Photosynthetic capacity in coral reef systems: Investigations into ecological applications for the underwater PAM fluorometer

A submersible pulse amplitude modulated (PAM) fluorometer was used to determine the effects of desiccation, ultraviolet radiation, changes in solar radiation and nutrient availability on the photosynthetic apparatus of a variety of marine plants (zooxanthellae, benthic microalgae and macroalgae) at Heron Island, Great Barrier Reef, Australia. The PAM measures photosynthesis as irradiance-dependent photosystem II electron transport. There were a number of interspecific and intraspecific variations in electron transport rate (ETR) based on physiological and morphological differences, and the plant's response to changes in environmental conditions. The highest ETR was found in the zooxanthellae of the clam Tridacna maxima, and the lowest in the calcified green macroalga Halimeda opuntia. Factors such as water velocity, ultraviolet radiation, solar radiation (total irradiance and spectral changes), desiccation, nutrient availability and algal pigment content were hypothesised as influencing intraspecific changes in ETR. A series of experimental manipulations were conducted to test these hypotheses. Reef flat algae was shaded to 50% of incident solar radiation and 0% of ultraviolet radiation. Samples of macroalgae were collected from the reef flat and 15 m depth and allowed to desiccate to determine if different populations of the same species could adapt physiologically to different environmental conditions. Reef flat samples were collected and incubated in seawater enriched in nitrogen and phosphorus to test for nutrient limitation. Significant differences in the ETR of the plants tested highlighted the impacts of various environmental parameters on photosynthetic capacity. Samples from regions with higher water velocities on the reef flat had significantly higher ETRs. Screening of ultraviolet radiation increased the maximum ETR of certain species, while prolonged periods of shading reduced the maximum ETR of some species more quickly than others. Desiccation responses were the same between deep collected and reef flat populations, although increased light and temperature did reduce the maximum ETR of the deep collected samples. Fertilisation responses varied between species. The results indicate that PAM fluorometry can be used as a tool for in situ non destructive assessment of the effects of various ecological parameters on photosynthetic activity in marine plants.

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