ON MODELS OF GROWTH AND PHOTOSYNTHESIS IN PHYTOPLANKTON

A model of the growth of a marine diatom (Sakshaug et al., 1989, Limnology and Oceanography, 34, 198-205) is examined. One equation describes the relationship between chemical composition and growth rate as a function of irradiance and daylength. It is valid for both nutrient-limited and nutrient-saturated growth. The model equation is rearranged to describe photosynthesis normalized to chlorophyll. The new equation is essentially the same as several other models. Its mechanistic basis is the variation of quantum yield as a function of the number of excess photons absorbed by a photosynthetic unit during the time it takes to process one photon. The mechanistic interpretation of the model could be deceptive because the general equation describes a composite response and does not represent any one growth state. Nonetheless, the reformulated equation is important because it shows that at a given temperature, the adapted rate of photosynthesis normalized to chlorophyll is a function solely of growth irradiance. The equation can be used to describe primary production in the sea as a function of insolation and chlorophyll in the water column. For comparison, the model of Ryther and Yentsch (1957, Limnology and Oceanography, 2, 281-286) is modified and found to fit obsercations as well or better than other formulations. Some date sets are not at all consistent with general models, however. Discrepancies may be due to taxonomic differences, temperature and vertical structure of phytoplankton biomass. It is also possible that changes in the photoynthesis-irradiance relationship associated with unbalanced growth are extremely important in determining primary production in perturbed environments. © 1990.