Characteristics of the sediment mussel matrix
The Limfjorden mussel site was characterized by a very patchy distribution of the mussel M. edulis (Fig. 4). Patch size ranged from small clumps of a few cm to large beds of mussels of >25 m. Based on the video transects the average coverage was estimated to be 27%. Average length of the mussels was 22.26 ± 0.32 mm. The length frequency distribution shows that mainly mussels of a single cohort are present (2-year old), with a few individuals of an older cohort (Fig. 5). Of the 25 box cores taken, nine contained none or only one individual of M. edulis. Average density of M. edulis was 3,911 ± 717 ind. m−2 (max. 10,000 ind. m−2); average biomass was 300 g AFDW m−2 (max. 1,011 g AFDW m−2).
The mussel site in the Oosterschelde was in a mussel culture plot of 4.5 ha area that was seeded with 5,000 tons fresh weight of mussels M. edulis in December 2003. Based on the video transects a coverage of >75% was observed (Fig. 4); box cores always contained mussels. Mussels had on average a length of 55.1 ± 0.66 mm; all belonged to a single cohort (Fig. 5). Average density of M. edulis was 616 ± 139 ind. m−2 (max. 1,525 ind. m−2). Average biomass was 615 g AFDW.m−2 (max. 1,315 g AFDW m−2).
Biogeochemical characteristics of the seafloor sediments
A significant effect of the presence of the mussels on grain size distribution and mud content (fraction <63 μm) was observed in all three study areas (Table 1; Fig. 6). In Limfjorden median grain size at the mussel site was 1.3× lower than at the control site. No mud was observed at the control site, whereas at the mussel site mud content averaged 13 ± 1.2%. At the small spatial scale within the mussel site, samples containing mussels had a significantly (p < 0.01) lower median grain size and higher mud content than non-mussel samples (Table 1). In the Oosterschelde, differences between the mussel site and the control site were more pronounced compared to Limfjorden, with a ~3× lower median grain size at the mussel site and a mud content of 41 ± 5.7% compared to zero (Table 1). In Ria de Vigo, the situation is more complicated as the sediment grain size changed along the depth gradient, but on average a higher mud content and lower median grain size was found along the transects in the mussel site than along the transects in the control site (Fig. 6). The difference is most expressed at the largest depths.
The chemical properties of the sediment showed similar trends as the trends observed for grain size and mud content, with significantly higher contents of POC, PON and phosphorus at the mussel sites than in the control sites (Table 1). In Limfjorden, values were 2 to 2.5 times higher at the mussel site and C:N ratio did not differ significantly between sites. At the small spatial scale within the mussel site, we observed also a significant difference (p < 0.01) with the m-site (samples with mussels) having a 1.5 to 2.5 times higher POC, PON and phosphorus content than the nm-site (samples without mussels). In the Oosterschelde larger differences were observed (factor 20, 15 and 50 for POC, PON and phosphorus, respectively; Table 1). C:N ratio was significantly higher at the mussel site. In Ria de Vigo POC, PON and phosphorus content varied with depth as did mud content (Fig. 6). Highest absolute POC, PON and phosphorus concentrations in our study were found in Vigo.
The chl a contents on average did not differ between the mussel bed and the control site in Limfjorden, whereas in the Oosterschelde chl a was ~40× higher at the mussel site (Table 1). In both Limfjorden and Oosterschelde, phaeopigment contents were significantly higher at the mussel site. At the small spatial scale within the mussel site in Limfjorden, we observed significantly higher chl a and phaeopigment contents at the m-site compared to the nm-site. In Vigo, compensating for depth, both chl a and (especially) phaeophytin a contents were higher at the mussel than at the control sites (Fig. 6).
Macrobentic diversity and community structure
Species richness and diversity
In Limfjorden, 23 species were observed at the mussel site; taxonomic groups included Annelida (74%, 17 sp.) and Mollusca (17%, 4 sp.), together with one species of echinoderms and one species of ascidians. Fourteen of these species were not found at the control site. At the control site, 19 species were identified; also here Annelida (63%, 12 sp.) and Mollusca (26%, 5 sp.) dominated, together with one species of echinoderms and one species of sea anemones. Ten species were not found at the mussel site. Epifaunal species were very rare at both sites: two species at the mussel site and one at the control site respectively. This was confirmed by the video transects. From these images we observed regularly the sea slug Philine aperta and the gastropod Hinia reticulata, and occasionally individuals of the crab Carcinus maenas and the sea star Asterias rubens. Species richness, diversity and evenness were similar at both sites (Table 2). At the small scale, however, within the mussel site a higher species richness was observed in the samples with mussels present, but with a significantly lower diversity and evenness (Table 3). In the m-site and the nm-site in total 20 and 12 species respectively were found.
In the Oosterschelde 54 species were identified at the mussel site; taxonomic groups included Annelida (58%, 31 sp.), Crustacea (26%, 13 sp.) and Mollusca (8%, 7 sp.), together with two species of echinoderms and one unidentified species of sea anemone. Forty of the identified species were not found at the control site. At that site 28 species were identified: Annelida (52%, 15 sp.), Crustacea (18%, 6 sp.), and Mollusca (15%, 4 sp.), together with two species of echinoderms. Thirteen species were not found at the mussel site. The number of epibenthic species was much higher in the mussel site compared to the control site (17 and 8 species, respectively). The video transects showed A. rubens and C. maenas as common species at the mussel site. Species richness, as well as diversity, was significantly higher at the mussel than at the control site, but evenness was lower (Table 2).
