Distribution of methane in the upper water layer of the northern Black Sea: Seasonal and daily trends and seawater-air emissions

We report on methane (CH₄) concentration measurements in the northern Black Sea area conducted during 6 cruises with R/V Professor Vodyanitsky from 2017 to 2019. Our work is a multi-season study at a uniform station grid covering an area of 88 × 10³ km² and including three latitudinal transects that comprises both surface and vertical profile water-column measurements. The main goal of the work was to assess the seasonal patterns of vertical CH₄ structure in the aerobic water column (upper 100 m) and its emission to the atmosphere.

In surface waters, the mean dissolved CH₄ concentration ranged from 2.6 nmol L⁻¹ detected in November 2018 to 11.5 nmol L⁻¹ measured in June–July 2018, respectively. Calculated CH₄ seawater-air fluxes and saturations were mostly positive (i.e. net flux to atmosphere), and winter fluxes (2.6 μmol m⁻² d⁻¹) were higher than summer fluxes (1.6 μmol m⁻² d⁻¹) due to the higher wind speed. The integral CH₄ flux from the whole study area (88 × 10³ km²) ranged from 84 to 235 kM day⁻¹.

It was shown that, on average, the methane concentration in the upper layer for deep-water stations where the seabed is located at depths >160 m (σt >16.2) was lower compared to stations at shallow water depths (28–140 m, σt <16.2). The most distinct difference was obtained for the summer season (June–July 2018) and a less significant difference – for spring (April–May 2019) and winter season (November–December 2018). During these seasons the water column was also considerably less saturated in CH₄ compared to the entire monitoring period. We observed subsurface maxima, which were generally located at the base of the thermocline and exceeded 100 nmol L⁻¹ at some stations. Exceptions were observed in October 2019 (cruise 110), when vertical CH₄ distributions were characterized by two-peaks at ∼20 and ∼50 m depth. The strong influence of the thermohaline structure on the water column CH₄ distribution has also been shown in studies of daily dynamics of CH₄ vertical profiles in the shallow water region. Despite the high variability of CH₄ concentrations, significant similarities in vertical distributions of CH₄ and chlorophyll-a for which sub-surface maxima coincided at some stations, are shown. Extremely high concentrations of CH₄ (up to 351 nmol L⁻¹) in the near-bottom water layer were revealed during all seasons at the station near the Dnieper paleo-channel at the northwestern edge of the study area. This enrichment is assumed to be caused by methane emissions from gas seeps densely located in this region.