Fucus of the Baltic

Fucus is the scientific name for the group of seaweeds that bladderwrack belongs. They are large, brown seaweeds that can live for many years. Typically residing in the intertidal zones of rocky seashores this group can be found throughout much of the northern hemisphere. The members of Fucus are commonly called to as wracks.

Underwater landscape in Brofjorden at Sandvik 6

An underwater Fucus) forest at Brofjorden. Sandvik. Sweden.

Their widespread distribution and the large number of species found residing in a tight zone has led to them being sought after seaweeds to be used in the study of many scientific concepts including zonation and competition.

Within the Baltic Sea four species are found, three native and one invasive. By native we mean that their North Sea ancestors entered the Baltic Sea naturally around 8000 years ago and they have been resident ever since (Ardehed et al., 2016). Whereas the invader was introduced to the enclosed sea ~100 years ago, possibly by human activity (Wikström, 2004). This invader is an alien to the Baltic Sea ecosystem and that could potential cause a harmful impact on the otherwise native community.


Tiny wrack (Fucus spp.) surrounded by barnacles.

The Baltic Sea is not the typical habitat of Fucus, and that’s a big reason why we find fewer species then you might expect on a normal coastline. Wracks are marine species with those inhabiting areas outside of the Baltic Sea normally living in true marine conditions. However all wracks in the Baltic Sea have some tolerance to the lower salinity conditions present in this semi-enclosed sea. This tolerance has a huge impact on where we can find each species. Each species can tolerate a different salinity range, with those that can tolerate lower ranges being found deeper into the inner Baltic Sea than those who can’t.

But who are these four Fucus of the Baltic Sea?

The most famous, and most widely spread of them, as you might guess, is bladderwrack. Bladderwrack can be found along most of the European coastline and throughout the Baltic Sea until the far northern parts of the Gulf of Bothnian and the eastern parts of the Gulf of Finland. Bladderwrack is able to tolerate a range of salinity from true marine conditions down to truly brackish conditions of the Baltic Sea. This is why we find it so extensively.


Bladderwrack (Fucus vesiculosus).

The second species, has a far narrower tolerance range, and as a species of marine origin it cannot be found when salinity gets too low. The typical form has a saw-like appearance, hence it’s name: serrated wrack. You’ll find serrated wrack all along the west coast of Sweden and along the southern east coast, but not quite as far north as Stockholm. Serrated wrack can also be found on the Swedish islands of Öland and Gotland.

Mostly serrated wrack and red hornweed at the North Harbor Lysekil

Serrated wrack (Fucus serratus ).

Up until quite recently, these two species were thought to be the only two native wracks. The extensive range of bladderwrack and fairly limited one of serrated wrack is the reason why for many years bladderwrack was viewed as the one and only species within the majority of the Baltic Sea. Accordingly, if you were in Finland or Estonia and you found some wrack, it was certainly bladderwrack.

However this might not be the case… In 2005, it all changed. A group of researchers from Sweden used the same techniques that I use within my own research to throw all previous assumptions up in the air. Enter Narrow wrack…

Narrow wrack (Fucus radicans).
Image from Tangbloggen.

For many years prior to this, researchers were commenting on a strange dwarf form of bladderwrack found in the Baltic Sea. This was assumed to be a morphotype (a group of distinctly different looking individuals within the same species) of bladderwrack. But by looking at the DNA of this dwarf the researchers found that it was not only morphologically distinct and but also genetically distinct (Bergstöm et al., 2005). This morphotype was afforded species level status. Thus now if you are in the northern parts of the Baltic Sea, in either Sweden or Finland, or even on the Estonian island of Saaremaa and find some wrack you could be looking at either bladder or narrow wrack.

Narrow wrack diverged from the same North Sea ancestors as those of Baltic Bladderwrack shortly after they entered the Baltic Sea (Ardehed et al., 2016). As such, narrow wrack is considered an endemic species to the Baltic. This means that it is both a native species to the Baltic Sea, but also it won’t be found anywhere else either.

An illustration of where you can find each Fucus within the Baltic Sea. The distribution of narrow wrack, bladderwrack, and serrated wrack are denoted by orange, green, and yellow respectively.
Image from Stockholm University DEEP.

So these are the three natives of the Baltic Sea: Bladderwrack, serrated wrack and narrow wrack. But who is the invader?

