Changing the appearance of underwater landscapes in the coastal zone of a large freshwater body under the influence of trout farming activities: a case study of Mustalakhti Bay, Lake Ladoga

Abstract

In Mustalakhti Bay of Lake Ladoga, which is used for placing trout farms, a study was conducted on the structure of bottom landscapes and the transformation of the lakebed surface under the influence of aquaculture activities. Field data for mapping bottom landscapes were collected using hydroacoustic surveys with the SOLIX 10 SI MEGA CHIRP side-scan sonar and underwater photography with the Limnoscout ROV. Standard sedimentological and biological studies of the lakebed were also carried out. The identification of distinct facies elements was based on morphometric features (depth zones, bottom slopes in different parts of the bay, and terrain elements), characteristics of bottom sediments (sediment type, fluid mud composition), surface appearance, and benthic biological communities (dominant benthic groups, presence/absence of macrophytes). The study resulted in bathymetric and landscape maps of the investigated bay, highlighting the characteristic features of individual landscape facies. A total of 8 facies were identified. The study demonstrated a significant negative impact of trout farms on the bay’s bottom. This is confirmed by: 1. substantial changes in the lakebed appearance detected through video footage; 2. visual changes in the surface observed in acoustic images (sonograms) from the side-scan sonar; and 3. increased sediment thickness and higher accumulation of organic matter, evidenced by sonar data and previously conducted chemical analyses of sediments in the farm areas. The data indicates that the features of the bay’s basin, specifically its steep slopes and the extensive shallow area with relatively large depths (20-35 m), create conditions for the accumulation of anthropogenically transformed sediments within the farm area and extending outwards by several tens of metres (a 100 m diameter zone around each farm). Conclusion: The use of a landscape approach with modern research tools, including acoustic and underwater photography, has provided a spatial assessment of the impact of trout farming on the condition of the studied section of the freshwater body.