Natural variability in photosynthetic energy conversion efficiency: A field study in the Gulf of Maine

The maximal change in the quantum yield of fluorescence (Δϕsat) is a quantitative measure of photosynthetic conversion efficiency of phytoplankton. Using a pump-and-probe fluorometer, we measured Δϕsat 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 Δϕsat is strongly related to the rate of supply of dissolved inorganic nitrogen and provide strong evidence that photosynthetic energy conversion can be nutrient limited in natural phytoplankton communities.