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The photosynthetic light response of Halophila stipulacea growing along a depth gradient in the Gulf of Aqaba, the Red Sea
Schwarz, A.-M.; Hellblom, F. (2002). The photosynthetic light response of Halophila stipulacea growing along a depth gradient in the Gulf of Aqaba, the Red Sea. Aquat. Bot. 74(3): 263-272. dx.doi.org/10.1016/s0304-3770(02)00080-3
In: Aquatic Botany. Elsevier Science: Tokyo; Oxford; New York; London; Amsterdam. ISSN 0304-3770, more
Peer reviewed article  

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Keywords
    Chloroplasts; Growth; Irradiance; Light absorption; Limiting factors; Nutrient deficiency; Photosynthesis; Sea grass; Shallow water; Halophila stipulacea (Forsskål) Ascherson, 1867 [WoRMS]; ISW, Aqaba Gulf [Marine Regions]; Marine

Authors  Top 
  • Schwarz, A.-M.
  • Hellblom, F.

Abstract
    Photosynthetic responses to irradiance were measured on the seagrass Halophila stipulacea growing along an extensive depth gradient (7-30m) 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 7m. At this depth, electron transport rates were highest in summer (35 µmol electrons m-2s-1) compared to spring and winter (20µmol electrons m-2s-1). Relative rates of electron transport at 30m were 60% lower than those at 7m 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 30m. Despite adequate light for photosynthesis, there was evidence for lower biomass at sampling depths of 7 and 17m compared to 24 and 30m. The potential for shallow plants to photosynthesis 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.

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