Influence of the population density of zooxanthellae and supply of ammonium on the biomass and metabolic characteristics of the reef corals Seriatopora hystrix and Stylophora pistillata

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Abstract
In May 1987, the populations density of zooxanthellae in the reef coral Seriatopora hystrix around Lizard Island (Great Barrier Reef) [Australia] varied within and between colonies. This naturally occurring variability made it possible to examine the effect of the population density of zooxanthellae on the physiological characteristics of S. hystrix its zooxanthellae. As the population density of zooxanthellae increased, the chlorophyll a content and maximum rate of photosynthesis of the zooxanthellae decreased. Phytoplankton studies suggest that cellular chlorophyll content will increase if cells are self-shaded or will decrease if cells are nitrogen-limited. To test the hypothesis that zooxanthellae in S. hystrix and Stylophora pistillata are nitrogen-limited at their highest population densities, colonies of S. hystrix and S. pistillata with high densities of zooxanthellae were incubated in aquaria to which ammonium (ca 10 to 40 .mu.M) was added at regular intervals. After 3 wk, the population density, chlorophyll a content and maximum rate of photosynthesis of the zooxanthellae had significantly increased, indicating that the biomass of zooxanthellae in reef corals can be limited by the availability of inorganic nitrogen to the association.