Northern Adriatic, one of the biologically richest areas of the Mediterranean Sea, has been experiencing dramatic ecological changes lately that affect not only its coasts and inhabitants, but also a fundamental part of marine life – phytoplankton. Although invisible to the naked eye, these microscopic organisms play a crucial role in sustaining life in the sea, as well as more broadly, as they participate in global climate processes. Recent long-term data analyses indicate changes in the composition, abundance, and seasonal dynamics of phytoplankton communities in this part of the Adriatic, all of which are connected with changes in the climate, human impact, and other environmental factors.

Phytoplankton – the invisible heroes of the deep sea

Phytoplankton is the primary producer of organic matter in the marine ecosystem. Through the process of photosynthesis, using sunlight and carbon dioxide, it produces oxygen and organic compounds that feed larger organisms. It is the basic link in the food chain – without it there would be no zooplankton, no small fish, nor large predators such as tuna or dolphins. Moreover, phytoplankton plays a key role in sequestering carbon from the atmosphere, thereby contributing to the regulation of the global climate.

In the northern Adriatic, where biological production is traditionally high thanks to shallow waters, the influx of nutrients from rivers, and intense water mixing, changes in the structure of phytoplankton also imply changes for the entire ecosystem. Long-term research shows that seasonal patterns of their appearance have changed, and the abundance of individual species has significantly decreased or increased, suggesting a profound environmental change.

Climate change as a catalyst for marine transformations

The rise in the average sea temperature, especially during the summer, is one of the main factors affecting the dynamics of phytoplankton. Warming of the surface layers of the sea reduces vertical water exchange, thereby limiting the influx of nutrients from deeper layers. In such an environment, species that can survive in nutrient-poor conditions have an advantage, while those requiring more nutrients lose dominance.

In addition to temperature, changes in sea salinity, increased UV radiation, changes in precipitation, and the influx of fresh water from rivers play an important role. All of this affects the delicate balance of nutrients and light, which are key factors for the photosynthesis and growth of phytoplankton cells.

Anthropogenic impacts and eutrophication

While on one hand there are phenomena such as a reduction in nutrients, on the other hand human activities further disrupt the balance. In coastal areas, especially those heavily exposed to tourism, the discharge of untreated wastewater into the sea occurs. Such discharges cause localized eutrophication – overfeeding of the sea that leads to explosive growth of phytoplankton, followed by a sudden die-off of cells, oxygen depletion, and the appearance of dead zones.

Furthermore, increased coastal urbanization, the construction of marinas and apartment complexes often involve the destruction of natural habitats such as Posidonia meadows, which play a key role in filtering the sea and stabilizing the coastline. The removal of such ecosystems leads to destabilization of the entire food chain.

Direct consequences on fisheries and the economy

Changes in the composition of phytoplankton have a direct impact on fisheries, as the availability of food for species with commercial value changes. If zooplankton populations decrease due to a lack of quality food, this results in a decline in the abundance of small fish such as anchovies or sardines. Consequently, predators such as mackerel, gilthead seabream, or sharks that depend on these species are also threatened. Such disturbances can, in the long term, endanger traditional fishing areas and cause economic damage to the local population.

Besides fisheries, the instability of the ecosystem also affects tourism. Phenomena such as sea blooms, especially those involving toxic species of phytoplankton, can damage the visual quality of the sea and cause concern among visitors, resulting in cancelled reservations and decreased tourism revenue.

Monitoring measures and accommodation in Rovinj

Monitoring the state of phytoplankton and environmental conditions is carried out continuously at several points in the northern Adriatic, and Rovinj is one of the key research locations. In this picturesque Istrian town, one of the most important centers for marine research is located. Accommodation in Rovinj can be found in seaside hotels, family apartments in the old town, or in modern tourist settlements, providing excellent infrastructure for those who want to combine science, nature, and relaxation.

