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).