Elemental composition of the coral Pocillopora damicornis exposed to elevated seawater ammonium

The elemental composition (C, N, and P) of zooxanthellae and host tissue from the coral Pocillopora damicornis (Linnaeus) was determined after maintenance in flowing seawater with 20-uM and 50-M ammonium enrichments for periods of 2 to 8 weeks. Compared with ambient seawater controls, total zooxanthellar nitrogen (ug N cm-2 colony surface) increased four-fold during exposure to 20-uM ammonium. This resulted from increases in N content of zooxanthellae and in zooxanthellae population densities. C: N ratios of zooxanthellae decreased from 19.7 (±4.0) to 10.3 (±3.0), and N: P ratios increased from 21.4 (± 3.1) to 30.4 (± 2.2) after 8 weeks in 20 uM ammonium. Zooxanthellae from the 8-week 50-uM ammonium corals had values of 8.9 (±0.6) for C: Nand 40.4 (±2.3) for N: P. Coral animal C, N, and P content were not affected by ammonium-enriched seawater. The C :N ratio of coral animal tissue was 5.2 (±O.O), and the N: P ratio was 20.1 (±0.2) after 8 weeks in 20-uM ammonium seawater. There were no changes in host C: N, N: P, or C: P with ammonium enrichment. Thus, most of the N from the elevated seawater ammonium is retained by the zooxanthellae of P. damicornis, rather than by the animal tissue. Accordingly, sustained high concentrations of ammonium are likely to result in increased N storage by zooxanthellae and to affect the relative size of zooxanthellar to animal N pools.