Some observations on the use of the in vivo fluorescence technique to determine chlorophyll‐a in natural populations and cultures of freshwater phytoplankton

SUMMARY. The variation in the ratio (R) between cellular fluorescence of chlorophyll‐a and unit extractable chlorophyll‐a was examined with natural populations of phytoplankton and unialgal cultures. Photo‐inhibition of in vivo fluorescence was observed al high values of irradiance with natural populations and in algal cultures, except with blue‐green algae. If samples were dark‐adapted (< 6 W m−2) before measurements were made, changes of fluorescence due to photo‐inhibition could be minimized. Measurements of in vivo fluorescence with dark‐adapted samples of phytoplankton, dominated by species showing large values of R, gave significant correlations (P< 0.001) with determinations of chlorophyll‐a concentration over periods of several weeks. The values for R for blue‐green algae were appreciably lower than for other algal groups. This, and their lack of photo‐inhibition effects at high irradiance values, are attributed to the association of most of chlorophyll‐a with photosystem I which fluoresces weakly compared to chlorophyll‐a in photosystem II. The significance of these findings for ecological studies is discussed.