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Frontal dynamics boost primary production in the summer stratified Mediterranean sea

机译：夏季分层地中海中的锋面动力促进了初级生产

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Bio-physical glider measurements from a unique process-oriented experiment in the Eastern Alboran Sea (AlborEx) allowed us to observe the distribution of the deep chlorophyll maximum (DCM) across an intense density front, with a resolution (similar to 400 m) suitable for investigating sub-mesoscale dynamics. This front, at the interface between Atlantic and Mediterranean waters, had a sharp density gradient (Delta rho similar to 1 kg/m(3) in similar to 10 km) and showed imprints of (sub-)mesoscale phenomena on tracer distributions. Specifically, the chlorophyll-a concentration within the DCM showed a disrupted pattern along isopycnal surfaces, with patches bearing a relationship to the stratification (buoyancy frequency) at depths between 30 and 60 m. In order to estimate the primary production (PP) rate within the chlorophyll patches observed at the sub-surface, we applied the Morel and AndrS (J Geophys Res 96:685-698 1991) bio-optical model using the photosynthetic active radiation (PAR) from Argo profiles collected simultaneously with glider data. The highest production was located concurrently with domed isopycnals on the fresh side of the front, suggestive that (sub-)mesoscale upwelling is carrying phytoplankton patches from less to more illuminated levels, with a contemporaneous delivering of nutrients. Integrated estimations of PP (1.3 g C m(-2)d(-1)) along the glider path are two to four times larger than the estimations obtained from satellite-based algorithms, i.e., derived from the 8-day composite fields extracted over the glider trip path. Despite the differences in spatial and temporal sampling between instruments, the differences in PP estimations are mainly due to the inability of the satellite to measure DCM patches responsible for the high production. The deepest (depth > 60 m) chlorophyll patches are almost unproductive and probably transported passively (subducted) from upper productive layers. Finally, the relationship between primary production and oxygen is also investigated. The logarithm of the primary production in the DCM interior (chlorophyll (Chl) > 0.5 mg/m(3)) shows a linear negative relationship with the apparent oxygen utilization, confirming that high chlorophyll patches are productive. The slope of this relationship is different for Atlantic, mixed interface waters and Mediterranean waters, suggesting the presence of differences in planktonic communities (whether physiological, population, or community level should be object of further investigation) on the different sides of the front. In addition, the ratio of optical backscatter to Chl is high within the intermediate (mixed) waters, which is suggestive of large phytoplankton cells, and lower within the core of the Atlantic and Mediterranean waters. These observations highlight the relevance of fronts in triggering primary production at DCM level and shaping the characteristic patchiness of the pelagic domain. This gains further relevance considering the inadequacy of optical satellite sensors to observe DCM concentrations at such fine scales.
机译：来自东部阿尔伯兰海（AlborEx）的独特过程导向实验的生物物理滑翔机测量值使我们能够观察到深叶绿素最大值（DCM）在密集密度前沿的分布，其分辨率（约400 m）适合用于研究亚中尺度动力学。在大西洋和地中海水域之间的交界处，该前沿具有陡峭的密度梯度（Delta rho类似于1 kg / m（3），类似于10 km），并且在示踪剂分布上显示了（亚）中尺度现象。具体而言，DCM中叶绿素-a的浓度沿等密度面显示出破坏的模式，在30至60 m的深度处有斑片与分层（浮力频率）有关。为了估算在亚表面下观察到的叶绿素斑块内的初级生产（PP）速率，我们使用光合作用主动辐射（PAR）应用了Morel和AndrS（J Geophys Res 96：685-698 1991）生物光学模型），并与滑翔机数据同时收集。产量最高的是与新鲜前部的圆顶状异戊二烯同时存在，这表明（亚）中尺度上升流携带的浮游植物斑块从较低到更高的照亮水平，同时输送营养。沿滑翔机路径的PP（1.3 g C m（-2）d（-1））的综合估算值比从基于卫星的算法获得的估算值大二至四倍，即从提取的8天复合场中得出在滑翔机旅行路径上。尽管仪器之间的时空采样有所不同，但PP估算的差异主要是由于卫星无法测量导致高产量的DCM斑块。最深（> 60 m深度）的叶绿素斑块几乎没有生产力，可能是从上层生产力层被动运输（俯冲）的。最后，还研究了一次生产与氧气之间的关系。 DCM内部主要产品的对数（叶绿素（Chl）> 0.5 mg / m（3））与表观氧气利用率呈线性负相关，这证实了高叶绿素斑块具有生产力。对于大西洋，混合界面水域和地中海水域，这种关系的斜率是不同的，这表明在锋线的不同侧面，浮游生物存在差异（无论是生理，种群还是群落水平都应作为进一步研究的对象）。此外，在中间（混合）水域中，光学反向散射与Chl的比率较高，这暗示着大型浮游植物细胞，而在大西洋和地中海水域的核心中则较低。这些观察结果突显了前沿在触发DCM水平初级生产和塑造中上层带特征性斑块方面的相关性。考虑到光学卫星传感器不足以在如此精细的规模上观测DCM浓度，因此这具有进一步的相关性。

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