Depth integrated nitrogen fixation from the northern Benguela upwelling system during Maria S. Merain cruise MSM18/4

Nitrogen fixation data from the cruise number MSM18/4 with research vessel \"Maria S. Merian\" from 24.07.-20.08.2011 (from Libreville to Walvis Bay) in front of Angola and northern Namibia. Samples taken by CTD- rosette sampler from different depths and incubated in glass bottles (535 ml) at light intensities that resemble the in situ light intensities of the sampling depth after Delta 15 N2 gas was injected to the sample. After the incubation time of 6 hours, the complete bottle content was filtered onto a pre-combusted Whatman GF/F filter. Filters were frozen, transported to the institute on dry ice and measured in a mass spectrometer for Delta15N. The principle of the method was described by Montoya et al. (1996) and calculation was done according to their spread sheet. From the data of the single depths, the nitrogen fixation per square meter within the upper 40 m of the water column was calculated. The methods are described in detail in a paper submitted by Wasmund et al. in 2014 to be printed in 2015. Some results are surprisingly below zero. This occurs if the Delta15N of the blank is higher than the measurement after incubation. It indicates that no nitrogen fixation occurred. Due to natural variability, the variability of the nitrogen fixation data is high. In an overall estimate, also over several cruises, negative and positive values compensate more or less, suggesting that nitrogen fixation is insignificant in the waters in front of northern Namibia and southern Angola.本数据集为2011年7月24日至8月20日期间，搭载"玛丽亚·S·梅里安号（Maria S. Merian）"科考船执行编号为MSM18/4的科考航次所获取的固氮作用数据，航次航线从利伯维尔前往沃尔维斯湾，作业海域位于安哥拉与纳米比亚北部近海。 采用CTD采水器（CTD-rosette sampler）于不同水深采集水样，将水样装入535 mL玻璃培养瓶中，注入δ¹⁵N₂标记氮气后，在与采样水深原位光强一致的光照条件下开展培养。经6小时培养后，将培养瓶内全部水样过滤至预先经高温灼烧的Whatman GF/F滤膜上，随后将滤膜冷冻保存，通过干冰运输至实验室，采用质谱仪（mass spectrometer）测定其δ¹⁵N值。该方法原理由Montoya等人于1996年发表，计算过程基于其公开的电子表格完成。 基于各采样水深的实测数据，研究人员计算得到水柱上层40米范围内单位面积的固氮通量。相关方法细节已由Wasmund等人于2014年投稿至学术期刊，并计划于2015年正式刊发。 部分实测结果呈现异常负值，该现象出现于空白对照样的δ¹⁵N值高于培养后样品的δ¹⁵N值时，代表该采样点未发生固氮作用。受自然变异影响，本数据集的固氮通量数据离散程度较高。在综合多次航次的整体评估中，正负值会相互抵消，整体结果显示纳米比亚北部及安哥拉南部近海海域的固氮作用并不显著。