Measurement of chlorophyll fluorescence reveals mechanisms for habitat niche separation of the intertidal seagrasses Thalassia hemprichii and Halodule uninervis
In Taiwan, Thalassia hemprichii dominates the upper intertidal zone, whereas Haloduleuninervis occupies the lower intertidal zone. We tested the hypothesis that T. hemprichii is better adapted to high irradiance and more resistant to air exposure than H. uninervis. The photosynthetic efficiency, damage, and extent of recovery were determined by measuring chlorophyll fluorescence using pulse amplitude modulated fluorometry. Both species growing in tidal pools, in response to high irradiance alone, revealed a small depression in maximal quantum yield of photosystem II (Fv/Fm) at noon. The second experiment compared the effect of air exposure alone and the combined effect of air exposure with high irradiance by interposing a shading screen on both species, growing in the intertidal zone over a diurnal cycle. Values of Fv/Fm of both the shaded and irradiated T. hemprichii remained high at low tide. However, H. uninervis exhibited a marked depression following air exposure and a synergistic depression under the combined effect. The experimental manipulations of exposure time demonstrated that the tolerance of T. hemprichii to the combined effect was longer and the recovery from air exposure following re-submersion was better than those of H. uninervis. Both species were more susceptible to the combined effect in the dry season than in the wet season. Our results suggest that air exposure is more important than high irradiance in constraining the distribution of H. uninervis in the upper intertidal zone. This was confirmed by transplantation experiments in which a rapid decline of H. uninervis was observed after transplantation into the upper intertidal zone. In the lower intertidal zone, measurements of the response of the photosynthetic electron transport rate to irradiance demonstrated that the transplanted T. hemprichii exhibited a sun-type response and H. uninervis a shade-type response.
KeywordsChlorophyll Fluorescence Diurnal Cycle Intertidal Zone Photosynthetic Efficiency High Irradiance
This study was supported by the National Science Council under grant number NSC90-2621-B-005-008 and Kenting National Park under grant number 092-301020100G-004. This manuscript was improved by the comments of Prof. S. Beer and one anonymous reviewer.
- Buchanan JB, Kain JM (1971) Measurement of the physical and chemical environment. In: Holme NA, McIntyre AD (eds.), Methods for the Study of Marine Benthos. IBP Handbook no. 16. Blackwell Scientific Publications, Oxford, UK, pp 30–58Google Scholar
- Dawes CJ (1998) Marine Botany, 2nd edn. Wiley, New YorkGoogle Scholar
- Fourqurean JW, Zieman JC, Powell GVN (1992) Relationships between porewater nutrients and seagrasses in a subtropical carbonate environment. Mar Biol 114:57–65Google Scholar
- Lin HJ, Shao KT (1998) Temporal changes in the abundance and growth of intertidal Thalassia hemprichii seagrass beds in southern Taiwan. Bot Bull Acad Sin 39:191–198Google Scholar
- Mukai H (1993) Biogeography of the tropical seagrasses in the western Pacific. Aust J Mar Freshw Res 44:1–17Google Scholar
- Sokal RR, Rohlf FJ (1995) Biometry, 3rd edn. WH Freeman, New YorkGoogle Scholar
- Strickland JD, Parsons TR (1972) A practical handbook of seawater analysis, 2nd edn. Fisheries Research Board Canada, OttawaGoogle Scholar