Taxonomic variability of phosphorus stress in Sargasso Sea phytoplankton

Low inorganic phosphorus (SRP) concentrations and high inorganic nitrogen to phosphor-us ratios suggest that phytoplankton production in the northwestern Sargasso Sea may be controlled to some extent by the availability of phosphorus. Phosphorus stress in marine phytoplankton was qualitatively assessed by using a single-cell enzyme-linked fluorescent (ELF) assay for the enzyme alkaline phosphatase, which is induced at low SRP concentrations. During the highly stratified summer period, similar to30% of the observed autotrophic eukaryotes in the surface waters were ELF-labeled, whereas in the well-mixed fall period, nearly 70% of the observed autotrophic eukaryotes in the surface waters were ELF-labeled. During the summer, autotrophic flagellates displayed significantly higher ELF-labeling than did both diatoms and dinoflagellates, and this labeling did not vary with depth, whereas in the fall, autotrophic flagellates and diatoms displayed statistically similar and decreasing percentages of ELF-labeled cells as a function of depth. This assay allowed for rapid assessment of the in situ physiological condition of individual autotrophic phytoplankton in the Sargasso Sea. By using this assay, we were able to identify taxonomic and potential seasonal variability of phosphorus stress within the autotrophic phytoplankton community.