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).