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A geographical and seasonal comparison of nitrogen uptake by phytoplankton in the Southern Ocean

机译：南部海洋浮游植物对氮吸收的地理和季节比较

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The impact of light and nutrients (such as silicate and iron) availability on nitrogen uptake and primary production vary seasonally and regionally in the Southern Ocean. The seasonal cycle of nitrogen uptake by phytoplankton in the Southern Ocean is not fully resolved over an annual scale due to the lack of winter in situ measurements. In this study, nitrate and ammonium uptake rates were measured using 15N tracers during a winter cruise in July 2012 and a summer cruise in February–March 2013. The winter cruise consisted of two legs: leg 1 extended from Cape Town to the ice margin along the GoodHope line and leg 2 stretched from the ice margin to Marion Island. The summer cruise was mostly focused on the subantarctic zone of the Atlantic sector. In winter, nitrogen uptake rates were measured at 55 and 1% of the surface photosynthetically active radiation (sPAR). The summer uptake rates were measured at four light depths corresponding to 55, 30, 10 and 3% sPAR. The integrated nitrate uptake rates during the winter cruise ranged from 0.17 to 5.20 mmol N m?2 d?1 (average 1.14 mmol N m?2 d?1) while the ammonium uptake rates ranged from 0.60 to 32.86 mmol N m?2 d?1 (average 6.73 mmol N m?2 d?1). During the summer cruise, the mean-integrated nitrate uptake rate was 0.20 mmol N m?2 d?1 with a range between 0.10 and 0.38 mmol N m?2 d?1. The integrated ammonium uptake rate averaged 4.39 mmol N m?2 d?1 and ranged from 1.12 to 9.05 mmol N m?2 d?1. The factors controlling nitrogen uptake in winter and summer were investigated. During the winter cruise, it was found that the different nitrogen uptake regimes were not separated by the fronts of the Antarctic Circumpolar Current (ACC). Light (in terms of day length) and ammonium concentration had the most influence on the nitrogen uptake. In the summer, increases in the mixed layer depth (MLD) resulted in increased nitrogen uptake rates. This suggests that the increases in the MLD could be alleviating nutrient limitations experienced by the phytoplankton at the end of summer.
机译：在南大洋，光和养分（例如硅酸盐和铁）的可用性对氮吸收和初级生产的影响随季节和区域而变化。由于缺乏冬季原位测量，南大洋中浮游植物对氮的吸收的季节性周期尚未在年度范围内得到完全解决。在本研究中，在2012年7月的冬季航行和2013年2月至2013年3月的夏季航行中，使用 15 N示踪剂测量了硝酸盐和铵的吸收率。冬季航行由两条腿组成：腿1延伸从开普敦到沿GoodHope线的冰缘，第2条腿从冰缘延伸到马里恩岛。夏季巡游主要集中在大西洋海域的亚南极带。在冬季，测得的氮吸收率为表面光合有效辐射（sPAR）的55％和1％。在对应55％，30％，10％和3％sPAR的四个光照深度下测量了夏季的吸收率。冬季航行期间的综合硝酸盐吸收速率在0.17至5.20 mmol N m ？2 d ？1 范围内（平均1.14 mmol N m ？2 > d ？1 ），而氨的吸收率介于0.60至32.86 mmol N m ？2 d ？1 （平均6.73 mmol N m ？2 d ？1 ）。在夏季航行期间，硝酸盐的平均积分吸收率为0.20 mmol N m ？2 d ？1 ，范围为0.10至0.38 mmol N m ？ 2 d ？1 。累计铵吸收速率平均为4.39 mmol N m ？2 d ？1 ，范围为1.12至9.05 mmol N m ？2 d ？1 。研究了冬季和夏季控制氮吸收的因素。在冬季航行中，发现不同的氮吸收机制没有被南极绕极流（ACC）的前沿分开。光照（以日长计）和铵浓度对氮的吸收影响最大。在夏季，混合层深度（MLD）的增加导致氮的吸收速率增加。这表明MLD的增加可以减轻夏季末浮游植物所经历的营养限制。

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