Closed beaches in the Tri-City. Cyanobacteria have arrived unusually early

The holiday season has begun on a disappointing note in the Gulf of Gdańsk. Red flags have appeared on beaches across the Tri-City, warning visitors that swimming is prohibited. The reason is cyanobacteria, which began blooming rapidly after a spell of record-breaking heat, releasing toxins that are harmful to human health.

Early invasion

Cyanobacteria are a familiar feature of almost every summer season in the Tri-City, but their peak bloom usually occurs at the turn of July and August. This year, however, the first bathing areas in the Gulf of Gdańsk were closed as early as the end of June. On 2 July, the situation became serious enough for the District Sanitary Inspector to ban swimming at six locations in Sopot.

Red flags are flying on most beaches in Gdańsk, from Jelitkowo to Świbno. Swimming is currently allowed only at Sobieszewo, Orle and Stogi. White flags have returned to bathing areas in Gdynia, where cyanobacteria were also present just a few days ago. As the situation remains highly dynamic, up-to-date information on water quality is available on the following websites:

• National bathing water service: https://sk.gis.gov.pl/

• Gdynia Sports Centre: https://gdyniasport.pl/pl/obiekty/plaze-i-kapieliska

• Gdańsk Sports Centre: https://kapieliskagdansk.pl/

Why are cyanobacteria dangerous?

Many tourists may be surprised by the way flag colours on Baltic beaches can change from one day to the next. How is it possible that cyanobacteria are present one day, while the beaches reopen the following day?

The authorities in Sopot explain that these harmful microorganisms cannot be effectively removed from the water, but they are quickly dispersed by waves and wind. They also disappear after rainfall and cooler weather. In the sea, where the water is constantly mixing and wind direction changes frequently, a surface bloom can shift within just a few hours. This is why continuous monitoring of bathing areas by the State Sanitary Inspection is so important.

The Polish Chief Sanitary Inspectorate also warns against swimming in waters affected by cyanobacterial blooms, stressing that skin contact with contaminated water or swallowing water containing cyanotoxins may cause:

• skin rash,

• itching and watery eyes,

• vomiting and diarrhoea,

• fever,

• muscle and joint pain.

It is worth noting that the greatest health risk comes not from the cyanobacteria themselves, but from the toxins they produce during periods of intensive growth. Large seaweed that can sometimes be seen floating in the sea is unrelated to this phenomenon and should not be a cause for concern.

The problem is likely to worsen

Cyanobacterial blooms usually occur during heatwaves and dry weather. They are also encouraged by the presence of nutrients in the water, particularly phosphates. As a result, climate change and rising temperatures may significantly increase cyanobacterial activity in the Baltic Sea.

This year, the first blooms were observed as early as mid-June off the coast of Finland. In an interview with the Trójmiasto.pl news portal, Dr Justyna Kobos of the University of Gdańsk admitted that researchers were surprised by such an early outbreak on this scale and with this level of intensity. She did not rule out the possibility that red flags will return to Tri-City beaches in July and August.

The outlook for the summer is further worsened by the continuous inflow of nutrients into the Baltic Sea from wastewater and agricultural areas. The level of eutrophication in this shallow and enclosed sea remains very high, creating favourable conditions for the growth of algae and cyanobacteria.