Nitrification rates in the NW Mediterranean Sea

During spring, ammonium oxidation and nitrite oxidation rates were measured in the NW basin of the Mediterranean Sea, from mesotrophic sites (Ligurian Sea and Gulf of Lions) to oligotrophic sites (Balearic Islands). Nitrification rates (average values for 37 measurements) ranged from 72 to 144 nmol of N oxidised/l/d, except in the Rhône River plume area where the rates increased to 264-504 nmol/l/d because of the riverine inputs of nitrogen. Maximal rates were located around the peak of nitrite within the nitracline at about 40 to 60 m and just above the phosphacline. At 1 station, relatively high values of nitrification (50 to 130 nmol/l/d) were also measured deep in the water column (240 m). Day-to-day variations were measured demonstrating the response within a few hours to hydrological stress (wind-induced mixing of the water column) and showing the role of hydrological characteristics on the distribution of nitrification rates. Because of the homogenous temperature (13°C) in the Mediterranean Sea, the spatial (geographical and vertical) fluctuations of nitrifying rates were linked to the presence of substrate due to mineralisation processes and/or Rhône River inputs. We estimate the contribution of nitrate produced by nitrification to the N demand of phytoplankton to range from 16% at mesotrophic to 61% at oligotrophic stations.

本研究于春季在地中海西北盆地开展氨氧化（ammonium oxidation）与亚硝酸盐氧化（nitrite oxidation）速率测定，采样站位覆盖从中营养（mesotrophic）海域（利古里亚海与里昂湾）到寡营养（oligotrophic）海域（巴利阿里群岛）的梯度范围。硝化速率（nitrification rates，37次测定的平均值）介于72~144 nmol N氧化/升/天，罗纳河羽流（Rhône River plume）区除外——该区域因河流输入氮素，速率提升至264~504 nmol/升/天。最大硝化速率出现在硝酸盐跃层（nitracline）内的亚硝酸盐峰值附近，水深约40~60米，且紧邻磷酸盐跃层（phosphacline）上方。在1个站位的深水层（240米处），也测得50~130 nmol/升/天的较高硝化速率。本次观测还记录了日变化特征，表明硝化速率可在数小时内响应水文胁迫（hydrological stress，风力引发的水体混合），同时揭示了水文特征对硝化速率分布的调控作用。由于地中海海域水温整体均一（13℃），硝化速率的空间（地理与垂向）波动与矿化（mineralisation）过程产生的底物或罗纳河输入的氮底物密切相关。本研究估算，硝化作用生成的硝酸盐对浮游植物（phytoplankton）氮需求的贡献占比，从中营养站位的16%到寡营养站位的61%不等。