ASLO/TOS Ocean Research Conference, Honolulu, HI, February 2004, p. 16.

Phosphorus uptake kinetics of different natural populations of bacterioplankton and eukaryotic phytoplankton in the North Pacific Subtropical Gyre

K. M. Björkman and D. M. Karl

University of Hawaii, Honolulu, USA

Phosphorus (P) uptake kinetic experiments were conducted on natural microbial populations in the oligotrophic North Pacific Subtropical Gyre (NPSG) at Station ALOHA (22.75N, 158W) and near the Hawaiian Islands in the summers of 2002 and 2003. To assess the maximum P consumption rate (Vmax) and half saturation constant (Km), time course incubations at several P concentrations were performed. Trichodesmium colonies, and mixed assemblages of diatoms and cyanobacteria (Rhizosolenia sp and Katagnymene sp) were also tested. The Km and Vmax for the Prochlorococcus-dominated community was 10-20 nM P and 4-6 nM P d^-1 respectively, similar to the mixed layer mean uptake rate at Station ALOHA (3.5 nM P d^-1, se. 0.2 , n=54). Km values for Trichodesmium and diatom/cyanobacterial communities were 5-10 fold higher. Summertime P concentrations in the mixed layer at ALOHA are typically 25-50 nM, conditions where Prochlorococcus is close to or at Vmax, but the larger phytoplankton will be performing at less than half their maximum uptake rate. This prevailing low nutrient regime allows the slow growing Prochloroccus community to remain dominant in the NPSG.