaba on three occasions between January and August 2000. Plant samples were collected for morphological and anatomical (chloroplast clumping) characterizations of the leaves and analysis of carbon and nitrogen content. The highest electron transport rates, calculated firstly from the quantum yield and irradiance and then after accounting for the proportion of incident irradiance absorbed, were found in plants at the upper depth limit of 7 m. At this depth, electron transport rates were highest in summer (35 μmol electrons m −2 s −1) compared to spring and winter (20 μmol electrons m −2 s −1). Relative rates of electron transport at 30 m were 60% lower than those at 7 m in winter and spring and 80% lower in summer. The irradiance at the onset of light saturation was also highest in the shallow growing plants, indicative of successful adaptations to high irradiance. Chloroplast clumping in leaves in situ in shallow water reduced the amount of light absorbed to 55% compared to 85% at 30 m. Despite adequate light for photosynthesis, there was evidence for lower biomass at sampling depths of 7 and 17 m compared to 24 and 30 m. The potential for shallow plants to photosynthesise at high irradiances correlated with their ability to clump chloroplasts, suggesting that factors other than high irradiances are likely to limit growth at shallow depths. Low tissue nitrogen content (<1.8%) indicates that nutrient availability also needs to be considered when determining constraints on growth of H. stipulacea in shallow water in the study region.