Sediments act as long-term sinks for heavy metals in aquatic systems but may also become secondary sources of contamination when environmental conditions change. Assessments based solely on total metal concentrations often fail to capture this dual role and may underestimate ecological risk. In this study, a speciation-based approach was applied to evaluate heavy metal mobility and ecological relevance in lake sediments from a forest catchment exposed to low direct anthropogenic pressure in northern Poland. Sediment samples were collected along transects representing the main hydrological zones of the lake basin and analysed using the BCR sequential extraction procedure. Metal speciation was integrated with mobility- and risk-based indicators (MI, RAC, RI, and PLI), supported by spatial analysis and multivariate statistical methods. Total metal concentrations indicated generally low to moderate contamination. However, speciation analyses revealed that cadmium and zinc were predominantly associated with mobile and potentially bioavailable fractions, resulting in elevated mobility- and risk-related indices. These fractions were concentrated mainly in the central depositional zone and in sediments influenced by outflow conditions, identifying areas particularly susceptible to remobilisation and ecological impact. Such spatial patterns were not evident when only bulk concentrations were considered. The results demonstrate that integrating metal speciation with indicator-based and spatial analyses provides a more sensitive and process-oriented assessment of sediment quality. This approach is especially relevant for lakes in forested or weakly disturbed catchments, where internal sedimentary processes play a dominant role in controlling metal behaviour and ecological risk. • Cd and Zn showed the highest mobility and ecological relevance (MI > 48%). • A moderate combined ecological risk (RI ≈ 164) reflecting diffuse atmospheric inputs. • Speciation-based indicators reveal sediment-related risks not evident from total concentrations. • The approach supports comparative assessment of small lakes and reservoirs. • Indicator-based evaluation enhances sediment quality monitoring at the catchment scale.
Marcin Sidoruk (Fri,) studied this question.