The influence of dissolved and particulate materials on underwater light fields in shelf seas: implications for primary production modelling

The effects that particulate and coloured dissolved materials had on the underwater light field were examined using a combination of in situ observations and radiance transfer modelling. Hydrolight (a commercially available radiance transfer software program) was used to validate in situ measurements of inherent optical properties (IOPs). A general set of specific optical cross sections were derived which allowed the nature of the underwater light field to be determined from the concentrations of chlorophyll (Chl), coloured dissolved organic matter (CDOM) and suspended minerals (MSS) The attenuation coefficient of irradiance at 490 nm (K_d490) was linearly correlated with the depth at which the surface irradiance reached 1% of its original value. A study was carried out to determine whether there were any advantages in taking hyperspectral measurements of the underwater light field as opposed to seven waveband multi-spectral measurements. It was found that there was no significant difference between hyperspectral and 7 waveband measurements at the surface, but there was a 20% difference at a depth equivalent to 5 optical depths. Finally, the light harvesting strategies of two different phytoplankton functional groups (diatoms and chlorophytes) were examined in waters with high concentrations of dissolved and particulate materials to assess whether the accessory light harvesting pigments in diatoms gave that group an advantage.