Every day, thousands of ships cross the oceans laden with cargo. Their holds carry containers, oil, or grain. But unwittingly, they also transport a hidden and dangerous cargo: millions of microalgae and other tiny organisms that travel in the ballast water, the water ships use to maintain stability during navigation.

Research led by the University of the Basque Country (EHU) has focused on three of Spain's most important ports—Algeciras, Bilbao, and Valencia—and uncovered a problem with enormous implications. The study shows that most of the phytoplankton present in the water of these ports is smaller than 10 microns.

The data is striking: in Algeciras, these organisms smaller than 10 microns represent, on average, 86% of the total phytoplankton biomass; in Bilbao, 78%; and in Valencia, 96%. The paradox is that, despite their absolute dominance and potential impact, this group is not regulated by the international regulations that control ballast water discharges. In short, it's as if the legislation has left out precisely the largest and, in some cases, most harmful group.

Serious flaws in the current legal framework

Since September 2024, the International Maritime Organization (IMO) has required ships to install treatment systems to reduce the number of living organisms they discharge into ports. However, the standard known as ‘D-2’ only considers two size categories: organisms larger than 50 microns and those between 10 and 50 microns. Everything below these sizes, such as pico- and nanophytoplankton, is ignored, with the exception of three bacteria that indicate fecal contamination.

The research conducted reveals that this “blind spot” is much more than a technical detail. Among the organisms smaller than 10 microns are species capable of producing toxins, forming harmful algal blooms, or surviving in extreme conditions inside ships' tanks. Once released into new ecosystems, they can spread rapidly and generate devastating impacts on biodiversity, fisheries, and human health.

The study also detected the persistent presence of 55 harmful algal species in the three ports. Of these, 36 are toxin-producing, and 19 have the capacity to generate large blooms that disrupt ecosystems. Some, such as Alexandrium and Pseudo-nitzschia, are well-known to those who monitor red tides in different parts of the world.

In fact, these species can cause everything from mass fish kills to shellfish poisoning, directly affecting public health and the fishing industry.

A Global Threat That Goes Unnoticed

Maritime trade moves more than 80% of the goods transported on the planet. Every year, ships displace some 10 billion tons of ballast water on their voyages. This movement is equivalent to moving around 7,000 different species every hour and causing a biological invasion every nine weeks, according to IMO estimates.

The phenomenon is not new. The zebra mussel in the Great Lakes of North America or the alga *Caulerpa taxifolia* in the Mediterranean are historical examples of how an accidentally introduced species can completely alter an ecosystem and generate costs in the millions. What is novel about the EHU's work is that it has systematically documented the magnitude of the problem in Spanish ports and, above all, has shown that international regulations are not designed to address this issue in its entirety.

Towards More Demanding Regulations

The study's conclusion is clear: the current legal framework urgently needs to be reviewed. On the one hand, the authors propose including a specific regulatory category for organisms smaller than 10 microns. On the other hand, they call for concrete limits for harmful algal species, regardless of their size, in the same way that limits are already in place for bacteria such as Escherichia coli and Vibrio cholerae.

In short, if we know that there are toxic species or species that produce algal blooms that are recurrently present in ports, it makes no sense that there are no specific restrictions for them.

The study also points out another weakness of the current system: the certification tests for ballast water treatment equipment are carried out under laboratory conditions that bear little resemblance to reality. In most ports, phytoplankton concentrations are much lower than those used in the tests, which creates a false sense of security regarding the effectiveness of the systems.

Science at the Service of Marine Biosecurity

To carry out this study, the research team combined three methodologies: optical microscopy, HPLC pigment analysis, and genetic sequencing using eDNA metabarcoding. This multi-method strategy made it possible to detect both the abundance and diversity of species present, including many that would go unnoticed with conventional techniques.

The combination of approaches made it possible, for example, to identify small species that are not on the radar of current regulations but are key to the dynamics of port ecosystems.

A Future Challenge

The study is part of the ECOTRANSEAS project, funded by the Ministry of Science and Innovation, in which the University of Cádiz and the Polytechnic University of Valencia collaborated, and is an example of how university research contributes to anticipating global risks.

In a context of climate change and increased maritime trade, the risk of dispersal of invasive species and harmful algae through ballast water will only increase. Therefore, the authors insist that it is necessary to adapt international regulations to current scientific realities and develop rapid detection tools that allow for timely action.

Reference:

Jone Bilbao, Christina Pavloudi, Sergio Seoane (2026) Unseen threat: Persistent picophytoplankton and harmful algae challenge ballast water management in major Spanish ports Marine Pollution Bulletin