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).