Summer blooms in the Baltic Sea

This summer, blooms of cyanobacteria were observed for more than two months period, from 29 June to 5 September. During 30 days from the 7 July, extensive blooms were seen, but the most massive accumulations stayed away this year. The Gulf of Bothnia, which usually blooms in August, also had an unusually prolonged bloom.

Following a few days with warm and sunny weather the first surface accumulations of cyanobacteria appeared in the Gulf of Finland 29 June. Signs of blooms were also seen east of Bornholm, and the next day surface accumulations were observed in the Gdansk Bay. The blooms were, as usual at this time, not very large.

Changing weather, changing blooms

A low pressure system dominated the weather on the first days of July. The bloom in the Gulf of Finland grew, but not until the 7 July more extensive surface accumulations were seen in the Baltic Proper. Up to the 12 July, a broad band with partly dense surface accumulations stretched from the Gulf of Finland past the western coast of Gotland towards Rügen.

On the 13 July a low with northerly winds passed and the algal belt was partly dissolved. A few cloudy days followed, but the 19 July, large blooms were again seen in the southern Baltic Proper as well as in a large part of the northern Baltic Proper. Much of the development for the rest of July was hidden under clouds, but the surface accumulations were still present.

The first week of August offered a solid high pressure over the Baltic Sea area which made the blooms increase in large parts of the Baltic Proper, for the first time also east of Gotland. Some media reports of algae drifting ashore along the Swedish east coast appeared during this week.

A low pressure system swept in over the Baltic Sea the 7 August, and remained until the 11. As it withdrew, this year’s large-scale blooms in the Baltic Proper had ceased. On the 23 August, a bloom unexpectedly turned up southeast of Gotland, and stayed until the 5 September.

The algal bloom in the Bothnian Sea earlier than usual

Algal bloom in the Bothnian Sea Blooms may occur also in the Bothnian Sea, here on 19 August. Photo: ESA (Envisat). Enlarge Image

The first signs of a bloom in the Bothnian Sea were seen already on the 22 July. At the end of the month the bloom had increased in size, but not formed any surface accumulations. They were visible on the satellite image the day after, the 1 of August. The bloom had its greatest extent around mid-August when a large surface bloom was floating in the centre of the Bothnian Sea. The bloom then declined, but remained until the end of the month.

The SMHI marine expeditions observes the most common species

At the SMHI expedition on the 17-22 June with the Swedish Coast Guard vessel KBV001 Poseidon the researchers found relative ample amounts of cyanobacteria of the genus Aphanizomenon when sampling in the southern Baltic, but not in blooming quantities. It would still take ten days before a bloom could be observed from satellite.

At the next expedition on 10-15 July, surface accumulations were observed at these stations and further northeast to the east of Gotland. The species Nodularia spumigena was commonly found in the surface accumulations, but Aphanizomenon and Anabaena dominated the water samples.

The last expedition of the summer on 1-7 August went through sheets of cyanobacteria filaments in much of the southern Baltic Proper as well as north of Gotland. Now, Aphanizomenon flos-aqua was again found to be the most abundant species.

Aphanizomenon och Nodularia The cyanobacterial species Aphanizomenon flos-aqua and Nodularia spumigena were abundant this summer. Photo: SMHI