Detection of changes in the hydrobiological parameters of the Oder River during the ecological disaster in July 2022 based on multi-parameter probe tests and remote sensing methods

Starting from the second half of July, hydrological conditions (low water levels and low flows) and atmospheric conditions (high temperatures, lack of precipitation) contributed to algae bloom in the Oder River basin. The mass extinction of fish in the middle and lower section of the Oder was probably the result of such environmental conditions. Because algae bloom is correlated with an increase in the content of chlorophyll in the river and on its surface, the authors focused on an analysis of changes in the chlorophyll amount in the longitudinal profile of the river. The key data was obtained by soundings carried out at an interval of about one kilometre during the research expedition, which took place on July 16–24, 2022. These data coincided with the values of the NDVI index (normalised difference vegetation index) based on 10-metre satellite images from the Sentinel 2 mission. Imagery from cloudless periods (mainly March, June, July and August) processed using the NDVI index was compared. Regularly elevated chlorophyll levels in the upstream section (occurring in each period) and temporarily elevated chlorophyll values as moving downstream were observed. This article confirms the initial conclusions of Kolada et al. (2022) that the mass fish kill in the Oder River in the summer of 2022 was conditioned by natural environmental pressure causing massive algae bloom and the release of their toxins. The article also proves that the use of satellite images for this type of analysis is the correct approach, but due to low resolution, they cannot be used as reference material. Changes in the hydrobiological parameters of the river, which are a direct indicator of the causes of the ecological disaster, are the data of the multi-parameter probe obtained two weeks before the first mass catches of dead fish.