Well this invader doesn’t have a readily available English common name but in Finnish it’s called häilyhauru. Perhaps not the easiest to pronounce so instead I will use it’s scientific name: Fucus evanescens. In fact this wrack is actually of dubious identity, potentially being either a species in its own right or a subspecies a different rockweed. Either way this seaweed invaded many European coasts including the Baltic Sea from its origins in the Arctic.  Perhaps surprisingly, the invasion of Öresund at the entrance to the Baltic Sea in 1966-72 was very well documented 1966-72 (von Wachenfeldt 1975). We know now that this wrack can be found throughout the Danish straits, Swedish sound, and the Baltic German coastline.

EUO © OCEANA Carlos Suárez CSE_4191

Häilyhauru (Fucus evanescens ).

So now we have it, the four Fucus of the Baltic Sea. They are important components of the Baltic Sea ecosystem, providing home and food for many other plants, animals, and seaweeds. Without these wrack the Baltic Sea would be a very different place, and more importantly I would be out of a research topic!


Ardehed, A., Johansson, D., Sundqvist, L., Schagerström, E., Zagrodzka, Z., Kovaltchouk, N.A., Bergström, L., Kautsky, L., Rafajlovic, M., Pereyra, R.T. and Johannesson, K., 2016. Divergence within and among seaweed siblings (Fucus vesiculosus and F. radicans) in the Baltic Sea. PLoS One11(8), p.e0161266.

Bergström, L., Tatarenkov, A., Johannesson, K., Jönsson, R.B. and Kautsky, L., 2005. Genetic and morphological identification of Fucus radicans sp. Nov.(Fucales, Phaeophyceae) in the brackish Baltic Sea 1. Journal of Phycology41(5), pp.1025-1038.

von Wachenfeldt T (1975) Marine benthic algae and the environment in the Öresund. Systematic Botany, Lund University:328.

Wikström, S.A., 2004. Marine seaweed invasions: the ecology of introduced Fucus evanescens (Doctoral dissertation, Botaniska institutionen).

The Baltic Sea

When I first came to Finland and started working with Baltic Sea bladderwrack I knew nothing about this unique sea. But if you ignore the fact that the Baltic Sea is so very different than the typical seas then you risks your ideas on the biology of any Baltic Sea plant animal or algae being wrong. So this month lets look at why the Baltic Sea is so unique.

Furuskär, a Baltic Sea island in the Tvärminne archipelago.

The most defining feature of the Baltic Sea is the low salinity. In simple terms the salinity is the saltiness of the water. In typical seawater you would expect a salinity of 30-35‰ whereas in the Baltic Proper you will not experience salinity any higher than around half this value. Despite being called a sea, it does not possess the characteristic salty seawater you’d expect from that name. But though the Baltic Sea is not like typical seawater, it is also not like typical ponds, lakes or river either. In fact it’s waters lies somewhere in between the two, being not quite sea or freshwater either. The easiest way to describe it is brackish, a sort of halfway water, with the average salinity being just ⅕ of typical seawater (Leppäranta & Myrberg, 2009).

But does this really make such a difference? Well it certainly does!

EUO © OCEANA Carlos Minguell 20130706_Puck Bay_052

Three-spined stickleback (Gasterosteus aculeatus). A common fish found within the Baltic Sea with a widespread distribution throughout the Northern Hemisphere in both fresh and salt water.

Plants, animals and algae have preferred environmental conditions that they can tolerate. This can be considered a tolerance range. Most of us like the summer heat of 30°C and don’t mind a winter of -1°C, but if we were then forced to live in 70°C or -60°C we probably wouldn’t last long. So we can tolerate the moderate temperatures, but the extremes are too much to survive. Well this is the same for salinity. For example, marine fish do well within seawater and lake fish likewise in freshwater, but if you took a lake fish to the sea or a sea fish to the lake they would not be able to endure the different conditions. Each is adapted to tolerate the habitat that they live in.

Some of you might be wondering about some of the rather high profile fish that move between the sea and rivers (ahem..salmon..). Well it is true that some fish can move between salinity conditions, however for the most part they must slowly acclimatise to the new conditions undergoing changes to their bodies to allow them to tolerate the new environmental conditions. Acclimatising to extremes is possible, but not very common. Generally, most plants, animals, and algae have a range they can tolerate and if subjected to conditions outside of this then the results end badly.

