Extreme Algal Bloom Finally Demystified by Croatian Scientists — Study Published by Springer Nature
The 2024 abnormal algal bloom event marked a significant ecological shift in the Northern Adriatic, reappearing after a 20-year hiatus. The article High-frequency observations during Adriatic mucilage event reveal unique phytoplankton traits and diversity response, published in Scientific Reports, Springer Nature, brings you the results of Croatian scientists’ research on the 2024 mucilage event.
Scientific Reports, Springer Nature, one of the leading scientific journals in the world, ranks as the third most-cited journal in the world (surpassing almost all niche journals). In 2024 alone, it recorded over 834,000 citations. Scientific Reports receives widespread attention in policy documents and the media, making it the perfect match for dissemination of results to oceanography specialists, data science and AI community as well as policy- and decision makers.
The research, conducted by the the Center for Marine Research Rovinj of the Ruđer Bošković Institute (IRB), utilised high-frequency sensors at oceanographic buoys, flow cytometry, complemented by SeaCras’ Coastal Intelligence digital system of satellite remote sensing of detection and classification, and the Algebra Bernays University for data science protocols and models validation, to characterise physical, chemical, and biological conditions during mucilage events in the Northern Adriatic in 2024.
The study revealed that this phenomenon is a complex biological response driven by specific environmental stressors and dominated by specialised phytoplankton traits, as well as large-scale drifting fronts classification and fine-scale water constituents concentration variations on scales even at 50 m in length.
Figure 1. Satellite image of the Northern Adriatic showing mucilage aggregates in September 2024. The satellite image is based on Copernicus Sentinel-2 Level-1 data acquired in 2024 and provided by the European Space Agency (ESA). Medium resolution Sentinel-2 processed images visualised with in-house Coastal Intelligence software (SeaCras, Croatia) showing concentration of chlorophyll-a shows areas with high phytoplankton.
Who Took Part in the Study
- Center for Marine Research, Ruđer Bošković Institute (Rovinj, Croatia)
- SeaCras Ltd. (Zagreb, Croatia)
- Algebra Bernays University
These partners, in a form of an innovative public-private endeavor, addressed the challenge of demystifying the enormous mucilage event that happened in summer of 2024 off the west coast of Istria — a shock for the local community and the numerous tourists and visitors of the destination. The scientists joined forces and resources, knowledge and technology to understand WHAT happened, WHY and WHERE it can have the most impact. Sadly, it’s mostly negative.
How the Research Was Conducted
The Center for Marine Research at the Ruđer Bošković Institute utilised a series of in-situ oceanographic, and meteorological measurements from their ODAS I and II buoys, alongside high-frequency flow cytometric observations. These data were systematically collected and processed depending on the method, from an hourly frequency to daily, with monthly averages, throughout 2024.
SeaCras complemented this single-point data using its Coastal Intelligence system, a digital platform for the detection and classification of sea surface and water column properties via satellite remote sensing. Several optical satellite payloads were deployed to characterise the physical, chemical, and biological conditions during the 2024 Northern Adriatic algal bloom events.
In addition to calculating mean concentrations over a larger area using ESA’s Sentinel-2 constellation data, Coastal Intelligence system processing was centered on the CIM ODAS I & II buoys. A grid of virtual sampling stations (or satellite-derived microlocation data points) was established to calculate the concentrations of water column constituents and sea surface properties on a finer spatial scale.
The fine-spatial processing by enabling virtual stations was conducted by using Planet Labs optical data. This approach enabled SeaCras and the Center for Marine Research to mitigate the effects of potential false-positive results and to better characterise the spatial heterogeneity of drifting mucilage aggregates.
Figure 2. View of the “mare sporco” phenomenon on the western coast of Istria in June 2024 (photo by Mihael Stojanović)
What We Previously Knew
- The Northern Adriatic Sea is a shallow and the northernmost part of the Mediterranean. It is under the direct influence of one of the largest freshwater inputs into the Mediterranean — the river Po.
- Meteorological and hydrological characteristics result in relatively long retention times of water masses in the Northern Adriatic.
- The combination of the aforementioned renders the area one of the most productive areas in the Mediterranean with strong west to east gradient. The area is well known for high nutrient concentrations and phosphate limitation throughout the year.
- This phenomenon called mare sporco (Italian for dirty sea) first attracted scientific attention in 1872 in the Gulf of Trieste.
So what are potential causes for this naturally occurring phenomenon to become abnormal? Are man-made influences to blame for triggering it or is it a climate-boosted occurrence?
What We Discovered About Algal Bloom
1. Environmental Drivers and Scale
- Climate Triggers: The formation of the algal bloom was directly linked to rising sea temperatures and significant salinity drops caused by freshwater discharge from the Po River.
- Massive Proportions: Coastal Intelligencesystem by SeaCras revealed that at its peak (June 2024), “macro-scale” mucilage fronts extended over 20 kilometers. While medium-spatial resolution imagery allowed for the evaluation of large-scale mucilage aggregates and the prediction of their migration, it could NOT estimate, within established confidence limits, the observed single-point, hourly variations of water constituents, provided by immersed real-time monitoring sensors on ODAS buoys.
- High-spatial and temporal variation analysis using high-resolution optical payloads revealed extreme spatial heterogeneity on a small scale across the sea surface, with distinct patches detectable at scales smaller than 20 meters. Notably, these algal bloom patches have an optical footprint significantly distinct from the hydrocarbon slicks known to appear in the Northern Adriatic due to anthropogenic fossil fuel activities.
