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

Abstract

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 N2‐fixing organisms. In this study, the naturally occurring cosmogenic radioisotopes, 32P (half‐life = 14.3 d) and 33P (halflife = 25.3 d), were measured and 33P/32P activity ratios were used to estimate radioactive P residence times at Sta. ALOHA (22°45′N, 158°00′W) in the NPSG from February 1999 to July 2000. The 33P/32P activity ratio in the total dissolved P pool varied considerably but systematically; high ratios correlated with periods of enhanced primary production (14C incorporation). Marine particulate 33P/32P 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 33P/32P closely follows primary production suggests that atmospherically derived 32P and 33P 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 on scales of weeks to months.