Seaweed biomass timeseries (1996/98-2012/13-2021):
Mean fresh weight (FW) was compared over three time points (1996/98, 2012/13 and 2021) and across the biomass dominant species and groups at relevant depth levels. Figure 3 illustrates the change in FW over time and statistical results of separate two-factorial ANOVAs for the factors time, depth and their interaction on FW are given in Table 1.
Overall seaweed biomass along the depth transect (0 – 15m) exhibited a significant interaction of time x depth (p = 0.006). When comparing 2021 data to earlier time points, the seaweed FW maximum at 2.5m was similar to 2012/13 (14.5 kg FW m-²). In contrast, in 1996/98 the seaweed FW maximum was approx. 50% lower and at 5m (6.7 kg FW m-²), explaining the interaction between time and depth over the three time points. The general pattern of seaweed FW distribution along the depth gradient was the same between 2012/13 and 2021 but very different compared to 1996/98. Additionally, seaweed FW significantly differed along the depth gradient (p< 0.0001) as time integrated seaweed FW increased 3.4 fold from 0m to the maximum at 2.5m (10.3 kg FW m-2) and then decreased to 52% (5m), 80% (10m) and 94% (15m) of the maximum, respectively ((2.5 = 5) > (5 = 0) > (0 = 10) > (10 = 15), p < 0.02, Tukey test).
As kelp FW (0 - 10m) constituted most of the overall seaweed FW, it showed similar responses and exhibited a significant interaction of time x depth (p = 0.003). Between 1996/98 and 2021 kelp FW at 2.5m increased significantly by 6.7 fold from 1.7 to 11.4 kg FW m-2 (p = 0.03, Tukey test). Consequently, the increase and upward shift in kelp FW maximum from 5 to 2.5m, which had already been observed between 1996/98 and 2012/13, remained the same in 2021. This relation is also reflected in significant differences along the depth gradient (p < 0.0001) as time integrated kelp FW was 9 and 4 fold higher at 2.5m (9.6 kg FW m-2) and 5m (4.2 kg FW m-2) than at 0m and 10m (both depths 1.1 kg FW m-2), respectively ((2.5m = 5m) > (10m = 0m), p < 0.007, Tukey test). In contrast to overall seaweed FW, kelp FW was also affected by the factor time (p< 0.021). Depth integrated kelp FW in 2021 (4.4 kg FW m-2) was double compared to 1996/98 (2.2 kg FW m-2) and slightly lower than in 2012/13 (5.5 kg FW m-2) but only the change between the 1st and 2nd study was significant ((2012/13 = 2021) > (2021 = 1996/98), p = 0.04, Tukey test).
Same as overall kelp FW, FW of the kelp species Alaria esculenta(2.5 – 10m) changed significantly over time (p = 0.018) and increased continuously from 0.5 kg FW m in 1996/98 over 1.2 kg FW m-2 in 2012/13 to 3.1 kg FW m in 2021 ((2021 = 2012/13) > (2012/13 = 1996/98), p = 0.017, Tukey test).
Similar to A. esculenta FW, FW of ‘Digitate Kelps’ (2.5 – 5m) was affected by the factor time (p = 0.02), but Tukey Posthoc test did not reveal significant differences between years. Furthermore, ‘Digitate Kelps’ FW changed with depth (p = 0.002) and time integrated FW was significantly 4 fold higher at 2.5m (6 kg FW m) compared to 5m (1.5 kg FW m) (p = 0.002, Tukey test).
Interestingly FW of Saccharina latissima (0 - 10m) showed a different trend than A. esculenta and ‘Digitate Kelps’ as there was a significant interaction between time x depth (p < 0.05). In 1996/98 S. latissima FW along the depth transect continuously increased and peaked at 10m (1.2 kg FW m-2). This pattern changed in 2012/13 and 2021 as the maximum of S. latissima FW was 1.6 fold higher and recorded at 2.5m (both years 2 kg FW m-2). Although S. latissima FW also changed significantly with depth (p = 0.04), these differences were not resolved via Tukey Posthoc test.
Understory seaweeds FW (0 – 15m) differed along the depth gradient (p = 0.005) as time integrated understory FW at 0m (1.9 kg FW m-2) was significantly 2.6 – 3.5 fold higher compared to 2.5m, 5m and 15m depth (0.7, 0.7 and 0.5 kg FW m-2, respectively) ((0m = 10m) > (10m = 15m = 2.5m = 5m),p < 0.03, Tukey test).
The group ‘Other Phaeophyceae’ (0 – 15 m), which excludes adult kelps, exhibited similar trends as understory seaweeds. There was an effect of depth (p = 0.01) as time integrated FW of ‘Other Phaeophyceae’ was significantly 2.6 – 12.8 fold higher at 0m (1.2 kg FW m-2) than at 5m, 10m or 15m depth (0.45, 0.4 and 0.1 kg FW m-2, respectively) ((0m = 2.5m) > (2.5m = 10m = 5m = 15m), p < 0.05, Tukey test).
In contrast to ‘Other Phaeophyceae’, the FW of Rhodophyta (0 – 15 m) showed a significant interaction of time x depth (p = 0.002). While there were no biomass dominant Rhodophyta at 0m in 1996/98 (Fig. 1), the FW at this depth significantly increased to 0.95 and 0.93 kg FW m, respectively in 2012/13 and 2021 (p = 0.04, Tukey test). Furthermore, the change in biomass distribution pattern over time is reflected at 2.5m and 5m where Rhodophyta FW was up to 93.5 fold higher in 1996/98 (0.37 and 0.40 kg FW m-2) compared to 2012/13 (0.006 and 0.03 kg FW m-2) or 2021 (0.004 and 0.07 kg FW m-2). Additionally, there was a significant effect of depth (p < 0.0001) as time integrated Rhodophyta FW decreased sharply from 0m (0.7 kg FW m-2) to 2.5m and 5m (both 0.1 kg FW m-2) followed by an increase at 10m and 15m (both 0.5 kg FW m-2) ((0m = 10m = 15m) > (5m = 2.5m), p < 0.03, Tukey test).
Chlorophyta were only rarely recorded along the depth transect (1996/98: 2.5m, 5m; 2012/13: 0m; 2021: 0m, 2.5m) and were therefore excluded from the statistical analysis.