Statistical analyses
The statistical analyses were performed in R version 4.2.2 (‘Innocent
and Trusting’, R Core Team, 2022). The original data from 1996/98
(https://doi.org/10.1594/PANGAEA.864321) and 2012-14 (provided by
I. Bartsch) were used for the time series analysis. Due to the
logistical constraints of the intensive scuba diving campaign along the
sublittoral gradient at Hansneset for all three time points of the time
series, only a limited number of replicates was obtained per campaign
(1996/1998: n = 2 (0m); n = 3 (5 – 15m); n = 4 (2.5m); 2012/2013: n = 3
(0 and 15m); n = 6 (2.5 – 10m); 2021: n = 3). The homogeneity of
variances was tested using the Levene’s test before each ANOVA. As the
absence of a species or group at a certain depth level results in zero
values for that depth, only relevant depth levels were included in the
statistical tests.
Time series analysis (1996/98-2012/13-2021): Separate
two-factorial ANOVAs were performed to assess the effects of the fixed
factors time, depth and their interaction for FW (log + 1 transformed
data) and LAI of the biomass-dominant species or groups. When the ANOVA
output revealed significant effects, a Tukey HSD post hoc test for
uneven n was applied. In case homogeneity of variances could not be
achieved by transformation but the results of the two-factorial ANOVA
were highly significant, a non-parametric Kruskal – Wallis test
followed by a pairwise Wilcoxon rank sum test was performed to test for
differences between depth levels. This study focused on investigating
changes over time and therefore the differences in species and group FW
and LAI across depth levels in 2021 alone were not examined
statistically.
Age and density comparison 2013 / 2021: To investigate
differences between timepoints and kelp species for the mean age and
density per m-² at 2.5m and 5m (2013 and 2021: n = 3),
separate two-factorial ANOVAs were performed and significant effects
were further investigated using a Tukey HSD post hoc test. For the
analysis of mean density per m², data was log + 1
transformed to achieve homogeneity of variances. Juvenile specimens
<1 year were excluded from the statistical analysis.
Adult kelp dry weight and biochemical investigations in 2021:Individual holdfast, blade and stipe DW, blade:stipe DW ratio as well as
blade %carbon, %nitrogen and C:N ratio was compared between kelp
species and across the relevant depth levels (2.5 – 5m) in 2021.
Homogeneity of variances (Levene’s Test) and normal distribution
(Shapiro-Wilk Test) could not be achieved through data transformation.
The effect of the factors depth and species on the individual parameters
were tested in separate non-parametric Kruskal-Wallis tests and by
pairwise Wilcoxon rank sum tests with Bonferroni correction to reveal
differences between species.
Carbon and nitrogen 2021: Mean extrapolated carbon and nitrogen
stored in kelp blade DW per m² in summer 2021 was
compared between depth levels and kelp species (2.5 and 5m; both n = 3)
using separate two-factorial ANOVAs.
PAR and Turbidity time
series: PAR and turbidity data were quality controlled according to an
adapted ARGO standard for stationary sensors (Fischer et al., 2021;
Waldmann et al., 2022). To analyze long-term changes and trends over the
sampling period, the residuals of the observed weekly PAR and turbidity
values to the expected PAR and turbidity values were calculated, using
the means of the observed weeks across all years as expected values. The
trend-analysis of PAR and turbidity over the sampling period was done by
simple linear regression over time using the R base functions “lm” (R
Core Team, 2019).