The seasonal dynamics of geochemical characteristics of sediments in the impact zone of the fish farm (Lake Ladoga, Russia)

This study focuses on the analysis of seasonal dynamics of physicochemical parameters (pH and Eh) and organic matter content in the sediments accumulated within the impact zone of the cage trout farm (Lake Ladoga, Russia). The Ladoga is the largest lake in Europe and there are more than 20 fish farms located in the northern part of the waterbody. The geochemical research is necessary for predicting the negative impact of the polluted sediments on both the cage fish and the aquatic ecosystem. The sampling was conducted during various seasons of the year (February, June, August, October) which were characterized by the different fish feeding regimes. The sediment cores were collected at 7 points, including the cage and reference sites (150 m and 600 m far from the farm). The results showed that the accumulation of organic material released from the studied fish farms was local and the wastes did not enter the sediments at the control sites. Furthermore, the accumulated organic layer can be compacted and partially mineralized during winter period. Nevertheless, strongly reducing conditions in sediments occur due to the accumulation of organic material and low rate of water exchange within the studied bay. Therefore, there is a potential risk of formation of toxic gases and release of accumulated nutrients and metals from sediments to water. It can have a negative impact on benthic and nekton organisms, in particular cage fish.