Temporal trends in diversity
The NMDS of the dissimilarity matrices based on beta diversity as measured by sedDNA and microscopy both displayed comparable trajectories of change from older to more recent samples. The temporal trends in alpha diversity were also similar between the two records, with both showing an increase in diversity from the 1970s which coincided with the intensification of nutrient enrichment. A trend that is driven by the accumulation of DNA degradation with age could be expected to be a monotonic decline in diversity with sediment depth (Dommain et al ., 2020). However, alpha diversity measured at the core surface was similar to that measured at the bottom of the core, and the temporal trends observed in the sedDNA record were accompanied by similar trends in the microscopy record. This provides evidence that the trends in diversity throughout the sediment core may represent a community response to environmental conditions, and not a trend that is primarily driven by an accumulation of DNA degradation with age. Previous studies have also shown that temporal trends in phytoplankton diversity as measured by sedDNA are consistent with environmental change and not necessarily DNA degradation (Capo et al ., 2017; Huo et al ., 2022; Zhang et al ., 2021). However, alpha genus diversity as measured by sedDNA was lower compared to that measured by microscopy. A lower diversity could be evidence of at least some DNA degradation, the extent of which may vary with conditions within the sediment (Tortiet al ., 2015).