In Ria de Vigo a diverse macrofaunal community was observed, and not all individuals were determined at species level. In the mussel site 159 taxa were identified in the three transects (18 samples): Polychaeta (57%), Crustacea (21%), Mollusca (6%) and Echinodermata (6%) dominated. Sixty-three of the identified taxa were not found at the control site. In the control site, 152 taxa were identified in the two transects (12 samples). The same phyla dominated. Sixty-eight taxa were not found at the mussel site. The species richness in the reference site was 31.4 ± 2.26 per sample and in the mussel site 25.6 ± 2.18, but no significant difference was observed between the two sites, because of the large variability. Diversity and evenness were significantly higher in the reference site. No significant relation with depth was observed.
Abundance and biomass
In Limfjorden and Oosterschelde, the total mean abundance and biomass were significantly higher at the mussel site than at the control site, but this was completely attributed to the presence of the mussels (Table 2). Without mussels the mean abundance and biomass were significantly lower at the mussel site than at the control site in Limfjorden, whereas in the Oosterschelde the opposite (but non-significant) was observed. At the small spatial scale in Limfjorden, within the mussel site, the abundance and biomass at the m-site were significantly higher than at the nm-site (Table 3), but this difference was not significant anymore when omitting the mussels.
In Oosterschelde, endofaunal abundance in the mussel site (504 ind. m−2) was higher compared to the control site (357 ind. m−2), but endofaunal biomass was significantly lower; epifaunal biomass on the other hand was significantly higher in the mussel bed (Fig. 7).
In Ria de Vigo, total abundance did not differ significantly between the reference site and the mussel site (Table 4). Total biomass at the mussel site was significantly lower than at the reference site, but a large variability was observed because of the dominance of a few large individuals. No significant relation with depth was observed.
In Limfjorden, SF dominated the mussel site in density and biomass. At the control site, SSDF (mainly Heteromastus filiformis) numerically dominated, SF (Ensis) dominated in terms of biomass. The number of SSDF (H. filiformis) was significantly lower at the mussel site compared to the control site, whereas the number of SDF was higher. At the small spatial scale within the mussel site, the m-site had higher densities of SDF and predators than the nm-site. Small SDF like Polydora sp. and Syllidia armata and small predators like Harmothoe sp. only occurred at the m-site and not at the nm-site (Table 3).
In the Oosterschelde, SF dominated the mussel site in terms of density and biomass. At the control site SSDF (Scoloplos armiger) and predators (Nephtys cirrosa) numerically dominated. The biomass of SSDF was significantly lower at the mussel site, whereas endofaunal SDF biomass was higher. The biomass of epifaunal predators and omnivores was much higher at the mussel site than at the control site (Fig. 7).
In Ria de Vigo small polychaetes numerically dominated at the mussel site, mainly SDF (51%, Prinospio sp., Aphelochaeta marioni, Ampharete sp.) and SSDF (31%, H. filiformis) (Table 4). Predators (9%) and especially SF (3%) were of minor importance. SDF (47%, Spio sp., Prinospio sp., A. marioni, Paraonis fulgens) also dominated at the reference site. SSDF (19%, H. filiformis) were less dominant than at the mussel site, whereas SF (15%, e.g. Ampelisca sp., Actiniaria, Ophiothrix fragilis) and predators (13%, e.g. Lumbrineris sp., Sphaerosyllis sp.) were more abundant. Biomass at both sites was dominated by a few large individuals of SF, e.g. Acanthocardia tuberculata and Cucumaria frondosa at the mussel site, and Cucumaria elongata, C. frondosa, and Dosinia exolata at the reference site.
Benthic community structure
The MDS ordination analysis for Limfjorden showed a clear separation of samples between the control and mussel sites. This separation remained after removal of M. edulis from the data matrix (Fig. 8, ANOSIM p < 0.01). Species that were responsible for the significant dissimilarity (66%, without mussels) were the polychaetes H. filiformis and S. armiger (characteristic for control site) and Nereis sp. (mainly Nereis succinea), Harmothoe sp., S. armata, Polydora sp. and Capitella capitata (characteristic for mussel site; Table 2). The polychaete Pectinaria koreni, common at both sites, showed significantly higher densities at the mussel site. In addition, the samples from the control site were more similar within the site (average similarity 71%) than samples from the mussel site (similarity 52%). The community structure inside the mussel site had a rather large variability. This was to a large extent explained by the significant difference in community structure between the m- and nm-site (Fig. 8, ANOSIM p < 0.01).
In the Oosterschelde, the MDS (excl. M. edulis) indicated that the community structure of the control and mussel site significantly differed from each other (Fig. 8, ANOSIM p < 0.01). Species responsible for the significant dissimilarity (91%) were S. armiger, N. cirrosa and Magelona papillicornis (characteristic for control site) and Oligochaeta, Pygospio elegans, Streblospio shrubsoli, C. capitata and Harmothoe sp. (characteristic for mussel site; Table 2). Additionally, several epibenthic species were only observed in the mussel site (e.g. the amphipods Amphilochus neapolitanus, Abludomelita obtusata, Microdeutopus anomalus,
Microprotopus maculatus and the decapods Athanas nitescens, C. maenas, Pinnotheres pisum; the latter living symbiotically in mussel shells).
In Ria de Vigo, the community structure significantly differed between the two sites (Fig. 8, ANOSIM p < 0.05), but both sites showed a large variability. The community of the mussel site was numerically dominated by H. filiformis and Prinospio sp. These two species also occurred at the reference site, but in lower abundances. Other species contributing to the dissimilarity between the mussel site and the reference site were Ampharete sp., Oligochaeta, C. capitata (characteristic for the mussel site) and Spio sp. and Ampelisca sp. (characteristic for the reference site; Table 4).