DETERMINATION OF NITRATE, PHOSPHATE AND ORGANICALLY BOUND PHOSPHORUS IN CORAL SKELETONS BY ION CHROMATOGRAPHY

Nitrate, phosphate and sulphate incorporated in the aragonitic skeletons of coral (Porites) were analyzed by ion chromatography (IC). All anion analyses were performed using a Dionex 4500i IC system with Dionex columns, conductivity detector and computer interface, The anions were separated on an AS4A separator column behind an AG4 guard column, and the background conductivity of the eluent was suppressed using an AMMSII membrane suppressor. The suppressor was continually regenerated using an anion membrane suppressor regenerant cartridge and H2SO4. The eluent used was 1.8 mM Na2CO3-1.7 mM NaHCO3. Bleached, oven-dried coral powder (200 mg) was dissolved in 0.5 ml of 30% HCl (Merck Suprapur) and diluted to 100 ml. Standards were also prepared in HCl, and the linearity was excellent for each anion (r(2) > 0.999) over the range 20-100 ng/g (ppb) for nitrate, 100-500 ng/g for phosphate and 0.5-12.0 mu g/g (ppm) for sulphate. All samples and standards were passed through a chloride-removal cartridge (Dionex OnGuard AG) prior to injection. This cartridge consisted of silver and its sole purpose was to remove Cl- which might otherwise overwhelm the separator column. Using a 250-mu l injection loop, the detection limits were approximately 5 ng/g for nitrate and 10 ng/g for phosphate. The analytical procedure variability was 9% for nitrate, 2% for phosphate and 0.5% for sulphate. The concentrations of these species in the corals varied from 0.2 to 18 ppm, 8 to 118 ppm and 560 to 590 ppm, respectively. Coral samples taken from three coral heads at the Grande Riviere Noire Bay (slabs 1Ks, 5Ks, 6Ks) averaged 4.5 +/- 0.5 ppm nitrate and 55 +/- 1 ppm phosphate, compared with 2.2 +/- 0.5 ppm nitrate and 16 +/- 1 ppm phosphate for the samples taken further out in the lagoon (slabs 7Ks and 11Ks). Thus, the nitrate and phosphate concentrations clearly distinguish between corals taken from the bay, which are closer to anthropogenic N and P inputs, from samples collected from the lagoon, which are further from such N and P sources. To estimate the possible importance of organically bound phosphorus, samples were also treated with H2O2. Following dissolution in HCl, 1 ml of 30% H2O2 (Merck Suprapur) was added, and the samples were heated at 50 degrees C for 3 h. The instrument was calibrated using standards prepared in HCl-H2O2. The phosphate concentrations measured in the samples dissolved in HCl-H2O2 were up to 100% higher than those dissolved in HCl alone, suggesting that organically bound phosphorus contributes significantly to the total phosphorus concentrations of the corals.