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 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.

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

“…The foreseen increase in the BGC-Argo network (Bittig et al, 2019;Claustre et al, 2020) will move in that direction. However, the need to elongate life floats will probably result in a decrease in the sampling cycle frequency from 5 to 10 d (Roemmich et al, 2019). Moreover, the potential effectiveness of the present BGC-Argo missions in covering the Mediterranean bioregions has been demonstrated at least for chlorophyll (D'Ortenzio et al, 2020).…”

“…While quality assessment advancements have improved our confidence in model results (Hipsey et al, 2020), intrinsic limitations still exist because of unrepresented processes and uncertainty in parameterization and numerical approximation (Dowd et al, 2014). On the other hand, emerging observation systems have provided valuable information on the biogeochemical state and processes in the ocean (Chai et al, 2020;Claustre et al, 2020;Groom et al, 2019;Muller-Karger et al, 2018;Roemmich et al, 2019). However, observations can be sparse and unevenly distributed in time and space (in situ), limited to the ocean surface (satellite remote sensing) and generally affected by calibration and measurement errors (Bittig et al, 2019;Xing et al, 2020).…”

Using Argo Floats to Characterize Altimetry Products: A Study of Eddy-Induced Subsurface Oxygen Anomalies in the Black Sea

“…It would be relevant to apply a similar methodology to carefully calibrated additional Argo data variables such as chlorophyll (Ricour et al, 2021), nitrate, particulate organic matter, and hydrogen sulfide (Rasse et al, 2020). However, the lesser number of available profiles and the seasonality of biogeochemical cycles would prevent a direct application of the broad-scale, annual perspective adopted here.…”

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 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).…”