Disparities between Phaeocystis in situ and optically-derived carbon biomass and growth rates: potential effect on remote-sensing primary production estimates

The oceans play a pivotal role in the global carbon cycle. Unfortunately, the daily production of organic carbon, the product of phytoplankton standing stock and growth rate cannot be measured globally by discrete oceanographic methods. Instead, optical proxies from Earth-orbiting satellites must be used. To test the accuracy of optically-derived proxies of phytoplankton physiology and growth rate, standard ex situ data from the wax and wane of a Phaeocystis bloom in laboratory mesocosms were compared with hyperspectral reflectance data. Chlorophyll biomass could be estimated accurately from reflectance using specific chlorophyll absorption algorithms. However, the conversion of chlorophyll (Chl) to carbon (C) was obscured by the observed increase in C : Chl under nutrient-limited growth. C : Chl was inversely correlated (r2 = 0.88) with Photosystem II quantum efficiency (Fv/Fm), the in situ fluorometric oceanographic proxy for growth rate. In addition, the optical proxy for growth rate, the quantum efficiency of fluorescence ϕ was linearly correlated to Fv/Fm (r2 = 0.84), but not – as by definition – by using total phytoplankton absorption, because during nutrient-limited growth the concentrations of non-fluorescent light-absorbing pigments increased. As a consequence, none of the three proxies (C : Chl, Fv/Fm, φ) was correlated to carbon or cellular phytoplankton growth rates. Therefore, it is concluded that although satellite derived estimates of chlorophyll biomass may be accurate, physiologically-induced non-linear shifts in growth rate proxies may obscure accurate phytoplankton growth rates and hence global carbon production estimates.