Atlantic Salmon

Atlantic salmon (Salmo salar) heading up the Tyne River. UK.

So back to the Baltic Sea. As a brackish sea, marine life are out of their comfort zone, but so too are the freshwater species. This means that many marine species that you would find around the coasts of Europe cannot live in the Baltic Sea just as freshwater species in continental Europe cannot either. This results in species paucity: only a few inhabitant species. In fact the adjacent North Sea has ~10X more species than the Baltic Sea (Elmgren and Hill, 1997). Characteristic marine organisms such as starfish and sea urchins are missing from most of this sea, though other species that you wouldn’t expect from a sea which are pretty abundant such as the common reed.

Example distribution of plants, animals, and seaweeds in the Baltic sea. Notice that marine animals become less common in northern areas whereas freshwater animals are absent from the southern areas. (Furman et al., 2014).

In Fucus terms; the group of seaweeds that bladderwrack belongs to; you could expect to find anywhere up to 3 or 4 species on a typical British rocky shore, whereas in the Baltic Sea you would be lucky to find more than one. In fact, if you searched throughout the whole Baltic Sea you’d only find three native Fucus species: bladderwrack, narrow wrack and serrated wrack.


Bladderwrack (Fucus vesiculosus). Torslanda. Västra Götaland County. Sweden.

Though the salinity within the Baltic is always brackish, not every location is the same. In fact the salinity ranges from (20-)10‰ (depending on where you decide the Baltic Sea starts) down to less than 1‰ occurs (Waern, 1952). This range is actually quite predictable, forming rather well defined gradients.

But why do we see this gradient?

Salt water can only flow into the Baltic Sea from one relatively narrow location in the south over the Danish belts and the Swedish sound. These channels are shallow, narrow sills that restrict the flow of saltwater into the Baltic Sea. Because seawater is denser than fresh and brackish water it does not readily transfer over these shallow sills, with only a few major inflows happening per year. This means that the input of high salinity seawater into the Baltic Sea is rare. To add to this, the Baltic Sea is surrounded by land. This area of land surrounding the Baltic Sea that drains water into the sea is four times larger than the sea itself (Zillén et al., 2008). From this large area of land, freshwater can runoff into the Baltic Sea and consequently reduces the salinity. The largest single input of freshwater occurs in the Gulf of Finland by the river Neva, equating to 15% of the total Baltic river inflow (Alenius et al., 1998; Kuosa and Myrberg, 2009). Other large river inputs include the Vistula, the Daugava, the Nemunas, the Kemijoki, the Oder and the Göta Älv (Furman et al., 2014). Because the large rivers occur in the north or east and the only saltwater input comes from the south a well-defined south-north salinity gradient is produced.

Map of the Baltic Sea and its sub-basins. The three Danish straits and Swedish Sound (Little Belt, Öresund, and Storebælt‎) are located in the south west in the Belt Sea and Öresund. (Leppäranta and Myrberg, 2009).

So this all means is that the Baltic Sea is a semi-enclosed brackish system, where high salinity water is only provided in the south and freshwater input from the land dilutes the sea into the brackish conditions we see. But how long does water stay in the Baltic? Well it takes approximately 40 years for Baltic Sea water to be renewed (Leppäranta & Myrberg, 2009).

The Baltic Sea is not the only environment to be considered brackish, saltmarshes and estuaries can also frequently be considered brackish. These environments share similar characteristics of salinity, however the Baltic Sea has many other features to add to its uniqueness.


A typical saltmarsh.

The Baltic Sea has a rather characteristic, elongated shape, ranging approximately 1,300km from north to south and 1,000km from west to east covering an area of 412,560km2 (Gerlach, 1994; Seifert & Kayser 1995). The sea actually lies across climatic zones, covering both maritime temperate zones and continental sub-arctic climate zones (Leppäranta & Myrberg, 2009). That means that different locations throughout the Baltic Sea will experience differing climatic conditions. Since the Baltic Sea reaches into the sub-arctic zone combined with the brackish water, ice is a common occurrence in winter. During the winter months some 40% of the Baltic Sea is covered in ice, with the greatest amount being seen between January and March (Finnish Meteorological Institute, 2017a).

