Aligning Molecular and Microscopy Methods for Biofouling Detection of Farmed Seaweed (Alaria esculenta and Saccharina latissima)

Seaweed biofouling by epibiont taxa, including bryozoans, hydrozoans, amphipods, bivalves, and gastropods is a key constraint in kelp yield and quality. The taxa were traced in the plankton samples using eDNA COI metabarcoding. Sampling took place at the KelpCrofters Seaweed Farm (Pabay, Skye, Scotland) monitoring two kelp species (S. latissima and A. esculenta). Sampling was carried every month, starting from the seeding date on the 16th of September 2021 and ended in 30th of July 2022. At each time point, samples were collected from 3 GPS points which served as replicates. At each GPS point, samples of zooplankton, eDNA and fronds were collected as described below. Harvesting of both species took place on 22 May 2022. To extend insight into epibiont settlement behaviour beyond the harvesting of hatchery-seeded lines, further sampling of S. latissima was carried out on wild-seeded kelp growing on ropes attached to farm buoys. Peak wild seeding in this region typically occurs between December and February (personal communication), and epibiont activity during this period was therefore considered comparable to what would occur on the twine-seeded (hatchery-seeded) lines.