Water temperature
The average on-site water temperature during the sampling period showed a typical bell-shaped curve, with a mean value of 14.03 °C (SD 1.4) over the sampling days in June, 16.70 °C (SD 0.2) in July, a maximal value of 18.05 °C (SD 0.4) in August and slightly lower values of 17.43 °C (SD 0.4) in September (Fig. 3).
Substratum types and algae
Substratum type
Of the 41 sampling stations, eight were classified as substratum type rock, 13 stations as cobbles and 20 stations as large pebbles. All substratum types were found at each site and every transect direction, whereas the category large pebbles was predominant in the northeasterly direction.
Laminaria hyperborea
A dense (20–50 m−2) substratum coverage of large kelp (mainly L. hyperborea) was observed in June, July and August in the categories cobbles and rock (Table 1). The growth of kelp in the area was generally closely associated with these two substratum categories, and no or only few kelp plants were found on the substratum category large pebbles at any time (Table 1). In September, the kelp died back in the entire area, dropping quickly to a level of only few to no plants m−2.
In terms of structural complexity, L.
hyperborea reached its maximum stipe length (>50 cm) in June to August, with an overall height of the entire plant of approximately 2–3 m. In June and July, the leaves were broad and without fouling, and the stipes had a diameter up to 3 cm with complex and broad holdfasts. In September, the older plants began to collapse, and only the multiannual holdfasts, sometimes with the stipes, remained; thus, the structural complexity of the kelp habitat significantly decreased.
Over the months, the Laminaria density showed no significant effect on the fish (ANCOVA, F = 2.669, df = 2, p = 0.0725) or crustacean abundances (ANCOVA, F = 0.73, df = 2, p = 0.484). Because the substratum types influenced the fish abundances significantly, we analysed a possible impact of the Laminaria density over the different substratum types. The density of Laminaria is negatively correlated with the fish abundance (ANCOVA, F = 4.898, df = 2, p = 0.0086), and the substratum large pebbles with the lowest density of Laminaria contained the most fish.
Red algae
Following the same pattern as the kelp, a dense growth of bushy or branched red algae was observed on the rocky and cobble substratum in June to August, while no or sparse growth was found in the substratum large pebbles (Table 1). In contrast to the kelp pattern, red algae plants remained present in September on the category rock, whereas in the areas with cobbles and large pebbles, only sparse or no red algae were found.
The density of red algae had no significant effect on the fish (ANCOVA, F = 0.287, df = 1, p = 0.593) and crustacean (ANCOVA, F = 0.294, df = 1, p = 0.589) abundances over the months. Furthermore, no effect of red algae was observed over the substratum types on fish abundances (ANCOVA, F = 0.577, df = 1, p = 0.4487).
Fish and crustacean abundance
A total of 510 fish and 2,708 macro-crustaceans were counted over the entire sampling period. With a total of 328 quadrats (1 × 1 m) analysed, a mean fish density of 1.55 ind. m−2 (SD 1.0) and a mean crab density of 8.26 ind. m−2 (SD 3.6) were calculated and averaged throughout the entire study.
When analysing the temporal (month) and spatial (substratum categories) effects in detail, a significant increase in the fish and crustacean abundance was observed over the months, with the highest average values of 2.79 (SD 1.9) fish m−2 (ANOVA, F = 19.83, df = 3, p < 0.0001) and 12.84 (SD 5.5) crustaceans m−2 (ANOVA, F = 42.57, df = 3, p < 0.0001) in September, over all of the substrate categories.
The fish were significantly more abundant in the large pebbles substratum to cobbles and rock (Fig. 3a, ANOVA, F = 5.41, df = 2, p = 0.008), and this effect was most prominent in September (Bonferroni post hoc test: p < 0.01, Fig. 3a). Summarised over all of the months, the large pebbles region contained 42.3 % (SD 2.2) of the total fish abundance and therefore significantly more fish (Nemenyi post hoc test, k = 2, p = 0.05) than the cobble substratum (27.2 %, SD 3.0). In terms of the fish abundance, the rocky substratum (30.5 %, SD 1.8) contained intermediate values and did not significantly differ from either of the other substratum types (Fig. 4a).
In contrast, for the overall crustacean abundance, no significant substratum effect could be found (Fig. 3b, ANOVA, F = 0.93, df = 2, p = 0.4015). The highest percentages of crustaceans were counted in the category cobbles (36.64 %, SD 2.3), followed by large pebbles (34.82 %, SD 4.6) and rock (28.54 %, SD 3.9; Fig. 4b).
Species-specific distribution
Ctenolabrus rupestris
The goldsinny (C. rupestris) was the most abundant fish species in the area, with an average abundance of 0.5 ind. m−2 (SD 0.2). The goldsinnies were almost equally distributed throughout all of the three substratum categories, with 37, 33 and 30 % of their occurrence in the categories rock, cobbles and large pebbles, respectively (Fig. 5a). Most of the goldsinnies were counted in August and September, followed by July and June (Fig. 6a).
