WHY WE WILL CONTINUE TO MEASURE PHYTOPLANKTON PHOTOSYNTHESIS DURING THE NEXT 50 YEARS

Banse’s letter in the ASLO Bulletin is correct as far as it goes. However, it does not include the value of C measurements for measuring P, the ‘assimilation number’ or ‘index.’ This is a measure of the rate of photosynthesis, not the amount. The units for this value are derived from dividing the C measurement by the standing stock of chlorophyll and Pmax values in units of mg C (mg Chl a hr) generally range from < 1.0 to ca. 10 (Parsons et al. 1990). Providing the chlorophyll a is measured on the same sample as the C uptake, the value is independent of grazing. This is a valid measure of the physiological state of the phytoplankton and will reflect light and nutrient conditions. For modeling purposes, it can be used in conjunction with assumed values of chlorophyll a to create scenarios of geographical differences in primary production.