Dynamics of the Deep Chlorophyll Maximum in the Black Sea as depicted by BGC-Argo floats

Abstract

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.

Deep chlorophyll maximum and nutricline in the Mediterranean Sea: emerging properties from a multi-platform assimilated biogeochemical model experiment

“…In addition to confirming previous findings on the spatial variability in the DCM layer characteristics in the Mediterranean Sea, the assimilated simulation presented in this work provides a full 3D and time-varying description of a number of biogeochemical variables, allowing us to further investigate potential relations between the vertical distributions of phytoplankton, nutrients, light availability, and other physical forcings. The use of the results of assimilated simulations that include along-depth observations can integrate investigations on the DCM and its dynamics that to date have been based on observations, which can be sparse and not evenly distributed in time (Navarro and Ruiz, 2013;Ricour et al, 2021).…”

Correction of Biogeochemical-Argo Radiometry for Sensor Temperature-Dependence and Drift: Protocols for a Delayed-Mode Quality Control

“…Downwelling irradiances at various wavelengths have been implied in the analysis of the bio-optical behavior of the global ocean [ 4 ] and the dynamics of dissolved organic matter [ 5 , 6 ], and for the validation of space-based ocean color measurements and products [ 7 , 8 , 9 , 10 , 11 , 12 , 13 ]. Besides, E d and PAR have been widely used to understand particulate organic carbon fluxes and export [ 14 , 15 , 16 ], to study phytoplankton dynamics [ 17 , 18 , 19 , 20 , 21 , 22 , 23 , 24 , 25 ], and to improve numerical and radiative-transfer models [ 26 , 27 ].…”

Seasonal Stages of Chlorophyll‐a Vertical Distribution and Its Relation to the Light Conditions in the Black Sea From Bio‐Argo Measurements

“…described the response of the vertical distribution of Chla on the storm impact. Patterns of the seasonal and interannual variability of the vertical distribution of Chla were briefly discussed in Journal of Geophysical Research: Oceans Stanev et al, 2017), while the recent study of Ricour et al (2020) investigate the variability of the subsurface maximum of the Chla in the Black Sea. The light attenuation at 490 nm, and its relation with Chla have been revealed by Organelli et al (2017).…”