Macroalgal biomass
For the quantitative diving campaign, a main transect was installed in the sublittoral off Hansneset, marking the vertical gradient between 2.5 and 15m depth below chart datum. Proceeding laterally ±15m from this main transect, complete destructive sampling was performed by scientific diving at 2.5, 5, 10 and 15m depth. At each depth horizon the attached macroalgal biomass of all kelps and understory seaweeds was collected within randomly chosen 1 x 1m quadrats (n = 3). Additional sampling was performed in the infralittoral fringe level (0m) using 50 x 50cm quadrats (n = 3) and data was extrapolated to 1m² before analysis. Algae were sampled in net bags and kept in seawater filled barrels for transport. In the laboratory, biomass samples were stored in flow through seawater tanks before further processing.
Algal material was sorted to species level, blotted dry with cotton towels and fresh weight (FW) as well as dry weight (DW) was determined (Mettler Toledo PB3002_S/FACT Delta Range, Max 600g/3100g, d = 0.01g/0.1g, Germany). Adult kelps were separated into holdfast, stipe and blade prior to weight measurements. Juvenile individuals with a stipe length ≤5cm were weighed as whole individuals. Dry weight of species was recorded after drying over night at 70°C in a drying oven (Termaks, Series TS9000, Model TS9135). Due to logistical reasons, the DW could not be documented for every adult kelp specimen. For adult kelp DW representatives of each species and stipe length category were dried depending on their occurrence to represent the properties of the local kelps as realistic as possible (Alaria esculenta : n = 8 (5–15cm, 31–50cm, >80cm); n = 10 (16–30cm), n = 9 (51–80cm); ‘Digitate Kelps’: n = 9 (5–15cm), n = 10 (16–30cm, 51–80cm), n = 14 (31–50cm), n = 3 (>80cm); Saccharina latissima : n = 3 (5–15cm), n = 8 (51–80cm, >80cm)). The resulting regression formulas between FW and DW expressed high determination coefficients (majority R² > 0.9) and were used to calculate the DW of the remaining specimens (Appendix 1). All adult kelp individuals ≥ 2 years collected from 2.5m and 5m depth were analyzed for holdfast, blade and stipe DW as well as blade:stipe DW ratio.
Biomass dominant species were organized in two categories as well as six groups (Kelps: A. esculenta , ‘Digitate Kelps’, S. latissima ; Understory seaweeds: ‘Other Phaeophyceae’, Rhodophyta, Chlorophyta) and the historical datasets were reanalyzed accordingly for comparison. The following species are classified in the group ‘Other Phaeophyceae’ (brown algae): Battersia arctica , Chorda filum , Chordaria flagelliformis , Desmarestia aculeata ,Desmarestia viridis , Dictyosiphon foeniculaceus ,Ectocarpus sp., Fucus distichus , Halosiphon tomentosus , Haplospora globosa , Laminaria solidungula , young Laminaria spp., Pylaiella sp., Sacchorhiza dermatodea , Scytosiphon sp., Sphacelaria plumosa . In the group of Rhodophyta (red algae) the following species are pooled:Coccotylus truncatus , Cystoclonium purpureum ,Devaleraea ramentacea , Euthora cristata , Odonthalia dentata , Palmaria palmata , Phycodrys rubens ,Plocamium cartilagineum , Porphyra sp./Pyropia sp.,Ptilota sp., Rhodomela sp., Turnerella penneyi . Some specimens of Devaleraea ramentacea were densely covered by brown Elachista fucicola epiphytes which could not be removed but were considered negligible relative to host biomass. The group Chlorophyta (green algae) contains the following species:Acrosiphonia spp., Chaetomorpha melagonium ,Kornmannia leptoderma , Spongomorpha spp., Ulva sp. Calcified coralline algae were present along the depth transect but were excluded from identification as in the previous studies.