Alkaline phosphatase activity detected in distinct phytoplankton communities in the northern Gulf of Alaska

Alkaline phosphatase activity (APA) has been proposed as a proxy to determine phosphorus (P) stress in marine phytoplankton. Phytoplankton typically meet P requirements by incorporating dissolved inorganic P (DIP), but can use reservoirs of dissolved organic P (DOP) by expressing alkaline phosphatase (AP). Enzyme-labeled fluorescence (ELF) was used to determine group-specific APA within 4 phytoplankton communities associated with mesoscale eddies, which are common to the northern Gulf of Alaska (nGoA). Nutrient stress in relation to eddies in the nGoA has not been well established and these anti-cyclonic eddies may be influencing P status in phytoplankton by isolating distinct biological communities. In the nGoA (fall 2007), APA was greatest in the northwest transitional waters of the Sitka eddy and the western edge of the Kenai eddy compared to coastal (shelf), eddy core waters, and open ocean waters. The APA signal was dominated by picoplankton (0.6 to 5 μm) and nanoplankton (5 to 20 μm), primarily nanoflagellates, though AP-producing phytoplankton were not the dominant phytoplankton at any station. APA did not correlate with DIP levels, and macronutrient concentrations suggest that DIP was not limiting for the bulk phytoplankton assemblage. Dissolved organic P (DOP) was not measured. These results suggest that P status within a community is more nuanced than typical Redfield or in organic nutrient indices indicate and that organic P sources could influence community composition of phytoplankton.