Yellow sea phenomenon off Šibenik reveals new insights into Adriatic ecosystem

Martinska near Šibenik (Photo credit: Ruđer Bošković Institute)

At the beginning of April 2026, the sea off Martinska near Šibenik briefly turned yellow, creating a striking surface layer that, from a distance, resembled pollution.

However, scientists confirmed the phenomenon was caused by pollen, not oil, harmful algal blooms or industrial contamination.

Carried by wind from surrounding vegetation, the fine yellow particles settled on the Adriatic Sea, forming a visible trace of springtime atmospheric activity.

For researchers at the Ruđer Bošković Institute, pollen is more than a seasonal allergen. It connects land, air and sea, transporting organic matter and chemical compounds into marine environments.

The observations form part of the POLLMAR project, supported by the Croatian Science Foundation, which investigates how pollen enters the Adriatic and influences coastal biogeochemical processes.

In April 2026, a field microcosm experiment was carried out at the Martinska Research Station near Šibenik under Dr Slađana Strmečki Kos.

Scientists created 12 seawater incubations, adding varying amounts of Aleppo pine pollen collected nearby. The samples were submerged for five days under natural conditions to observe how pollen affects seawater chemistry, microbial activity and organic matter release.

Earlier research by the team identified 89 different pollen types in the surface microlayer of the sea and recorded up to 205,000 pollen grains per litre of water during peak spring conditions.

These findings suggest that the sea surface is an active interface where atmospheric particles accumulate and interact with marine processes, rather than a passive boundary.

The Adriatic is naturally low in nutrients, meaning even small atmospheric inputs can influence primary production, the process by which microscopic organisms form the base of the marine food web.

Understanding how pollen contributes to these dynamics is increasingly important as climate change alters flowering seasons, wind patterns and the intensity of pollen transport across the Mediterranean.

Researchers say the aim is to better understand how land-based biological material affects marine systems at a chemical and ecological level.

The results from the POLLMAR project are expected to improve knowledge of how the Adriatic responds to changing atmospheric conditions and broader climate-driven shifts across coastal environments.