Effect of nutrient enrichment in the field on the biomass, growth and calcification of the giant clam Tridacna maxima

Abstract

Nutrients were added separately and combined to an initial concentration of 10 lM(ammonium) and/or 2 lM (phosphate) in a series of experiments carried out with the giant clam Tridacna maxima at 12 microatolls in One Tree Island lagoon, Great Barrier Reef, Australia (ENCORE Project). These nutrient concentrations re- mained for 2 to 3 h before returning to natural levels. The additions were made every low tide (twice per day) over 13 and 12 mo periods for the ®rst and second phase of the experiment, respectively. The nutrients did not change the wet tissue weight of the clams, host C:N ratio, protein content of the mantle, calci®cation rates or growth rates. However, ammonium (N) enrichment alone signi®cantly increased the total population density of the algal sym- biont (Symbiodinium sp.: C . 3.6 á 108 cell clam)1, N . 6.6 á 108 cell clam)1, P.5.7 á 108 cell clam)1, N + P . 5.7 á 108 cell clam)1; and C.4.1 á 108 cell clam)1,N . 5.1 á 108 cell clam)1, P . 4.7 á 108 cell clam)1, N + P . 4.5 á 108 cell clam)1, at the end of the ®rst and second phases of the experiment, respectively), although no di€erences in the mitotic index of these populations were detected. The total chlorophyll a (chl a) content per clam but not chlorophyll a per cell also increased with ammonium addition (C . 7.0 mg chl a clam)1, N . 13.1 mg chl a clam)1, P . 12.9 mg chl a clam)1, N + P . 11.8 mg chl a clam)1; and C . 8.8 mg chl a clam)1, N . 12.8 mg chl a clam)1; P . 11.2 mg chl a clam)1, N+P . 11.3 mg chl a clam)1, at the end of the ®rst and second phases of the experiment, respec- tively). The response of clams to nutrient enrichment was quantitatively small, but indicated that small changes in inorganic nutrient levels a€ect the clam±zooxanthellae association.