Phosphorus cycling in the North Pacific Subtropical Gyre using cosmogenic 32P and 33P

The North Pacific Subtropical Gyre (NPSG) has been hypothesized to be in transition from a nitrogen (N)-limited system to one predominantly limited by phosphorus (P) as a result of a two-decade-long selection for N-2-fixing organisms. In this study, the naturally occurring cosmogenic radioisotopes. P-32 (half-life =14.3 d) and P-33 (half-life = 25.3 d). were measured and P-33/P-32 activity ratios were used to estimate radioactive P residence times at Sta. ALOHA (22degrees45'N. 158degrees00'W) in the NPSG from February 1999 to July 2000. The P-33/P-32 activity ratio in the total dissolved P pool varied considerably but systematically; high ratios correlated with periods of enhanced primary production (C-14 incorporation). Marine particulate P-33/P-32 activity ratios were similar to those found in the source (i.e. rain). Smaller size classes had longer apparent residence times. The observation that the activity ratio of P-33/P-32 closely follows primary production suggests that atmospherically derived P-32 and P-33 atoms track the most "bioavailable" pool of P within the NPSG ecosystem. These preferred substrates were removed from the dissolved phase via plankton uptake during periods of high productivity. Our results suggest that the soluble nonreactive P pool, which is substantially larger than the soluble reactive P pool, is a potentially important source of P to organisms and that its utilization can vary significantly oil scales of weeks to months.