Fluorescence assessment of the maximum quantum efficiency of photosynthesis in the western North Atlantic

Abstract

The maximum quantum efficiency (ø II) of phytoplankton photosystem II photochemistry was assessed using a pump and probe fluorometer on an offshore-onshore transect from the oligotrophic blue waters of the western Sargasso Sea to the eutrophic waters of lower Delaware Bay. Low values of the ratio of variable to maximum fluorescence ( F v/ F m) of dark adapted samples provided evidence for reduced quantum efficiences surface waters ( z = 10 m) of the Sargasso Sea. In contrast, higher values of F v/ F m in estuarine samples indicated near maximal ø II in lower Delaware Bay, with intermediate values of F v/ F m observed over the continental shelf. Variations of F v/ F m in surface samples (5 or 10 m depth)_were associated with changes in hydrographic features that were consistent with the hypothesis that reduced values of ø II in Sargasso Sea surface waters were due to nutrient limitation of phytoplankton photosynthetic energy conversion. In contrast, F v/ F m of samples collected from the chlorophyll maximum layer were independent of distance from shore and water mass characteristics.