Temporal trends in community composition
Chlorophytes, dinoflagellates, ochrophytes and bacillariophytes were
well-represented in both the sedDNA and microscopy records (Fig. 3A-D).
Generally, data derived from sedDNA and microscopy showed broadly
similar long-term trends for these phyla, but with some differences in
the exact timing of the onset of change. Of these dominant phyla,
chlorophytes and dinoflagellates made up the largest proportion of the
sedDNA community. Chlorophytes were initially present in the sedDNA
record with a relative abundance between 0.03 and 0.10 from 1916 to
1994. Their relative abundance then increased abruptly to between 0.20
and 0.27 in more recent samples from 1997 to 2021. In the
microscopy-based monitoring record, chlorophytes had a low occurrence
initially, but increased sharply from the 1980s to become the group with
the highest occurrence (Fig 3A).
In the sedDNA record, dinoflagellates had a relative abundance less than
0.01 until 1970. Their relative abundance then increased to two distinct
peaks in 1980 when they reached a relative abundance of 0.17, and in
2000 when they reached a relative abundance of 0.25. Dinoflagellates in
the microscopy record had three main peaks in 1967, 1986 and 2002 when
they reached an occurrence of 2.35, 2.67 and 2.92, respectively (Fig.
3B).
The relative abundance of ochrophytes in the sedDNA record was below
0.006 and relatively stable until the 1980s when there was a slight
increasing trend to the 2000s. The occurrence of ochrophytes in the
microscopy record remained below 1.00 until 1983, but then increased
throughout the 1980s and 1990s to their highest occurrence of 4.08 in
2001 (Fig. 3C).
Bacillariophytes had the lowest relative abundance of the four phyla
analysed in the sedDNA record which was consistently below 0.003. There
was a general increasing trend in the relative abundance of
bacillariophytes from the 1970s, although there was some scatter around
this trend. In the microscopy record, bacillariophytes displayed a
slight decreasing trend to the 1980s, and then increased to a period of
higher occurrence from the 1990s. Bacillariophytes had the highest
occurrence of any phylum in the microscopy record until 1980, after
which the only phylum with a higher occurrence were chlorophytes (Fig.
3D).
There was a significant positive correlation between the sedDNA and
microscopy GAM trends for all four phyla (p < 0.001). The
correlation between the two records was strongest for ochrophytes, with
an r value of 0.93, followed by 0.76 for chlorophytes and
bacillariophytes, and 0.75 for dinoflagellates (Fig. 3A-D). All GAM
trends for each dataset were significant (p < 0.01), and
statistics associated with the GAMs are provided in Supplementary
Information, Table S4 and S5.
Charophytes, cryptophytes and haptophytes were also recorded by
microscopy. However, charophytes and haptophytes were only detected with
a relative abundance greater than 0.001 in three sediment core samples,
and cryptophytes were absent from the sedDNA record. The GAM-fitted
trends in occurrence as measured by microscopy for charophytes,
cryptophytes and haptophytes are presented in Supplementary Information,
Fig. S3.