ALBERT

Description: 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 environment that is the Black Sea. The retrieval of chlorophyll a (chl a) 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 coloured dissolved organic matter (CDOM) which contributes to the fluorescence signal and produces an apparent increase in the chl a 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 chl a 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 the chl a profiles are consistent with HPLC data. In the Black Sea, the DCM begins to form in March, throughout the basin, at a density level set by the previous winter mixed layer. During a first phase (April–May), the DCM remains attached to this particular layer. The spatial homogeneity of this feature suggests a hysteresis mechanism, i.e. that the DCM structure locally influences environmental conditions rather than adapting instantaneously to external factors. In a second phase (July–September), the DCM migrates upward, where there is higher irradiance, which suggests the interplay of biotic factors. Overall, the DCM concentrates around 45 % to 65 % of the total chlorophyll content within a 10 m layer centred around a depth of 30 to 40 m, which stresses the importance of considering DCM dynamics when evaluating phytoplankton productivity at basin scale.

Description: Within the frame of a study on the deep chlorophyll maximum in the Black Sea, water samples were collected conjointly to the deployment of a new Biogeochemical-Argo float (WMO 6903240). The sampling took place onboard the R/V Akademik (Bulgarian Academy of Sciences – Institute of Oceanology) in the western Black Sea (station 307, 43.16°N and 29°E) on March 29, 2018. Water samples were obtained using a CTD carousel with twelve 5 L Niskin bottles. Samples were taken at 12 different depths between the surface and 1000 meters, and were considered to be co-located in time and space with the float deployment. Seawater samples were vacuum filtered through 47 mm diameter Whatman GF/F glass fibre filters (0.7 micrometer pore size). After filtration, filters were stored in liquid nitrogen and kept at -80°C before being analyzed by High-Performance Liquid Chromatography (HPLC). HPLC analyses allow the determination of pigments and by extension the chlorophyll a (chl a) with the best known precision. Using this information on chl a, our objective was to validate, using chl a concentrations derived with HPLC, an algorithm that correct the chl a fluorescence measured by Biogeochemical-Argo floats in the Black Sea by removing the contamination due to the fluorescence of dissolved organic matter.