Study design
In the present study macroalgal biomass and species distribution
investigations from 1996/98 (Hop et al., 2012) and 2012-14 (Bartsch et
al., 2016) were repeated between 21 June and 15 July 2021. A detailed
overview of response variables and a comparison of methods used in the
previous studies is given in Bartsch et al. (2016). To guarantee a
comparable dataset and facilitate future monitoring at the study site,
the survey methods followed the established protocols given in Bartsch
et al. (2016) and Paar et al. (2016). In 2021, two successive diving
campaigns were conducted of which the first was a semi-quantitative
(visual) investigation on the depth distribution and abundance of
dominant brown algae between 2m and 20m depth. In contrast, the second
campaign was an extensive quantitative study with complete destructive
sampling at 2.5, 5, 10 and 15m depth and multiple associated
measurements of the collected material (fresh and dry weight, leaf area
index, kelp demography, blade carbon and nitrogen content). In both
diving campaigns sampling depths were measured by a bottom timer
(Scubapro Digital 330) and afterwards corrected to chart datum (lowest
astronomical tide) according to the local tide calendar
(https://www.kartverket.no/en/at-sea/).
Macroalgal fresh weight and the lower depth distribution of biomass
dominant brown algae were analyzed across all three timepoints (this
study, Bartsch et al., 2016; Hop et al., 2012). Mean age and density per
m² of adult kelps as well as the leaf area index of biomass dominant
species or groups was investigated in 2021 and compared to 2012/13 data
(Bartsch et al., 2016). Furthermore, new aspects targeting the
ecological differences between kelp species (holdfast, stipe and blade
dry biomass as well as blade carbon and nitrogen content) were analyzed
in the recent study.
In the Arctic and sub-Arctic, two morphologically very similar digitate
kelp species (Laminaria digitata and Hedophyllum nigripes )
may grow side by side and only DNA barcoding can reveal secure species
identification (Dankworth et al., 2020), which has not been conducted
here. Thus, this species complex is referred to as ‘Digitate Kelps’
throughout this study, although the ecology of these two species is
probably quite different (Franke et al., 2021).