Field and laboratory studies on the effect of particle size and composition on optical backscattering measurements in hydrothermal plumes

We describe the performance of an inexpensive but highly sensitive light ba ckscattering sensor (LBSS) suitable for use in deep-sea waters, where parti cle concentrations are typically < <similar to> 0.1 mg/l. Laboratory calibr ations using aluminosilicate particles and latex spheres show that the conc entration-normalized backscattering of the LESS, Kbs, is greatest for parti cles with a diameter close to the wavelength of light emitted by the LESS ( 0.88 mum), declining by more than a factor of five for particles < 0.1 <mu> m or > 10 mum. Field studies indicate that in hydrothermal plumes dominated by fine-grained metal precipitates the 95% confidence interval for predict ing mass concentration with an LESS ranges from +/- 0.004 mg/l for 0.03 mg/ l suspensions to +/- 0.008 mg/l at 0.12 mg/l. Comparisons among a group of 19 LBSSs found between-instrument variability to reach 70%, but normalizing the raw data using a laboratory calibration procedure reduced the maximum variability to similar to 8%, Published by Elsevier Science Ltd.