Unseen threat: Persistent picophytoplankton and harmful algae challenge ballast water management in major Spanish ports

Ports are critical hubs for global trade, yet they are also significant hotspots for the unintentional introduction and dispersal of aquatic organisms, primarily via ballast water. While the International Maritime Organization's (IMO's) Ballast Water Management Convention aims to prevent the transfer of harmful aquatic organisms and pathogens, this study reveals persistent shortcomings in its effectiveness, particularly concerning phytoplankton. The present research analyzed phytoplankton communities in three major Spanish ports (Algeciras, Bilbao, and Valencia) across three sampling campaigns in 2023. Employing a multi-method approach, including microscopy, HPLC pigment analysis, and eDNA metabarcoding, the study evaluated biosecurity risks and regulatory limitations. A key finding was the consistent dominance of picophytoplankton (<10 μm) in total biomass across all ports, averaging 86 % in Algeciras, 78 % in Bilbao, and 96 % in Valencia. However, despite their overwhelming abundance, ecological relevance and the potential of several of them for being harmful (e.g. Phaeocystis globosa ), the IMO's D-2 Performance Standard explicitly excludes this size range from its discharge criteria. Additionally, 55 harmful algal species (36 toxin-producing, 19 high-biomass) were persistently identified in all three ports, highlighting another critical regulatory gap, as the current regulations lack specific discharge limits for harmful taxa. The study also revealed a significant mismatch between natural port phytoplankton concentrations and Ballast Water Management System certification testing conditions, with most samples not reaching required challenge levels. Thus, results underscore the urgent need to revise IMO standards, advocating for new discharge criteria for phytoplankton smaller than 10 μm and implementing taxon-specific regulations for harmful algae. • Phytoplankton <10 μm, unregulated by IMO, dominated Spanish ports. • Harmful algae, lacking specific regulation, were consistently present in port waters. • BWMS tests do not reflect real port phytoplankton concentrations. • Regulatory gaps facilitate the unnoticed spread of potentially harmful phytoplankton. • A new <10 μm size class and taxon-specific limits for harmful taxa are needed.