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.


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

AC-9 Issues:

  • Protocols to follow at Wetlabs:, 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

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