Hotspots of human impact on the seafloor in the Southwestern Baltic Sea

The Southwestern Baltic Sea is a notable example of intense human impact on the seafloor in a confined shallow marine environment. Various marks left by dredging, dumping, fishing, anchoring, among others, reflect the different pressures on the seafloor. These activities represent cumulative stressors for marine ecosystems, potentially leading to habitat modification or loss. Characterizing and quantifying the extent of the different pressures is essential for understanding the system and evaluating the environmental status outlined by different legal frameworks. Here, the effects of different human activities on the seafloor in exemplary hotspots in the SW Baltic Sea are visualized and assessed. Actual anthropogenic marks are compared with assessments based on commonly accessible regional and local information from remote sensing data and institutional sources. About 36% of the seafloor of the investigated area is influenced, mainly by bottom trawling, propeller scouring, anchoring, and dumping. More than 91% of the human footprint corresponds to trawl marks, mainly affecting soft substrates. In addition, from 15% to 47% of the seafloor is disturbed in selected ‘detail areas' within the hotspots. Comparisons with indirect data used for regional pressure estimation demonstrate how hydroacoustic data can enhance assessments of seabed physical pressures. However, quantitative comparisons are challenging, especially when information on human activities is limited or when seafloor recovery rates in relation to the frequency of anthropogenic pressures are unknown. • Anthropogenic features on the seafloor in the SW Baltic Sea are identified and quantified using available hydroacoustic data. • Bottom trawling accounts for >91% of human impact, mainly affecting fine-grained sediments. • Physical pressures were quantified and compared at hotspots using hydroacoustic and HELCOM data. • Seafloor regeneration rates must be considered when comparing fishing abrasion datasets. • Human-induced seafloor features can persist over a decade, indicating slow regeneration.