- Our virtual estimation protocol, based on data comparisons with single-point, in-situ measurements of water constituents from sensors on ODAS buoys has, for the first time, demystified the alternating variations in optical properties recorded at these stations. The 20-meter variation in the concentrations of water constituents (e.g., chlorophyll-a (Chl-a) or total organic carbon (TOC)) worked in favour of the Center for Marine Research’s hypothesis: within the aggregates themselves, there is a complex distribution of ‘living’ and ‘dead’ phytoplankton within the carbohydrate (hydrocarbon) matrix.
Figure 3. The CIM ODAS II buoy with water constituents sensors
2. Phytoplankton Dynamics and “Unique Traits”
- Taxonomic Shifts: The study identified a progression of specific microphytoplankton taxa — Cerataulina pelagica, Cylindrotheca closterium, Thalassionema spp., and Gonyaulax fragilis — which dominated different phases of the event.
- Morphological Changes: Unlike healthy autumn blooms characterised by high diversity and chain-forming colonies, mucilage-associated blooms were defined by low diversity and a shift toward single, thicker, and more complex cells.
- Biomass Increases: Microphytoplankton biomass and abundance surged as mucilage advanced toward the eastern Adriatic coast, suggesting a self-reinforcing biological cycle.
3. Physiological and Chemical Anomalies
- Oxygen Depletion: A remarkable drop in surface water oxygen was recorded at the onset of the event. This raises critical questions about the role of microbial respiration and the potential for localised hypoxia.
- Organic Release: The accumulation of mucilage, essentially a polymer of exuded sugars, was likely fueled by both the active secretion of organic matter from blooming species and the degradation of dead cells.
- The Role of Cyanobacteria: While microphytoplankton (diatoms and dinoflagellates) were the primary drivers of the aggregate phases, cyanobacteria remained a constant but non-varying background presence, indicating they were not the catalyst for the event.
Figure 4. Mucilage aggregates in the Adriatic during the 2024 extreme algal bloom
Digital Twin as Key Technology Advancement in Detecting Algal Bloom
The integration of different types of sensors, in situ sampling, and satellite imaging revealed subtle phenomena at the study locations, as well as specific patterns across the entire Northern Adriatic. By combining these data sources within a Digital Twin of the sea, researchers were able to simulate conditions in near real time, test different scenarios, and better understand complex marine dynamics. This approach significantly improves the ability to predict changes, support decision-making, and respond more effectively to environmental risks.
Figure 5. High spatial resolution PlanetScope optical imagery over coastline of the city of Poreč. The data were processed and visualised using the in-house Coastal Intelligence software developed by SeaCras, Croatia. Image on the left shows migrating mucilage aggregates.
Who else can benefit from these findings?
Our insights highlight the key role of microphytoplankton and single cells in mucilage dynamics and the influence of environmental factors like temperature, wind and freshwater inputs on phytoplankton structure and biomass in coastal ecosystems.
- Advancing the knowledge and capacities of the scientific community and sharing expertise with industry partners.
- Tourism industry — early warning and short term forecasting of the such radical event for dissemination and public engagement
- Fishermen in activity planning and choosing of appropriate tools. For example nets are almost impossible to use when an algal bloom event is in its intense phase.
- Governing bodies and environmental agencies in estimating impact on environment and the influence on the fish stock for that and upcoming years.
The Next ‘Big Thing’ for The Partners
The partnership between the Center for Marine Research in Rovinj, part of the Ruđer Bošković Institute (IRB), and SeaCras is continuing in several directions where the capacities of both sides can be increased in greater synergy;. e.g. through the development of various early-warning systems based on the concept of a digital twin of the sea, seabed mapping, and public health applications for detecting different pollutants that harm coastal areas and the sea as a whole, such as microplastics, fecal contamination, and more.
In this way, Croatia is demonstrating strong internal capacities in this area. Moreover, it is pushing boundaries and setting standards of good practice for public–private partnerships.
About the Team
SeaCras is a Croatian company specialised in monitoring the sea through digital technologies, using satellites and artificial intelligence. SeaCras’ Coastal Intelligence software covers detection and analysis of a range of marine biochemical indicators, marine emissions, such as MARPOL pollutants, as well as underwater noise and GSG.
The company has been internationally recognised as one of the top five maritime technologies in the world by the World Economic Forum in 2024, and in 2025 was ranked among the top 10 most prominent European space startups by VC investors, according to Sifted.
Also in 2025, SeaCras was included in the Batch 6 of the Cassini Business Accelerator for the European Union’s space sector. Cassini Business Accelerator is an elite program by the European Space Agency and EUSPA, under governance by the Directorate-General for Defence and Space (DG DEFIS) of the European Commission.
Figure 6. From sea to screen: Ruder Bošković Institute researchers in the field work (left) and the SeaCras team analysing satellite data from the ‘virtual station’ (right).
The Center for Marine Research in Rovinj, part of the Ruđer Bošković Institute (IRB), leads a team of scientists that participates at a global level in the most important projects, such as the Euro-Argo, part of a global mission, and consistently brings excellence to the research and development of operational oceanography in the Adriatic.
The achievement regarding the abnormal algal bloom is the result of a strong joint effort by colleagues across all participating institutions, who combined their expertise, data, and methodologies to design, implement, and deliver this study at the highest scientific and professional standards. Through close collaboration and continuous knowledge exchange, the team ensured that the research reflects both scientific excellence and real-world applicability.
Source/References
Vlašiček, I., Pfannkuchen, D.M., Tanković, M.S., Špadina, M., Kopal R. Pfannkuchen, M. et al. High-frequency observations during Adriatic mucilage event reveal unique phytoplankton traits and diversity response. Sci Rep, Springer Nature (2025). https://doi.org/10.1038/s41598-025-31369-4
