Natural activities of 32P and 33P and the 33P32P ratio in suspended particulate matter and plankton in the Sargasso Sea

A procedure is presented to determine the turnover time of P in the pelagic food web at the Bermuda Atlantic Time-series Study (BATS) station in the Sargasso Sea (31°50′N, 64°10′W). This method is based on observations made in suspended particulate matter and net-plankton of cosmic ray produced 32P (half-life, 14.3 days) and 33P (half-life, 25.3 days). Measurements were made of the specific activities of 32P (i.e. 32PP) and 33p (i.e. 33PP) and the 33P32P activity ratio in particulate matter collections by low-level β counting. The specific activities of 32P were higher by one-to-two orders of magnitude compared to earlier studies in more eutrophic sites. This large difference is mainly due to the extremely low concentrations of stable P off Bermuda, which in addition made possible the determination of the activity of the weak β emitter 33p. Our study, over a 1 year period, shows an increase in the 33P32P ratio in higher trophic levels relative to our previously reported ratios in the rain, reflecting P turnover time in plankton on timescales of days to weeks. Model P turnover times in zooplankton of 60–80 days, a zooplankton grazing flux of 90 mg C m−2 day−1 and an upper limit fecal material flux of 27 mg C m−2 day−1 were determined. These results show promise for the use of cosmogenic 32P and 33P in rainwater and marine plankton for the study of zooplantkon dynamics in the upper ocean.