Lower depth distribution of dominant brown algal species
The depth distribution of biomass dominant brown algae (Laminariales species, Sacchoriza dermatodea , Desmarestia spp.) was semi-quantitatively (visually) investigated by scientific divers in five parallel transects off the coastline covering the vertical gradient between 2m and 20m depth. The target distance between transects was 5m and transects were distributed over a horizontal width of approximately 30m. A 1 x 1m quadrat divided into four 50 x 50cm subquadrats was placed on the ground or above the kelps at every depth meter along the transect and species occurrence was documented as attached frequency within each subquadrat. This resulted in a relative frequency of 0 – 4 per depth and transect. Additionally, the visual presence of the species in the close surroundings of each quadrant was documented. In summary this generates in a maximum presence score of 5 for each species per replicate. The target replication was five per depth but due to depth corrections to chart datum this resulted in n = 3–8 for each depth and species. In replicates with more than 50% coverage of kelps, it was not possible to place the quadrat on the ground. Here the depth was corrected by the mean height (78cm) of the local kelp canopy (data not shown). To enable a comparison between the publications of Hop et al. (2012), Bartsch et al. (2016) and the present study, scores were transformed into percentage cover classes and the abundance of species was classified as rare (5-15% ≙ score 1-3), common (16-60% ≙ score 4-12), subdominant (61-80% ≙ score 13-16) and dominant (81-100% ≙ score 17-20). In 1996/98 these semi-quantitative classes referred to a combination of biomass and %-coverage values while in 2014 and 2021 a combination of relative frequency and presence/absence data was applied. Furthermore, investigated depth levels varied between time points (1996/98: 20 – 8m; 2014: 20 – 8m; 2021: 20 – 2m) andDesmarestia spp. includes D. aculeata and D. viridis as in Bartsch et al. (2016).