Integrating Cavity Absorption Meter measurements of dissolved substances and suspended particles in ocean water

We have developed a new device to measure the separate contributions to the spectral absorption coefficient due to a pure liquid, due to the particles suspended in it, and due to the substances dissolved in it. This device, t he Integrating Cavity Absorption Meter (ICAM), is essentially independent o f scattering effects in the sample. In April 1993, a prototype of the ICAM was field tested on board the research vessel USNS Bartlett. A major part o f the cruise track included criss-crossing the area where the Mississippi f lows into the Gulf of Mexico at various ranges from the mouth of the river; thus samples were collected from areas of blue, green, and brown/black wat er. We evaluated 35 seawater samples collected with 5-1 Niskin bottles from 22 locations to determine absorption spectra (380-700 nm) of suspended par ticles and dissolved substances (gelbstoff). Results validate the ICAM as a viable tool for marine optical absorption research. Gelbstoff absorption a t 432.5 nm ranged from 0.024 to 0.603 m(-1). Over the spectral region 380 - -> 560 nm, gelbstoff absorption by each of the samples could be accurately fit to a decaying exponential. The particle absorption spectra are generall y characteristic of those of phytoplankton and exhibit a local maximum at 4 30-440 nm. Absorption values at 432.5 nm ranged from similar to zero to sim ilar to 1.0 m(-1). Some samples with moderate particulate absorption, howev er, did not show the characteristic local maximum of phytoplankton in the b lue and instead resembled the characteristic decaying exponential of detrit us with a shape similar to that observed in the gelbstoff. The ratio of gel bstoff to particulate absorption at 432.5 nm ranged from 0.46 to 152. (C) 2 000 Elsevier Science B.V. All rights reserved.