Sample processing
After storage of ca. 8 months at -20°C, the 12 samples were shaken and mixed with a spoon to suspend DNA, after which two times 200 ml ethanol were filtered using a 100 ml microfunnel unit with a 0.45µM GN6 Metricel membrane. Filters were stored at -80°C. Subsequently, bulk samples were processed largely following the protocol outlined by Aylagas, Mendibil, et al. (2016). In short, samples were six to 13 times decanted in the lab using tap water. Decanting was repeated until no animals were recovered from the samples. All animals remaining on a 1mm sieve were collected and stored in ethanol. The remaining material after decantation was sorted in a cleaned tray and heavier animals such as bivalves were picked with clean tweezers and added to the decanted material in ethanol. From each of the four locations, one replicate was identified morphologically (ZVL-A, 120-B, 840-C, 330-C) to the lowest taxonomic level possible prior to grinding. Specimens were identified up to species level, except for juveniles which were identified to the genus level and specimens belonging to Nemertea, Anthozoa and Oligochaeta, which were only identified up to phylum, class and order level, respectively, following the certified protocol for macrobenthos identification (according to ISO16665:2014 and NMBAQCS’s ”Guidelines for processing marine macrobenthic samples: a Processing Requirements Protocol” Version 1.0). In total, 24 samples were available for further molecular processing (12 ethanol samples and 12 bulk samples).