Relative age class distribution in 2021:
The relative abundance of adult kelps (≥ 1 year) peaked at 2.5m in A. esculenta , ‘Digitate Kelps’, S. latissima and the overarching group ‘All Kelps’. Furthermore, 2.5m was the only depth with a balanced age structure in 2021. At 5m the relative abundance of juveniles sharply increased to ≥ 95% in all three kelp species and the overall kelp density reached its maximum with 690 ind. m-2. This pattern became even more pronounced at 10m where 100% of all ‘Digitate Kelps’ and S. latissima individuals were juveniles and A. esculenta was the only kelp species for which a few old individuals (5% 4-9years) were present. Compared to the density peak at 5m, the species-specific densities at 10m decreased to mean densities similar to 2.5m but mostly composed of juveniles. Of
Age and density per m2 of adult kelps between 2013 and 2021:
The age of adult kelps ≥1 year at 2.5m and 5m was significantly different between the three kelp species (p = 0.004 and p= 0.02, Table 3). At 2.5m the time integrated age of A. esculenta(4.2 years m-2) was 1.5 fold higher compared toS. latissima and ‘Digitate Kelps’ (both 2.7 years m-2) (A. esculenta > (S. latissima = ‘Digitate Kelps’), p ≤ 0.01, Tukey test). The species-specific age differences became even more pronounced at 5m as the time integrated age of A. esculenta remained at 4.2 years m-2 and was thereby double of S. latissima (2.1 years m-2) and 2.5 fold higher compared to ‘Digitate Kelps’ (1.7 years m-2) (A. esculenta> (S. latissima = ‘Digitate Kelps’), p ≤ 0.05, Tukey test). The age of adult kelps at both depths was not significantly affected by the factor time or the interaction of time x species (Table 3).
Similarly, the density of adult kelps at 2.5m and 5m did not change significantly over time, across species and there was no interactive effect of time x species. An interesting exception is the overall density of adult kelps per m-2 at 5m which exhibited differences between years (p = 0.007) as species integrated density was significantly 2.5 fold higher in 2013 (17.1 ind. m-2) compared to 2021 (6.8 ind. m-2).
Kelp holdfast, stipe and blade dry weight in 2021
Holdfast, stipe and blade DW, and blade:stipe DW ratio of all adult kelps (≥ 2 years) collected in 2021 was compared between A. esculenta , ‘Digitate Kelps’ and S. latissima from 2.5m and 5m depth. Figure 8 illustrates the differences between species at 2.5m and 5m. Statistical results of non-parametric Kruskal-Wallis tests for the factors species and depth are given in Table 4.
Holdfast DW of adult kelp individuals ≥2years at 2.5 and 5m depth varied significantly between species (p < 0.05) as integrated holdfast DW in ‘Digitate Kelps’ (1.9 ± 1.8 g DW) was 1.2 – 1.6 fold higher than in S. latissima (1.6 ± 1.2 g DW) and A. esculenta (1.2 g ± 1.1 DW), respectively. Nonetheless, the Wilcoxon test did not reveal significant differences in holdfast DW between species.
Interestingly, stipe DW of adult kelps also varied significantly across kelp species (p < 0.001) but, in contrast to holdfast DW, depth integrated stipe DW was highest in S. latissima (9.9 ± 4.5 g DW) and A. esculenta (9.7 ± 7.2 g DW) while it was significantly lower in ‘Digitate Kelps’ (6.2 ± 6.7 g DW) ((S. latissima = A. esculenta ) > ‘Digitate Kelps’,p < 0.002, Wilcoxon test) (all: mean ± SD).
Blade DW did neither exhibit significant differences between kelp species (p = 0.2) nor across depth levels (p = 0.7).
Similar to the stipe DW, the blade:stipe DW ratio of adult kelps was different between species (p = 0.002) and changed with depth (p < 0.001). In contrast to the other parameters, depth integrated blade:stipe DW ratio in ‘Digitate Kelps’ (4.1 ± 2.8) was significantly 2.1-2.4 fold higher than in A. esculenta (2 ± 2.6) and S. latissima (1.7 ± 1.4), respectively (‘Digitate Kelps’ > (S. latissima = A. esculenta ), p< 0.001, Wilcoxon test). Species integrated blade:stipe DW ratio was 2.6 ± 2.2 at 2.5m and 2.7 ± 3.9 at 5m (p = 0.002, Wilcoxon test) (all: mean ± SD).