In this month’s ‘In Depth’ feature, we present the findings of a new ICM-CSIC-led study that synthesises almost three decades of monitoring of toxic and harmful phytoplankton along the Catalan coast.
The marine environment is not static, nor are the communities that inhabit it. A synthesis of almost three decades of monitoring of toxic and harmful phytoplankton in Catalonia shows that the dynamics of these microalgae have changed in markedly different ways along the Catalan coast: some species have become more frequent or expanded their distribution, whereas others that were once common have declined.
Harmful algal blooms (HABs) are events caused by microalgae that can produce toxins or generate other adverse effects on ecosystems, human health or economic activities. Not all HABs are toxic, and not all involve high cell abundances. Monitoring and surveillance programmes must therefore be adapted to the species, habitats and periods of risk.
An article by researchers at the Institute of Marine Sciences (ICM-CSIC), recently published in the journal L’Atzavara, reviews this historical record and analyses the main ecological patterns observed, to improve coastal monitoring and management.
A coastline with different degrees of confinement
One of the main conclusions of the study is that offshore and coastal phytoplankton communities differ in their composition and dynamics. Moreover, these communities are not distributed uniformly along the coastline. Water residence time and exchange determine which species can proliferate and accumulate in each area. More enclosed environments—such as harbours, bays, coastal lagoons and highly sheltered beaches—have very limited water exchange. This favours cell retention, allowing certain harmful species to persist in these environments.
‘The marine environment is not a homogeneous system. Offshore communities differ markedly from coastal communities, and each harbour, bay or beach has its own particular conditions. This heterogeneity determines which species can proliferate in each location and must be considered in monitoring and management programmes,’ explains Esther Garcés, an ICM-CSIC researcher and lead author of the study.
This heterogeneity has a direct impact on aquaculture. Alfacs Bay is one of Catalonia’s main bivalve-producing areas. Continuous monitoring is essential: when toxin concentrations exceed the statutory limits, harvesting is temporarily suspended to protect consumers’ health.
An integrated view of the coastal environment
Traditionally, HABs have been studied by collecting water samples and examining them under a microscope. Over recent decades, new molecular sequencing techniques have been introduced that can detect species occurring at very low abundances or that are virtually indistinguishable by microscopy.
Monitoring has also been extended to benthic species living on macroalgae, as well as to the resting cysts of various species that accumulate in sediments. If sampling were restricted to the water column, these components would remain undetected. The research team therefore emphasises that integrating processes occurring in the water column and on the seabed is essential for effective risk management.
‘Molecular techniques have considerably expanded our detection capacity, but they must be complemented by microscopic observations, quantitative methods and toxicological analyses,’ explains Albert Reñé, an ICM-CSIC researcher and co-author of the study. ‘Growing concern about benthic blooms requires us to consider the coastal environment as an integrated system and not to restrict surveillance to conventional water-column sampling.’
Marked seasonality
Seasonal bloom patterns vary among species. In confined areas, these patterns appear to be largely influenced by hydrodynamics, temperature and other environmental variables, which can modify the onset, duration and intensity of bloom events.
‘Time series show that not all species follow the same trend,’ explains Nagore Sampedro, an ICM-CSIC researcher and co-author of the study. ‘For example, Alexandrium minutum has increased the frequency and intensity of its blooms in several harbours and confined areas, whereas Alexandrium pacificum and Gymnodinium impudicum, which were common in the late 1990s and early 2000s, have declined markedly.’
‘These trends can only be detected through monitoring time series maintained over several decades. Without systematic long-term data, it would be impossible to distinguish interannual variability from a sustained ecological trend,’ she adds.
The invaluable role of long-term time series
The main contribution of this work is to demonstrate the value of sustained long-term monitoring. Because there is no single cause or direct solution applicable to all bloom-forming species, one of the most effective protective measures is to establish early-warning systems based on long-term time series and adapted to each coastal environment. Only then is it possible to identify recurrent patterns, detect changes in species distribution and seasonality, and assess how communities respond to environmental variability.
Almost three decades of collaboration between ICM-CSIC and the Catalan Water Agency of the Government of Catalonia have made it possible to identify ecological patterns, detect changes in species distribution and seasonality, and progressively refine surveillance tools. This knowledge is essential for anticipating risks, guiding coastal management and protecting public health through evidence-based measures.