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).