Relationship between total alkalinity and salinity during different seasons in the coastal waters of the Northern Bulgarian Black Sea - an example from the IO-BAS Shkorpilovtsi monitoring station

The Black Sea remains poorly studied regarding its carbonate system parameters and ocean acidification status. This is partly due to its characteristics as a semi-enclosed sea with significant riverine inflow and naturally high pH and total alkalinity (TA) levels, which generally suggest a stable buffering capacity. Such an assessment is necessary given the increasing anthropogenic pressures like climate change and increasing atmospheric CO₂ and the rapid environmental changes observed in recent years. 

With this study, we aim to contribute the understanding of the Black Sea carbonate system. The presented results are based on in-situ data spanning three consecutive years, with a sampling frequency of twice per month. Salinity was measured using a WTW TetraCon 925 (0.1 resolution), while total alkalinity (TA) was determined according to ISO 9963-1:1994 (high-precision open-cell potentiometric titration with an NIST-calibrated pH electrode). This objective is further driven by our use of autonomous observations and carbonate system sensors such as the continuously operating FerryBox system with a membrane-based pCO₂ sensor at the IO-BAS Shkorpilovtsi research station

The observed relationships between salinity and alkalinity confirmed our hypothesis that the expected high correlation between these two parameters is absent in Black Sea coastal waters (r=0.08; r²=0.01). The correlation coefficient varies seasonally, and is highin summer (r=0.73; r²=0.53), but low in autumn (r=0.32; r²=0.16), when warm, nutrient- and biota-rich waters are characterized with maximum seasonal salinity (18.2 ‰),but lowest alkalinity (3.255 mmol/l). We also observed a low correlation during the spring and winter seasons. The spring salinity-alkalinity relationship is particularly interesting due to the sporadic influence of the Danube River on the North Bulgarian Black Sea coast, affecting salinity (average calculated minimum for the year of 16.2 ‰; absolute spring minimum of 13.8 ‰) and alkalinity (average calculated maximum for the year of 3.382 mmol/l). The winter patterns show similar trends but are driven by different factors: a decrease in salinity due to reduced evaporation of cold water and a slight recovery in alkalinity following the sharp autumnal decline.

These conclusions are discussed in the context of concurrently collected data on nutrients, both dissolved and suspended in the seawater and the comparison with FerryBox data on pH and pCO₂.