Hydroids of kelp beds
Three sites were located at different distances to active tidal glaciers and differed with respect to glacier disturbance levels. There were clear differences in sedimentation rates of mineral particles. Both mineral and organic sedimentation rates were highest in Isbjørnhamna—the most affected by the tidal glacier (146.53 mg dm−3 day−1 ± SD 103.68 mg dm−3 day−1 and 20.36 mg dm−3 day−1 ± SD 12.24 mg dm−3 day−1, respectively) and reached the lowest value in Gåshamna—the site free from direct impact from glaciers (12.73 mg dm−3 day−1 ± SD 4.75 mg dm−3 day−1, and 5.27 mg dm−3 day−1 ± SD 2.59 mg dm−3 day−1, respectively). Numerous icebergs (including large grounded ones) were observed at Hyrneodden during the entire sampling period (5 weeks), while at Isbjørnhamna, the icebergs were observed in lower numbers and only occasionally. No icebergs were observed at Gåshamna.
Hydroids were present on collected macroalgal thalli occurring with a frequency of 79% in samples at Gåshamna, 63% at Isbjørnhamna, and 28% at Hyrneodden.
The species accumulation curves plotted against the sampling effort for the three sites did not stabilize toward asymptotic values (Fig. 3). The 95% confidence intervals for the observed number of taxa at Isbjørnhamna and Gåshamna overlap indicating no significant difference in the total species richness between the two sites. The total number of species was the lowest in Hyrneodden and differed significantly from the other two sites.
The maximum total species richness was noted at Isbjørnhamna (23 species) (Fig. 4). However, there were 12 species occurring at low numbers (uniques and duplicates). A total of 19 species were observed at Gåshamna (with three species at low number of records), while only ten species were recorded at Hyrneodden (with half of them being uniques or duplicates—see “Methods”) (Table 2).
There were significant differences in hydroid sample species richness among the studied sites (Kruskal–Wallis test: H = 83.85, P < 0.05). The post-hoc pair-wise tests revealed significant differences in sample species numbers between two pairs of sites: Gåshamna—Isbjørnhamna and Gåshamna—Hyrneodden (Mann–Whitney U-test, P < 0.05), whereas hydroid sample species richness at Isbjørnhamna and Hyrneodden did not differ significantly (Mann–Whitney U-test: P > 0.05).
The highest mean sample species richness was found at Gåshamna (Fig. 5).
The highest mean frequency of hydroid species occurrences was also noted at Gåshamna (10.52%), while at Isbjørnhamna and Hyrneodden values were lower (3.79 and 2.55%, respectively). The most common species occurred with the frequencies exceeding 20% at Gåshamna and Isbjørnhamna, while no species were recorded with frequency over 10% at Hyrneodden (Table 3).
Hydroids in deeper subtidal
Hydroids were present in 79% of grab samples taken at the Outer stations and in 7% of samples taken at the Inner stations. The total number of observed hydroids taxa was 20. All of them were recorded at the Outer site (fjord entrance), while at the Inner site (glaciated bay) only two species were noted (Fig. 4). Most of the species (18 species) belonged to the order Leptothecata. Anthoathecata was represented by only two species (E. annulatum and Sarsia sp.) present only at the Outer site.
Sample species richness varied significantly between the two sites (Kruskal–Wallis test: H = 23.58, P < 0.05) and reached on average 5.5 species per sample at the Outer transect and 0.1 species per sample at the Inner transect (Fig. 6).
Hydroid species composition varied between samples of the Inner and Outer sites. Symplectoscyphus tricuspidatus was the most common species at the Outer site (frequency 67%) (Table 3). Abietinaria pulchra, Eudendrium annulatum, Sertularia schmidti, and Sertularia cupressoides) occurred at frequencies over 50%. At the Inner site, both Gonothyraea loveni and Lafoeina maxima occurred with 7% frequency.
Most of the specimens collected were not attached to any substrate. Due to the sampling methods delicate colonies of hydroids are often broken off, although if the coenosarc and hydrant are present the specimen was treated as live-collected. Among the few that were found with substrata, they attached to rocks, bryozoans, two species of other hydrozoans, shells of bivalves and barnacles.
In dredge samples, species richness changed according to the location within the fjord transect. It was lowest (one and zero species per sample) for stations (F and G respectively) in the glaciated bay and sharply rising with increasing distance from the inner fjord. The highest values were at stations A and B (14 and 15 species per sample correspondingly) at the fjord entrance (Fig. 4). The only species collected in the inner fjord was S. argentea. Only one representative of the order Anthoathecata, E. annulatum, occurred exclusively at the outer stations.
The differences in species composition among the sites under the influence of glacial impact (Inner, F and G) and those located in the outer part of the fjord, not affected by the glacial impact (Outer, A, B, D, and E), were significant and high (one-way ANOSIM pair-wise test: R = 0.58, P = 0.01).