Abstract
Kelp farming in the North Atlantic is emerging as new sustainable source of food, nutraceuticals and biostimulants. Seaweed biofouling by epibiont taxa, including bryozoans, hydrozoans, amphipods, bivalves, and gastropods is a key constraint in kelp yield and quality. The planktonic stages of these species belonging to meroplankton precedes visible settlement on seaweed fronds. Thus, the precise taxonomic detection of the planktonic stage and the assessment of temporal alignment with the biofouling stage could act as an early warning helping farmers plan harvesting times. To achieve this, monthly samples of plankton and kelp fronds were collected from a Scottish kelp farm during a kelp farming period (September 2021-July 2022). The taxonomic identity of kelp biofouling epibionts was established by microscopy and DNA-barcoding. The taxa were then traced in the plankton samples using both microscopy counts and eDNA-metabarcoding. Taxonomic identification of kelp epibionts via COI-based barcoding agreed with microscopy findings in 14 out of 19 specimens and helped refine the taxonomic resolution from family and genus down to species level. Out of the 20 epibiont species identified from the kelp fronds, 10 were identified via microscopy at phylum, class or genus level and 10 were identified via eDNA metabarcoding at species level. Microscopy counts of hydromedusae, bryozoan cyphonautes and bivalve larvae peaked in the plankton two weeks before the maximum coverage on Saccharina latissima fronds. Maxima of eDNA reads and microscopy abundances did not coincide temporally for the same taxa. Depth-stratified frond surveys showed that kelp segments that were deeper in the water column harboured the highest hydrozoan colonisation, indicating either depth-related colonisation processes, kelp age-related exposure effects or UV avoidance. The results indicate that the combination of molecular diagnostics and conventional microscopy improves the sensitivity and specificity of biofouling detection, facilitating earlier and more informed farm management decisions.