Besides hosting numerous scientific teams, Rovinj is also an educational center that allows visitors to get to know the richness and fragility of the Adriatic marine ecosystems more closely.

Invasive species – silent enemies from afar

One of the problems that further burden local phytoplankton communities is the presence of invasive algae and other organisms that enter the Adriatic via ships' ballast water or through aquaculture. These species often have no natural enemies, spread rapidly, and outcompete native species. Invasive algae can alter the structure of the sediment, compete for light and nutrients, and in some cases produce harmful toxins that affect entire food chains.

Their suppression requires international cooperation and constant monitoring, as well as increased awareness among sea users, particularly boaters, fishermen, and marina owners. Education about the threat posed by invasive species must be made available to the broader public to encourage responsible use of marine resources.

Ocean currents and the role of the northern Adriatic

The northern Adriatic is not an isolated system, but represents an important node in the circulation of water within the entire Adriatic Sea. Changes occurring in its water column and ecosystems can have an impact on the central and southern Adriatic as well. Since phytoplankton reacts strongly to changes in the physical properties of the sea, its dynamics often reflect deeper changes that are yet to be manifested in higher trophic levels, including coastal communities and fish populations.

This part of the sea is particularly sensitive due to its shallowness and enclosure. This means that changes in temperature and salinity manifest more quickly than in deep oceans, and the ecosystem's recovery after stressful events takes longer. Therefore, monitoring the sea is not only of scientific interest but also a strategic issue for the protection of the entire Adriatic coast.

Phytoplankton diversity as a biological indicator

Phytoplankton communities consist of hundreds of species, each with different needs, sensitivities, and roles in the ecosystem. Over many years, scientists have managed to map the seasonal cycles of appearance of certain groups, such as diatoms, dinoflagellates, and cyanobacteria. Each species responds to changes in temperature, salinity, light intensity, the presence of nitrates, phosphates, and other nutrients. Changes in the dominance of certain groups often signal broader environmental changes.

When species that are characteristic of the spring months begin to dominate in the summer or disappear entirely, it indicates a disrupted balance. Similarly, the appearance of species that are not normally present in a certain area – often associated with transport via ballast water – can indicate the spread of atypical ecological conditions.

Collaboration between science and society in marine conservation

Although the scientific community plays a key role in analyzing and interpreting data, successful marine conservation is not possible without cooperation with the local population, the tourism sector, and decision-makers. Cross-sectoral cooperation is necessary, which includes regular reporting on the state of the sea, educating the local population about the importance of preserving natural resources, and actively involving citizens in marine observation through citizen science.

Cities such as Rovinj are already developing models for involving the public in monitoring and reporting ecological changes – from sea blooms to changes in fish abundance or unusual water coloration. Such tools enable the collection of valuable data in real time, which can then be compared with laboratory analyses and satellite images.

Transition towards sustainable marine utilization

Changes in the structure of phytoplankton communities are not an isolated problem, but a symptom of a broader crisis of sustainability of marine resources. Investments in technologies for wastewater treatment, stricter control over the discharge of industrial waste, and the introduction of green maritime policies are key elements of the transition towards sustainable marine management. In this process, the Adriatic Sea could become a model of good practice, provided that existing scientific insights are translated into concrete protective measures.

Rovinj, as a research and tourist center, is an ideal place for the development of models for the sustainable coexistence of humans and the sea. With a rich network of accommodation capacities, including eco-camps, tourist settlements, and private seaside apartments, the city has the potential to host educational workshops, scientific camps, and conferences dedicated to marine protection. Such offerings can add extra value to the tourist offer and contribute to the development of sustainable tourism that respects the natural cycles of the sea.

Although changes in phytoplankton may not be visible to the naked eye, their effects are felt throughout the entire ecosystem. Understanding these changes, monitoring them, and applying knowledge in practice are the only ways to preserve the sea that not only nourishes and employs but also connects generations of people with its beauty and importance.