Abstract. The Deep Chlorophyll Maximum (DCM) is a well known feature of the global ocean. However, its description and the study of its formation are a challenge, especially in the peculiar Black Sea environment. The retrieval of Chlorophyll a (Chla) from fluorescence (Fluo) profiles recorded by Biogeochemical-Argo (BGC-Argo) floats is not trivial in the Black Sea, due to the very high content of Colored Dissolved Organic Matter (CDOM) which contributes to the fluorescence signal and produces an apparent increase of the Chla concentration with depth.

Here we revised Fluo correction protocols for the Black Sea context using co-located in-situ High-Performance Liquid Chromatography (HPLC) and BGC-Argo measurements. The processed set of Argo Chla data (2014–2019) is then used to provide a systematic description of the seasonal DCM dynamics in the Black Sea, and to explore different hypotheses concerning the mechanisms underlying its development.

Our results show that the corrections applied to Chla profiles are consistent with HPLC data. In the Black Sea, the DCM is initiated in March, throughout the basin, at a pycnal level set by the previous winter mixed layer. The DCM then remains attached to this particular layer until the end of September. The spatial homogeneity of this feature suggests a self-sustaining DCM structure, locally influencing environmental conditions rather than adapting instantaneously to external factors.

In summer, the DCM concentrates around 50 to 65 % of the total chlorophyll content around a depth of 30 m, where light conditions ranged from 0.5 to 4.5 % of surface incoming irradiance.

In October, as the DCM structure is gradually eroded, a longitudinal gradient appears in the DCM pycnal depth, indicating that autumnal mixing induces a relocation of the DCM which is this time driven by regional factors, such as nutrients lateral loads and turbidity.

中文翻译：

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BGC-Argo浮标描绘的黑海深层叶绿素最大值动态

摘要。最高的叶绿素（DCM）是全球海洋的众所周知的特征。然而，其描述和对其形成的研究是一个挑战，尤其是在特殊的黑海环境中。在黑海中，从生物地球化学Argo（BGC-Argo）浮标记录的荧光（Fluo）谱中检索叶绿素a（Chla）并非易事，因为有色溶解有机物（CDOM）含量很高荧光信号，并随着深度的增加产生明显的Chla浓度。

在这里，我们使用位于同一地点的高效液相色谱（HPLC）和BGC-Argo测量，针对黑海环境修订了Fluo校正协议。然后，使用经过处理的Argo Chla数据集（2014-2019年）对黑海的DCM季节性变化进行系统描述，并探索有关其发展机理的不同假设。

我们的结果表明，应用于Chla谱图的校正与HPLC数据一致。在黑海，DCM于3月在整个盆地启动，其前次冬季混合层所设定的比海平面水位较高。然后，DCM会一直附加到该特定图层，直到9月底。此功能在空间上的同质性表明了DCM结构的自我维持，局部影响环境条件，而不是瞬间适应外部因素。

在夏季，DCM会在30 m的深度集中约50％至65％的总叶绿素含量，其中光照条件范围是表面入射辐照度的0.5％至4.5％。

在十月份，随着DCM结构逐渐被侵蚀，DCM的深部深度出现了纵向梯度，这表明秋季混合会引起DCM的重新定位，这是由区域性因素（例如养分侧向负荷和浊度）驱动的。