Evaluation of Si-32 as a tracer for measuring silica production rates in marine waters

The radioisotope Si-32 was evaluated as a tracer for measuring silica production rates in marine waters. Si-32 is a weak beta-emitter that decays to P-32 allowing the isotope to be used to determine simultaneously the production rate of biogenic silica and organic phosphorus. High specific activity Si-32, 42,000-52,000 Bq (mu g Si)(-1), was produced by the proton spallation of KCl. Reasonable tracer additions result in only nanomolar increases in the ambient silicic acid and orthophosphate concentrations during production rate experiments. Protocols for quantifying the amount of Si-32 and P-32 in samples by liquid scintillation counting (LSC) and gas-flow proportional counting (GFPC) are presented. Both protocols eliminate the 4-month wait for secular equilibrium between Si-32 and P-32 required by earlier methods. GFPC is especially useful as the instrumentation is lightweight and portable allowing real-time analysis of samples at sea. Real-time analysis by GFPC at sea and postcruise analysis by LSC gave results that agreed to within 17 and 7% for Si-32 and P-32, respectively. Subsequent intercalibrations of the LSC and GFPC methods indicated that agreement to within 0.1-4% for both isotopes can be achieved. Parallel incubations of samples from Monterey Bay, California, using the radioisotope Si-32 and the stable isotope Si-30 gave silica production rates that generally agreed to within 30%. No significant bias was observed between rates obtained with the two tracers. The greater sensitivity and ease of analysis of the radioisotope makes Si-32 the tracer of choice for future studies of silica production in the ocean.