Cyanobacteria forming a summer water bloom in the Baltic Sea

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Harmful algal blooms are a growing problem in both fresh and brackish water, as well as in seawater. They develop in response to increased eutrophication of waters, but also as a result of imbalances in ecosystems. The occurrence of blooms depends on environmental conditions and the adaptive capacity of the organism in question. In the seas, blooms are usually formed by diatoms, dinoflagellates or haptophytes, while cyanobacteria, green algae and euglenins can develop en masse in freshwater. The Baltic Sea is the largest body of brackish water in the world, and thus very unique. Because of its salinity, averaging 7 PSU (the average for the world’s seas is 35 PSU; freshwater has a salinity of less than 0.5 – 1 PSU), both marine and freshwater organisms are found here, including cyanobacteria, whose summer blooms can be truly extensive and cover an area of up to 200,000. km2.

Cloudy water, or what thrives in the Baltic in summer?

Taxonomically, cyanobacteria (Cyanobacteria, cyanobacteria) belong to bacteria. Most species show great adaptability to environmental conditions – including. can use a broad spectrum of light, are resistant to unfavorable oxygen conditions and tolerate high pH values. They occur as coccal cells (single or colony-forming) and in the form of trichomes/niches. Although individual cyanobacterial cells are often small (tenths of a µm to a few – several µm), their colonies and strands are generally characterized by large sizes.

The ability to form blooms on the water surface is limited to species that, due to buoyancy or the ability to move in favorable conditions, have the ability to accumulate near the water surface, causing turbidity or forming dense dross. Typically, surface blooms are formed by species that regulate their buoyancy due to the presence of gas vacuoles in the cells. Based on paleontological studies, it was concluded that episodes of mass occurrence of cyanobacteria in the waters of the Baltic Sea probably took place as early as 7,000 BC. [1]. The phenomenon of cyanobacteria blooms, along with the increasing eutrophication of the Baltic Sea and rising water temperatures due to global warming, is intensifying.

The most prominent component of cyanobacterial blooms in the Baltic Sea is the very large, colony-forming cyanobacteria Nodularia spumigena and Aphanizomenon flosaquae (Figure 1). Both species belong to diazotrophs, that is, they have the ability to fix atmospheric nitrogen through the production of special cells – the heterocyte. In addition, these species can regulate their buoyancy in the water depths because they have gas vacuoles. Aphanizomenon forms densely packed, haystack-like colonies that are usually several mm long. Nodularia, on the other hand, builds colonies of about the same size, but much less packed. It can also form aggregates as large as 10 cm. However, they are so impermanent that they cannot be retrieved from the water surface without disturbing the structure and destroying the integrity. Even quite small aggregates of Nodularia disintegrate when attempting to relocate, so turbulent environmental conditions (ripples, mixing of water) may have a greater impact on them than on those formed by Aphanizomenon. Nodularia aggregates, especially when they are quite old, deoxygenated and close to decomposition, can be colonized by diatoms, zooplankton or bacteria.

Other diazotrophic species found in the Baltic Sea and possessing gas vacuoles include freshwater cyanobacteria of the genus Dolichospermum. However, they are usually found in small numbers, in the coastal zone, and appear as individual trichomes.

Wodne Sprawy 15 2023 1
Fig.1. Cyanobacteria species forming blooms in the Baltic Sea
Author: Justyna Kobos

Deeper less, shallower more, or cyanobacteria unevenly distributed

An unusual feature of cyanobacteria blooms in the Baltic Sea is that they are spatially distributed, and the surface location and size can change rapidly. The clustering of bloom formation may be explained to some extent by hydrodynamic conditions that cause local increases in nutrients in regions of so-called upwelling, but the mechanisms responsible for this phenomenon are still very poorly understood. By having gas vacuoles and forming aggregates, Nodularia and Aphanizomenon drift to the surface when the water is calm. They can then form extensive deposits of surface blooms that look like streaks, dross or spilled paint (Figure 2).

Wodne Sprawy 15 2023 2
Fig. 2. Cyanobacteria – a surface bloom observed during a cruise on the Oceanographer ship in August 2022.
Author: Anna Krakowiak

During periods of mixing, the distribution of cyanobacteria in the water column may become more even, but as soon as the water column stabilizes, they drift toward the surface again. Such a mass can be transported by the wind over quite long distances, far from where the bloom has formed, as confirmed by satellite transmission data (Figure 3).

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Fig. 3. Satellite images showing the area of the Baltic Sea affected by the mass occurrence of cyanobacteria in July 2023.
(Source: https://www.smhi.se/data/oceanografi/algsituationen)

Blooms not only unsightly but potentially dangerous – blue-green algae in the Baltic Sea

Due to wave action and wind, blooms can move toward the shore and accumulate in the bathing area. Unfortunately, the occurrence of cyanobacteria blooms in the coastal zone poses a real threat to the health of recreational water users, as these organisms can produce toxins. It is believed that during cyanobacterial blooms, when toxins are released into the water, their harmful effects on humans occur through various routes: ingestion of water or foam, direct contact with the skin or conjunctiva, or inhalation of aerosol, as well as wind-dispersed particles of dried cyanobacterial cells. The species Nodularia spumigena produces the hepatotoxin nodularin, which has a strong effect primarily on the digestive system. Species of the genus Dolichospemum can also produce hepatotoxins (microcystins) and, in addition, toxins from the group of neurotoxins affecting the nervous system (anatotoxins). Baltic strains of Aphanizomenon have not been found to produce toxins – unlike freshwater strains. However, whenever a cyanobacteria bloom is observed in the coastal zone, bathing areas are closed due to the potential to cause health problems (Figure 4).

cyanobacteria
Fig. 4. Cyanobacteria bloom observed in Gdynia on July 10, 2023
Author: Justyna Kobos

Fortunately, blue-green algae blooms in the coastal zone do not appear throughout the coast during the season, and when they do, it is only for a few days. Each year, about 170 coastal bathing beaches are officially open, guarded and surveyed in accordance with the requirements of the Chief Inspectorate of Environmental Protection. The season usually runs from mid-June to the end of August, or about 75 days. In recent years, blue-green algae have been the cause of bathing site closures mainly in the inner Gulf of Gdansk. The largest number of closed bathing areas (58) and the longest (up to 14 days) were in 2018 [2]. During the current holiday season, an intense blue-green algae bloom was observed for only 1 – 3 days at 24 bathing sites (Figure 5). When planning a trip to the sea, you should not be afraid that your entire vacation will be spoiled by blue-green algae, as the likelihood of hitting a bloom is less than we think.

image 13
Fig. 5. Coastal bathing areas closed due to blue-green algae in June and July 2023
Source: own compilation based on the Bathing Service

Source:

[1] Cegłowska M., Toruńska-Sitarz A., Kowalewska G., Mazur-Marzec H., 2018, Specific chemical and genetic markers revealed a thousand-year presence of toxic Nodularia spumigena in the Baltic Sea. Marine Drugs, 16(4), 116; http://www.mdpi.com/1660-3397/16/4/116

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