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

The maximal change in the quantum yield of fluorescence (A&,,) is a quantitative measure of photosynthetic conversion efhcicncy of phytoplankton. Using a pump-and-probe fluorometer, we measured A&, along the 100-m isobath in the Gulf of Maine in June 1987. The hydrographic regime was charactcrizcd by a nutrient-rich, vertically mixed region in the northcast and a nutrient- depleted, stratified region to the southwest. The results reveal that A&,, 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.