Phytoplankton light absorption of cultures and natural samples: comparisons using two spectrophotometers.

Here we present laboratory measurements of phytoplankton absorption for cultures and natural water samples using two different spectrophotometers, an Ultrapath system and a double beam spectrophotometer equipped with an integrating sphere (Lambda 850). The Ultrapath system provides simplified optics with high throughput efficiency, portability, and is relatively less expensive in comparison to conventional spectrophotometers. A more robust algorithm for correction of pathlength amplification (β) for particles retained on filter paper was determined for Lambda 850 in comparison to the Ultrapath. The Lambda 850 β algorithm (ODs(λ) = 0.405 [ODf(λ)] + 0.475 [ODf(λ)]2; r2 = 0.973; n = 7395) showed species and size dependence as indicated by the LISST 100X and HPLC chlorophyll-a concentration data. A better agreement was observed between the two spectrophotometers for filter paper measurements (r2 = 0.991; slope = 0.958; n = 130 for cultures and r2 = 0.978; slope = 0.957; n = 349 for natural samples), than for suspensions (r2 = 0.960; slope = 0.915; n = 92 for cultures and r2 = 0.960; slope = 0.921; n = 27 for natural samples). The differences in measurement of suspensions between the spectrophotometers could be attributed to volume scattering function and acceptance angle of the waveguide detector.