Contrasting Seasonal and Spatial Distribution of Native and Invasive Seaweed Revealed by Targeting Species-specific EDNA

Aim: , an invasive seaweed, has spread widely during the last century, impacting on local seaweed communities through competition and disturbance. Early detection of can help on its control and management. Environmental DNA (eDNA) has proved successful for early detection of aquatic invasive species but its potential use for seaweed remains understudied. We used a species-specific eDNA qPCR approach to investigate the spatial distribution, abundance, and coexistence of the invasive and three native species (, , and ) in the Cantabrian Sea.

Location: Bay of Biscay, Northern Atlantic Coast of the Iberian Peninsula; two ports, a beach and a rocky cliff.

Methods: We designed species-specific primers in barcoding regions targeting short fragments of the rbcL gene for the invasive species, and the elongation factor Tu (tufA) gene for the native species, to assess their spatial and seasonal distributions using quantitative real-time PCR in samples collected during summer, autumn, and winter.

Results: We found seasonal differences in the presence of the invasive and two of the native species, but did not detect at any point. Species distribution patterns produced with qPCR targeting species-specific eDNA coincided with the known distribution based on previous conventional sampling, with a seasonal alternance of and , and a marked dominance of invasive in ports, which are known hotspots for invasive species.

Main Conclusions: Our results demonstrate the utility of using eDNA for early detection and monitoring of invasive seaweed. Native and invasive spp. displayed significant seasonal and spatial differentiation that needs to be taken into account in risk management. Regular monitoring of ports and adjacent areas using eDNA should help to assess the potential expansion of invasive and the need for management interventions to avoid the displacement of native seaweed.