Nuisance algae in ballast water facing international conventions. Longitudinal and transversal analysis of European ships employing eDNA Metabarcoding.

Ballast water is one of the main vectors of transport of nuisance species among marine ports. Although treatment or interchange seems to reduce the risks of containing harmful species, today there is not a 100% efficient treatment to avoid the spread of dangerous species. The understanding on what routes encompass a higher risk is important for designing targeted surveillance measures. In this study we have analysed ballast water from seven ships arriving to the port of Gijon (south Bay of Biscay, Spain) just after the entry into force of the International Ballast Water Management Convention in 2017. DNA Metabarcoding was employed for identification of exotic species and harmful algae. One ship carried DNA of 20 risk species inside the ballast water while other three ships contained DNA of only one risk species and for the other three ships, risk species were not detected. These results demonstrate the importance of keeping continuous ballast water surveillance until completely safe methods to remove undesired species are applied and agreed among the stakeholders and governments.