Effects of Nutrient Enrichment and Water Motion on the Coral Pocillopora damicornis'

Exposure of the hermatypic coral Pocillopora damicornis (Linnaeus) to elevated levels of dissolved inorganic phosphorus did not affect the colony or the zooxanthellae. Exposure to elevated levels of dissolved inorganic nitrogen and inorganic nitrogen + phosphorus led to an increase in algal density, and as a result, to an increase in the chlorophyll concentration. These latter two experimental enrichments slowed skeletal growth rate of the corals, probably because of a decrease in the photosynthetic rate of the algae and perhaps a decrease in the translocation of photosynthetic products from the algae to the coral. The algae probably used the photosynthetic energy for their own increased growth. Experimental manipulation of water motion used in these experiments did not affect the coral or the symbiotic algae . damicorn is decreases the density of zooxanthellae (based on cellular C :N :P ratio), wherea s addition of nitrogen did not have an effect on algal standing stock (Snidvongs 1987). Water motion affects the rate of exchange of materials (including nutrients) across the coral-seawater interface and thus can influence respiration, photosynthesis, and calcification rates . In unstirred conditions net photosynthesi s, respir ation, and dark calcification of Acropora formosa were reduced compared to those of corals exposed to higher rates of water motion (Dennison and Barnes 1988). The growth, reproductive rate, and mortality of the hermatypic corals Pocillopora damicornis and Montipora verrucosa are also influenced by water motion (Jokiel 1978). Dennison and Barnes (1988) hypothesized that water motion affects coral metabolism by altering the thickness of the boundary layer adjacent to the animal tissue, thus altering diffusion rate of dissolved substances, particularly CO2 and 02. Diffusion rates may limit the rates of photosynthesis and calcification. Evidence of competition among algae for CO2 was suggested by high algal densities in nutrientenriched colonies of Stylophora pistillata. Photosynthetic rates on a per-algal-cell basis were inversely correlated with algal density, which increased in response to nutrient enrichment (Dubinsky et al. 1990).