Marine Biology, vol: 129,4 (1997)
Effect of nutrient enrichment in the field on the biomass, growth and calcification of the giant clam Tridacna maxima
Ambariyanto, Hoegh-Guldberg O.
Abstract
Nutrients were added separately and combined to an initial concentration of 10 ?M (ammonium) and/or 2 ?M (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 remained 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 first and second phase of the experiment, respectively. The nutrients did not change the wet tissue weight of the dams, host C:N ratio, protein content of the mantle, calcification rates or growth rates. However, ammonium (N) enrichment alone significantly increased the total population density of the algal symbiont (Symbiodinium sp.: C = 3.6 ·108 cell clam-1 N=6.6 · 108 cell clam-1, P=5.7 · 108 cell clam-1, N + P = 5.7 · 108 cell clam-1; and C = 4.1 · 108 cell clam-1, N = 5.1 · 108 cell clam-1, P = 4.7 · 108 cell clam-1. N + P = 4.5 · 108 cell clam-1, at the end of the first and second phases of the experiment, respectively), although no differences 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.l 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 first and second phases of the experiment, respectively). The response of clams to nutrient enrichment was quantitatively small, but indicated that small changes in inorganic nutrient levels affect the clam-zooxanthellae association.
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