Phosphorus stress of phytoplankton in the Taiwan Strait determined by bulk and single-cell alkaline phosphatase activity assays

Phosphorus stress of marine phytoplankton was determined by using both bulk and single-cell alkaline phosphatase activity (APA) assays during the summer (July to August 2004) and winter (March 2005) along 2 transects (A and B) in the Taiwan Strait. During the summer of 2004, time-series variations in APA were monitored on a daily basis for 6 d along a third transect (F). Results showed that surface APA along Transects A and B ranged between 1.14 and 7.30 nmol l(-1) h(-1), respectively with a mean of 4.43 nmol l(-1) h(-1). High soluble APA contribution indicated that phytoplankton in this area suffered long-term phosphorus (P) stress. Percentages of enzyme-labeled fluorescence (ELF-labeled) representative phytoplankton increased with bulk APA during the time-series investigation and compared favorably with phosphate concentration. Dinoflagellates had high percentages of ELF labeling during both seasons, suggesting that they were under severe P-stress. The predominant dinoflagellates were Skeletonema costatum, Chaetoceros spp., Asterionellopsis glacialis, Thalassionema nitzschioides and Pseudo-nitzschia spp. During the summer, A. glacialis, T nitzschioides and Pseudo-nitzschia spp. were labeled with ELF, which meant they were also P-stressed during that time of year. Percentages of algal cells labeled with ELF were lower during the winter, suggesting that the relatively high, soluble reactive phosphorus supply softened phytoplankton P-stress in this period. Species such as Rhizosolenia spp. and T nitzschioides were for some reason still P-stressed during the winter. Both field and lab data suggested that bulk and single-cell APA should be used together to provide comprehensive assessment of phytoplankton P-stress from the community level to the individual taxon level in natural phytoplankton communities.