Features of the Intra-annual Variability of the Hydrochemical and Hydrobiological State of Varna Lakes in the Period 2022–2023

This study continues the presentation [1] of the results from long-term (June 2022–July 2023) observations in Varna lakes. The analysis focuses on data from in situ measurements taken with multiparameter sondes at two points, covering the following parameters: i) oxygen saturation, dissolved oxygen, fluorescence, turbidity, and blue-green algae; these are continuous time series recorded at a depth of 1.5 m below the lake surface by anchored autonomous buoys; ii) OS and DO, turbidity, pH, chlorophyll-a, and BGA – vertical profiles of the water column from the surface to the bottom received during 11 periodic control measurements. As a result, the peculiarities of intra-annual variability and the vertical distribution of the observed parameters have been identified, along with a number of cause-and-effect relationships. Oceanographic factors (temperature, salinity, and content of the two oxygen forms) play a key role in the analysis. Special attention is given to extreme situations. Cases of a strongly negative impact of the Provadiyska river on the ecological status have been identified. The unstable ecological condition is evidenced by recorded periods of high (up to 300%) OS or oxygen deficiency and hypoxia, high concentration of Chl-a, as well as turbidity (exceeding by two orders of magnitude the standard sensor calibration scale).