Phosphorus cycling in the North Pacific Subtropical Gyre using cosmogenic 32P and 33P
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چکیده
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 5 14.3 d) and 33P (halflife 5 25.3 d), were measured and 33P/32P activity ratios were used to estimate radioactive P residence times at Sta. ALOHA (228459N, 1588009W) 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. Phosphorus (P) is an essential nutrient for all living organisms. Compared to studies of carbon (C) or nitrogen (N), two additional bioelements, the distributions and dynamics of inorganic and organic P pools in seawater are less well characterized. Recent evidence suggests that P and trace elements (especially iron) may play a significant role in limiting primary production and nitrogen fixation in selected marine habitats (Martin et al. 1990; Krom et al. 1991; Karl et al. 1997; Behrenfeld and Kolber 1999; Wu et al. 2000; Karl et al. 2001; Sañudo-Wilhelmy et al. 2001). This is particularly true in the western North Atlantic, where enhanced dust deposition has resulted in severe P limitation (Wu et al. 2000). In the North Pacific Subtropical Gyre (NPSG), there is also a hypothesized climate-induced transition from N to P limitation, potentially due to increasing blooms of N2-fixing organisms (Karl et al. 2001). As a result, it is essential that we understand the composition and the residence times of the various P pools within oligotrophic regimes because they may directly affect the magnitude of global primary production and, hence, nutrient cycling and export in the world’s oceans. 1 To whom correspondence should be addressed. Present address: University of South Carolina, Department of Geological Sciences, 700 Sumter Street, Columbia, South Carolina 29208 (cbnelson@
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تاریخ انتشار 2002