Potential indicator taxa: Nematoda and Polychaeta
There were 175 Nematoda OTUs and 38 Polychaeta OTUs in the OTUBM table. Polychaeta are commonly used for morpho-taxonomic analysis for benthic impacts of fish farming; however, interestingly here we found a much stronger correlation between Nematoda and pore-water sulphides than Polychaeta. The correlation between Nematoda OTU richness and pore-water sulphide concentration was -0.86, whereas that between Polychaeta OTU richness and pore-water sulphide concentration was -0.38 (Figure 4). The GLMM analyses showed a positive relationship between Nematoda OTU richness and distance from cage edge, but no relationship was identified between Polychaeta OTU richness and distance (Figure S4). Across all samples, Nematoda OTU richness increased along distance from cage edge. Within each farm, the correlation coefficients between Nematoda OTU richness and distance from cage edge varied from 0.55 to 0.86 (Figure S5).
Capitella polychaetes were found in both morphotaxonomic macrofaunal data and eDNA metabarcoding data; they were present in 29/42 and 25/42 of sampled stations as revealed by macrofaunal and eDNA metabarcoding analyses, respectively (Figure 5). Presence/absence ofCapitella at each station of BS, MI and VP agreed between macrofaunal and eDNA data. There was one station at MR (90 m) whereCapitella was present in macrofaunal data but not eDNA data; conversely, there was one station at BI (60 m) where Capitellawas present in eDNA data but not macrofaunal data. Patterns at PP were markedly different: macrofaunal analyses found Capitella at six stations, with a proportion in Polychaeta individuals ranging from 0.38% to 82.5% among stations, whereas Capitella were present only at two stations in eDNA data. Capitella was not detected at reference stations in either macrofaunal or eDNA datasets. The correlation coefficient between pore-water sulphide concentration and relative abundance of Capitella in eDNA data was 0.28 (Figure 4)