Abstract
Biogenically produced calcite represents a significant source of light scatter in the ocean, poorly defined in space and time. We describe an automated photometer for measuring 90 degree(s) light scatter due to calcite on time scales as short as a minute. Raw seawater continuously flows through a 1 mL glass cuvette illuminated by a helium-neon laser. A photodiode is used to measure the 90 degree(s) light scatter of particles in the incident beam. During each sampling interval, a calibrated addition of weak acid lowers the seawater pH to dissolve the calcite and another 90 degree(s) light scatter measurement is made. Standard curves are prepared using calcium carbonate coccoliths from the coccolithophore Emiliania huxleyi. The relationships between acid-liable light scatter and coccolith abundance given an r
of 0.96 - 0.99. The light scatter photometer is coupled to a fluorometer, temperature and salinity probe to relate the suspended calcite concentration to algal distributions and hydrography. We show some examples of its performance at sea.
