The following are notes from the IOP subgroup of the SIMBIOS science team held at the August 1997 SIMBIOS Science Team meeting at Solomons Island, Maryland.

Contents:

• Filter Issues
• CDOM Issues
• Beta and bb
• AC-9 Issues
• San Diego Absorption Workshop

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Filter issues:

Questions raised concerning a possible species specify absorption. Others asserted it is rather a size distribution (synechococcus below 1 micron may less than the wavelength of light)

• On filter issue: Errors may be spectrally dependent -or- different cells may act differently on different filters -or- both may occur.
• GF/C filters may have a smaller porosity than GF/F.
• G. Mitchell: "Increased detritus may lead to increased absorption at 750 (Italians reported)
• Report on TR method: measure transmission and reflectance, solve for correction of absorption from filter scattering (may not be needed in case I waters)
• Report on FTF (freeze transfer technique) of Bricaud et al., appears to agree well w/ integrated sphere data, difficult procedure

CDOM issues:

The index of refraction is seen to exhibit a larger value between fresh and saltwater at larger wavelengths, this leads to need to 'zero' scan data by the null wavelength

• Group suggests 850 nm as the null wavelength for CDOM scans (and a 350-850 scanning range) but not all instruments may go to 850
• Group suggest reporting the raw optical density (OD) scans along with needed processing info
• Glenn Cota suggested the use of a power law curve to pre-smooth low absorbing CDOM (as) scans, re-attachment at a UV wavelength (325nm), then null wavelength correction
• Heidi Sosik has performed extensive tests to examine the effects of long term storage of samples. Report status unknown.

Beta and bb:

• Balch addressed de-bubbling techniques
• Pope & Fry data available at http://spg.ucsd.edu/~lab

AC-9 Issues:

• Protocols to follow at Wetlabs: www.wetlabs.com, under news or instruments
• Use Pope & Fry pure water std.
• To submit, first apply the temp and sal correction to both 'a' and 'c' and provide raw and fully processed data
• Submitters should document scattering correction method used. Three primary types:
  1. subtract null wavelength (at 715nm)
  2. assume Epsilon(Cb(lambda) - aTS(lambda)) <-- useful if not good temp & sal correction available
  3. Epsilon = a(715)/c(715)-a(715) must be: 0.08 < Epsilon < 0.5 should be: 0.1 < Epsilon < .3 - small errors in 'c' & 'a' strongly affect this value.
** If scan has 670 > 650, assume phytoplankton main component, if slowly decreasing w/ wavelength, assume sediment
• Definitions:ar - absorption raw cr - beam attenuation raw Submit: ar, cr, temp, sal
• Calibrate instrument on its frame due to torquing problem w/ AC-9 (new meters have better design and don't suffer from this problem)
• Use clean DI water under pressurized flow w/ a valve for back pressure when calibrating
• Filters on AC-9 for 'as' determination may need null wavelength correction applied
• Scott Pegau uses tygon tubing (saw no difference w/ teflon coated, except possibly in bluest waters)
• Outstanding IOP issues:
  1. aph & ad techniques vary by investigator
  2. length of storage of samples may or may not be a problem

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San Diego Absorption Workshop

Information on the absorption workshop held at Scripps during December 9-13, 1996

• All San Diego absorption workshop instruments compared well except the HP spectrophotometer.
• Workshop resulted in some convergence in Beta and null wavelength, more work needed
• Dave Karl suggested GF/F absorbs CDOM (per G. Mitchell)
• "For SIMBIOS purposes, people should specify their measurement methods" -G. Mitchell