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

The year‐to‐year seasonal variability of the vertical distribution of concentration of chlorophyll‐a (Chla) in the Black Sea, its relation to light (photosynthetically available irradiance [PAR]), and mixing conditions were studied based on the 6‐year (2014–2019) measurements of Bio‐Argo floats. Variations in PAR caused by changes in incoming solar radiation and light attenuation determine the position of the deep chlorophyll maximum (DCM). During all seasons, the DCM was located between isolume of daily integrated PAR of 0.080 and 10.0 mole photons m⁻² d⁻¹. Analysis of Bio‐Argo data allowed to distinguish seven different stages of Chla seasonal variability. In February, the minimum of Chla in the entire water column was often observed in areas with a deep mixed layer (Stage 1), followed by the March early spring peak, occupying both the mixed layer and the seasonal thermocline (Stage 2). In April–May, Chla decreased throughout the 0–40 m layer, which was accompanied by the maximum in light penetration and an increase in Chla in the deepest layers (Stage 3). In summer, two deep peaks of Chla were distinguished: the first, a thin and shallow peak in June, and the second, a large and deeper peak in August (Stage 4 and 5). The August peak was followed by the deepest seasonal minimum of Chla in September–October, coincided with the autumn rise of the zooplankton biomass (Stage 6). Autumn–winter convection in November–January led to the third seasonal peak of Chla (Stage 7). The study discusses the role of various abiotic and biotic factors in the formation of each of these stages.