Alpha diversity
Shannon’s diversity index at the genus level was used as a measure of
alpha diversity throughout the sedDNA and microscopy records. Both
records displayed a general increasing trend in alpha diversity from the
1970s which began to plateau from the 1990s (Fig. 2). Diversity in the
most recent sediment core section was 2.75, which was similar to that in
the oldest section with a diversity of 2.50. Alpha diversity as measured
by sedDNA ranged between 1.47 and 2.76, and was consistently lower than
that measured by microscopy, which ranged between 2.43 and 3.92. There
was a significant positive correlation between the sedDNA and microscopy
GAM-estimated annual values of alpha diversity between 1945 and 2010
with an r value of 0.75 (F1,64 = 81.45, standard error =
0.19, p < 0.001).