Lower depth distribution of kelp and kelp-like species over time
A comparison of semi-quantitative data on lower depth distribution and abundances of biomass dominant brown algae species over time is illustrated in Figure 5. An overall upward shift in lower depth distribution limit and a noticeable decrease in species abundance since 1996/98 is evident for all investigated species (A. esculenta , ‘Digitate Kelps’, S. latissima , Saccorhiza dermatodea ,Desmarestia spp.). This change is especially prominent in ‘Digitate Kelps’ and S. latissima . The lower distribution limit of ‘Digitate Kelps’ decreased severely by 10m over time from 15m in 1996/98 over 10m in 2014 to 5m in 2021. Similarly, the recorded depth for lowest occurring S. latissima specimens decreased by 7m over time from 16m (1996/98) over 14m (2014) to 9m (2021). For both taxa the shift in abundance which was already observed between the 1st and 2nd study continued in 2021 and these species were not classified as dominant or sub-dominant any more.
In contrast to the other two kelp species, the lower depth distribution limit of A. esculenta remained relatively unchanged and varied between 15m (1996/98), 18m (2014) and 13m (2021). However, also A. esculenta abundance decreased throughout the investigated time points as the species was sub-dominant at 12m in 1996/98 while this abundance class moved upwards to 8m and 9m in 2014 and 2021, respectively.
Similar to A. esculenta , the lowest distribution limits ofSaccorhiza dermatodea and Desmarestia spp. only changed slightly over time. However, despite their stability in depth distribution, these species exhibited a decrease in abundance, which was especially strong in Desmarestia spp. While Desmarestiaspp. had been mostly dominant to sub-dominant in 1996/98 between 20m and 9m, these abundance classes were not recorded at any depth level in 2021.