Natural Variability in Photosynthetic Energy Conversion Efficiency: A Field Study in the Gulf of Maine

The maximal change in the quantum yield of fluorescence $(\triangle \emptyset_sat)$ is a quantitative measure of photosynthetic conversion efficiency of phytoplankton. Using a pump-and-probe fluorometer, we measured $\triangle \emptyset_sat$ along the 100-m isobath in the Gulf of Maine in June 1987. The hydrographic regime was characterized by a nutrient-rich, vertically mixed region in the northeast and a nutrient-depleted, stratified region to the southwest. The results reveal that $\triangle \emptyset_sat$ is strongly related to the rate of supply of dissovled inorganic nitrogen and provide strong evidence that photosynthetic energy conversion can be nutrient limited in natural phytoplankton communities.