Light absorption by phytoplankton: development of a matching parameter for algal photosynthesis under different spectral regimes

A spectral matching parameter (absorption efficiency, A(e)) was developed to quantify the relationship between the light absorption spectra of phytoplankton communities and the spectral irradiance of their ambient light field. A(e) was defined as the ratio between the amount of radiation absorbed by the phytoplankton in situ and the amount absorbed in a spectrally flat light regime. This approach was applied to our measurements of spectral absorption for the phytoplankton communities in six lakes in High Arctic Canada that spanned a range of bio-optical conditions. A(e) values were calculated for the light spectrum down through the water column and for 11 types of artificial light source. Spectral matching varied among lakes and with depth. There was a significant linear relationship between the relative change in A(e) with depth and the diffuse attenuation coefficient K-d (r(2) = 0.52, P = 0.012 for K-d for the 400-700 nm waveband; r(2) = 0.78, P = 0.0003 for K-d at 440 nm). The tabulated values for the matching parameter A(e) allow the comparison of photosynthesis versus irradiance (P versus E) curves among studies using different light sources. A(e) estimates also facilitate the evaluation of chromatic adaptation in natural waters, and the calculation of spectrally adjusted, in situ primary production down through a water column from P versus E relationships under a single spectral regime.