Has it ever occurred to you that you went to the seaside, all excited to enter the water, but you were soon disappointed by the bubbly and discoloured vista of the coast? If yes, then you’ve faced the blooming of the sea, which is basically the rapid proliferation of microalgae, cyanobacteria or macroalgae. These events have been known since the Biblical times and have been occurring in regions characterized mainly by a high nutrient load also known by the term eutrophication. In the past, however, algae would bloom shortly in either spring or autumn or both, whereas now, these algal blooms have been observed much more frequently and for longer periods of time, with prevalence still in spring or autumn or both, suggesting that there might be other factors involved, such as global warming and altered river inflow.

A couple of weeks ago I visited the Slovenian coast, which is actually really short, measuring 46,6km altogether. The neighbouring countries are Croatia on the eastern side and Italy on the western side of the North Adriatic Sea. This upper section is delimited by the lowest lying cross-section which goes from Ancona, in Italy, to Zadar, in Croatia, and has an average depth of only 35m. Due to the high nutrient load that comes from rivers, among them the Po, aliments primary productors, and thus is considered the most productive basin of the entire Medtiterranean Sea. The water circulates superficially in a cyclonic way, entering the Adriatic Sea through the Strait of Otranto, flowing along the eastern coast to then turn around and go down along the western Italian coast. The small gyres within each section of the Adriatic vary depending on the season and intensity of the river inflows, but a peculiarity happens in the Northern section, where the north-eastern Bora wind contributes to the creation of North Adriatic Deep Water (NAdDW), a denser water that flows in depths along the western Adriatic and enters the Eastern Mediterranean as the most important source of bottom water.

But in the Gulf of Trieste, the poor current circulation and the presence of Trieste and Koper ports, where ships can unintentionally discharge organisms through the ballast water, makes the Gulf more exposed to introduction of alien species and frequent algal blooms. The species of algae can differ from place to place, and they can oftentimes produce thermostable toxic compounds that bioaccumulate in fish and shellfish tissues. This is potentially problematic because there are several mollusk farms within the Gulf, and some studies have observed that the farmed organisms accumulate, for example, a biotoxin produced by the dinoflagellate Dinophysis that causes diarrhetic shellfish poisoning (DSP) in humans. The second problem is that the discolouration of the sea doesn’t look natural, and sometimes this proliferation of algae either kills the other organisms living in proximity through the release of toxins, or through deoxygenation upon the species’ decomposition.

As I was speaking, I went to a coastal town called Izola with the intention of cycling around, sun tanning and freediving. Soon enough I was left irritated because the water did not look inviting at all. There were multiple stretches of red and orange, some bubbles here and there with decomposed jellyfish. Obviously, I decided to stay ashore and practice yoga instead. The following days, I had been actively searching through scientific reports in hope that I would find precise information about the event, but to be honest, there wasn’t much to be looking forward to, and so unsure of what to seriously take into consideration without any in situ research, I abandoned this little investigative project. A few days after, however, I saw a newspaper article with these bloody sea images from Izola, taken just around the same time as I was visiting. An expert from the Marine Biology Station of Piran reported that the bloom was provoked by a non-toxic dinoflagellate species, Noctiluca scintillans, that commonly blooms at the end of June and in autumn. This microalgal species becomes bioluminescent, if agitated mechanically. Being non-toxic, it means that Noctiluca does not produce harmful metabolites. However, it can be an object of problems due to the emission of a high concentration of ammonia into the water column, as well as anoxia creation upon degradation of the whole bloom.

I personally find it astonishing how one fact quickly replaces another, how unbeknown to us are these fine differences that we, or as least I, sometimes take for granted – the redness of that water in Izola was poison for my eyes and represented a complete disgust on the thought of going into the water, but if I would have known that there was almost nothing to be afraid of, I would have perhaps gone for that dive.