Pholis gunnellus
With an average abundance of only 0.24 ind. m−2 (0.1 SD), benthic gunnels (P. gunnellus) were clearly less abundant than goldsinnies, even though gunnels were the second most abundant species in the sampling area. Contrary to the suprabenthic goldsinny, P. gunnelus showed a distinct substratum affinity, with 50 % of the total number of sightings in large pebbles and only 31 and 22 % of the gunnels found in the substratum categories cobbles and rock (Fig. 5a). The high appearance in large pebbles was present in all of the months except June (Fig. 6a).
Taurulus bubalis
The long-spined sea scorpion (T. bubalis) was found in all of the substratum categories (Fig. 6a), with an average density of 0.19 ind. m−2 (SD 0.1), but was mostly found in the substratum large pebbles (44 % of the total number of sightings). Only 29 % of the fish were observed on the rocky substratum, and 27 % were found on the cobble substratum (Fig. 5a). However, both the substratum associations and the abundances of T. bubalis strongly varied among the months (Fig. 6a).
Pomatoschistus minutus
The sand goby (P. minutus; 0.19 ind. m−2, SD 0.2) was almost as abundant as the long-spined sea scorpion, and P. minutus was observed mainly in the substratum large pebbles (Fig. 5a). A total of 72 % of the sand gobies was observed in this substratum type, while only 17 % of the total number of this species was found in rocky habitats and 11 % was found in cobbles (Fig. 5a). P. minutus revealed a distinct seasonality, with no or only rare specimen observations in June and July and significantly higher mean abundances in August and September (Fig. 6a).
Callionymus lyra
We found only females or immature males of C. lyra (Wheeler 1978), with an average size of approximately 8–10 cm and a density of 0.17 ind. m−2 (SD 0.2; Fig. 5a). The dragonet was equally found in the substrata large pebbles (48 %) and rock (41 %) but only 11 % occurred in the substratum type cobbles (Fig. 5a). The dragonets showed the strongest seasonality during the sampling period; virtually, the only times dragonets were observed was in August and September (Fig. 6a).
Gobiusculus flavescens
The two-spotted goby (G. flavescens) was mainly present in September (Fig. 6a). During September, the gobies occurred with an average abundance of 0.15 ind. m−2 (SD 0.2) and were mainly distributed in the substrata cobbles (49 %) and large pebbles (47 %) but were only sparse in rocky areas (4 %; Fig. 5a).
Additional fish species
Besides the species described above, six other species that were less abundant were observed in the area. These species were Spinachia spinachia (n = 2 over the entire sampling period), Ciliata mustela (n = 1), Entelurus aequoreus (n = 1), Myoxocephalus scorpius (n = 4), Liparis spp. (n = 13) and Zoarces viviparus (n = 14). Because these species were only sighted occasionally, it was not possible to establish a reliable species–substratum association.
Galathea squamifera
The squat lobster (G. squamifera) was by far the most abundant crustacean species, with an average of 5.2 ind. m−2 (SD 2.3), and was more or less equally distributed over all of the substrata, with 30 % of the sightings in rock, 39 % in cobbles and 31 % in large pebbles (Fig. 5b). G. squamifera was observed in all of the months but showed a slight seasonality, with the highest occurrence in September (Fig. 6b).
Pisidia longicornis
With an average abundance of 1.9 ind. m−2 (SD 1.1), the long-clawed porcelain crab (P. longicornis) was the second most abundant crustacean species (Fig. 5b) and 49 % of the members of this species were observed in the category large pebbles. This species was also sighted in the category rock (23 % of the sightings) and in cobbles (28 %; Fig. 5b). Like the squat lobster, this species was most abundant in September, followed by August and July. In June, only a few members of this species were found (Fig. 6b).
Cancer pagurus
The edible crab (C. pagurus) was the third most abundant crustacean species, with an average abundance of 0.6 ind. m−2 (SD 0.1). Similar to the squat lobster (G. squamifera), the edible crab was found equally distributed in the substrata of cobbles (39 %), rock (29 %) and large pebbles (32 %; Fig. 5b). C. pagurus was observed in all of the months, with slightly higher abundances in August and September compared with June and July (Fig. 6b).
Pilumnus hirtellus
The hairy crab (P. hirtellus) was found in an average abundance of 0.4 ind. m−2 (SD 0.1) and showed the strongest substratum affinity, with 51 % of the individuals found in the cobble substratum followed by the rock category (31 %; Fig. 5b). The substrate large pebbles contained only 18 % of the observed members of this comparatively small species. The presence in the category cobbles was obvious in all of the months, except for September, when the hairy crab was mostly seen in the substratum rock (Fig. 6b). P. hirtellus showed no clear seasonality but was mainly seen in September, followed by July.
Additional crustacean species
In addition to the four decapod species described above, the following crustaceans were observed in lower numbers over the sampling period: Necora puber (n = 17), Liocarcinus spp. (n = 12), Pagurus spp. (n = 8), Carcinus maenas (n = 4) and Homarus gammarus (n = 2). Because these species were sighted only occasionally, it was not possible to establish a reliable species–substratum association.