Relationship of nitrogen isotope fractionation to phytoplankton size and iron availability during the Southern Ocean Iron RElease Experiment (SOIREE)
Relationship of nitrogen isotope fractionation to phytoplankton size and iron availability during the Southern Ocean Iron RElease Experiment (SOIREE)
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DOI：
10.4319/lo.2003.48.3.1058
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发表时间：
2003-05
影响因子：
4.5
通讯作者：
K. Karsh;T. Trull;M. Lourey;D. Sigman
中科院分区：
文献类型：
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作者：
K. Karsh;T. Trull;M. Lourey;D. Sigman
沉积物的15 N组成已被用作硝酸盐利用率在地表沃茨的代理，以评估在大气CO2浓度的冰川/间冰期变化的作用，南大洋输出生产。解释依赖于与浮游植物吸收和同化硝酸盐相关的时间恒定同位素效应（ε）。为了研究这种方法的可靠性，我们研究了溶解硝酸盐的15 N组成之间的关系，散装和粒度分级（200，70，20，5，1 μm）悬浮颗粒有机氮（PON），以及在南大洋铁释放实验（SOIREE）期间从沉积物捕集器中获得的沉降颗粒。我们发现浮游植物的氮同位素组成与细胞大小和铁的可用性的变化。在富铁斑块内外，δ 15 NPON随斑块大小的增加而增加>2‰。与未施肥的沃茨相比，铁施肥斑块中以大型硅藻为主的粒度级（20-70，70-200 µm）的δ 15 NPON进一步高出3-4‰。我们推测，这种铁反应可能是由于（1）硝酸盐利用率ε的变化或（2）铁刺激从铵基到硝酸盐基生产的转变。将大粒级的δ 15 N与硝酸盐的δ 15 N进行比较，表明ε值相对较低，为4-5‰，而季节性硝酸盐耗竭和出口生产的估计值为7-10‰。这表明，南大洋沉积物中较高的冰川δ 15 N可能部分反映了铁可用性的增加，主要细胞大小和可能的生长速率，这些影响必须在δ 15 N变化的任何定量标度中考虑，包括硅藻结合的δ 15 N，以达到硝酸盐利用的程度。
The 15N composition of sediments has been used as a proxy for nitrate utilization in surface waters to assess the role of Southern Ocean export production in glacial/interglacial changes in atmospheric CO2 concentration. Interpretation has relied on a temporally constant isotope effect (ε) associated with uptake and assimilation of nitrate by phytoplankton. To investigate the reliability of this approach, we examined the relationships between the 15N compositions of dissolved nitrate, bulk and size‐fractionated (200, 70, 20, 5, 1 µm) suspended particulate organic nitrogen (PON), and sinking particles obtained from sediment traps during the Southern Ocean iron release experiment (SOIREE). We found variations in phytoplankton nitrogen isotopic compositions with both cell size and iron availability. δ15NPON increased by >2‰ with increasing size, both within and outside the ironenriched patch. In comparison to unfertilized waters, δ15NPON within the iron‐fertilized patch was a further 3–4‰ higher in those size fractions dominated by large diatoms (20–70, 70–200 µm). We speculate that this iron response might result from (1) variation in ε of nitrate utilization or (2) an iron‐stimulated shift from ammonium‐based to nitrate‐based production. Comparing the δ15N of the large diatom—dominated size fractions to the δ15N of nitrate suggests relatively low ε values of 4–5‰, in contrast to estimated values of 7–10‰ from seasonal nitrate depletion and export production. This suggests that higher glacial δ15N in Southern Ocean sediments could, in part, reflect increases in iron availability, dominant cell size, and possibly growth rates, and these effects must be considered in any quantitative scaling of δ15N variations, including those of diatom‐bound δ15N, to the extent of nitrate utilization.