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).