Stockholm, Sweden

A view of the Baltic Sea from space. Sweden appears on the left and Finland in the upper right. The sea ice envelops the coastal islands with sprinkles of sea ice away from the coast. Credit: NASA image courtesy the MODIS Rapid Response Team at NASA GSFC.

The deepest part of the Baltic Sea is at Landsort Deep in the Baltic Sea Proper, reaching a maximum depth of 459m (Furman et al., 2014). Maybe this sounds pretty deep, but when you compare it to the Atlantic and Pacific Oceans with a maximum depth of 8710m at Milwaukee Deep or ~10900m at Challenger Deep respectively then it seems pretty shallow (Stewart & Jamieson, 2019). In fact the mean depth of the Baltic Sea is a measly 54m, that’s just over the length of an Olympic-size swimming pool (Furman et al., 2014). Moreover much of the Baltic Sea lies well above this depth, with the Gulf of Finland averaging 37m (Kuosa and Myrberg, 2009) and the Gulf of Riga just 27m (Szaniawska, 2018).

Saaremaa. Estonia. Gulf of Riga.

Despite its elongated shape, the shallow depth means that the total volume of the Baltic Sea is merely 21,631km3 (Seifert & Kayser, 1995); that equates to <0.1% of the total ocean volume on earth (Eakins & Sharman, 2010). So the Baltic Sea is just a drop in the ocean when compared to the Atlantic (23.3%) and the Pacific (49.4%) oceans.

Though being inconsequential in the grand scheme of the global oceans, the Baltic Sea has a huge effect on and is also hugely affected by human activities. Nine countries surround the semi-enclosed sea: Denmark, Germany, Poland, Lithuania, Latvia, Estonia, Russia, Finland and Sweden. The sea has been integral in the development of these countries and has supported many a livelihood.

Eutrophication is a negative impact on the Baltic Sea caused by human activity whereby excess nutrients are leaked into the sea. This can have many bad effects on the Baltic Sea, and has caused mass die-offs of bladderwrack in the past. Notice that most of the Baltic Sea is affected by eutrophication, with much of it being quite severe. (HELCOM 2010a).

To add to the peculiarity of the Baltic Sea, tides are absent. In the Baltic Sea you will not experience the daily fluctuation of seawater traveling up and down the shore. For plants, animals, and algae living on the shore this means that they are subjected to constant submergence, a very different experience than those outside of the Baltic Sea. Tides are minimal within the Baltic Sea because the body of water body is so small and the influence from the surrounding tides of the Atlantic and North Sea cannot penetrate into the mostly enclosed sea. The absence of tides is actually quite common in enclosed seas, being seen in both the Black and Caspian seas as well (Medvedev et al., 2016). The absence of tides also plays an important part in stabilising the defined salinity gradient. However if you are on the Baltic Sea coast you might notice that the water level does change from time to time. This is not because of the tide but actually due to air pressure, wind conditions, ice cover and the flow of water between the Baltic and North Sea (Finnish Meteorological Institute, 2017b).

So the Baltic Sea is a semi-enclosed, shallow, brackish sea with various climates, no tides, and a low number of species. As a study system this makes it pretty unique, and adds a further complexity to any research we do in this unique sea. As a marine biologist, I am grateful to have the opportunity to work in such a special place!

Fieldwork at Kõiguste field base (University of Tartu)


Alenius, P., Myrberg, K. & Nekrasov, A. 1998. The physical oceanography of the Gulf of Finland: a review. Boreal Environ. Res. 3:97–125.

Eakins, B.W. and G.F. Sharman, Volumes of the World’s Oceans from ETOPO1, NOAA National Geophysical Data Center, Boulder, CO, 2010.

Elmgren, R. & Hill, C. 1997. Ecosystem function at low biodiversity – the Baltic example. Cambridge University Press, Cambridge. 319–336 pp.

Finnish Meteorological Institute, 2017a. Ice season in the Baltic Sea.https://en.ilmatieteenlaitos.fi/ice-season-in-the-baltic-sea [date accessed 28/09/2020]

Finnish Meteorological Institute, 2017b. Sea level variations on the Finnish coast. https://en.ilmatieteenlaitos.fi/sea-level-variations [date accessed 28/09/2020]

Furman, E., Pihlajamäki, M., Välipakka, P. & Myrberg, K. 2014. The Baltic Sea–Environment and Ecology.

Gerlach, S.A. 1994. Oxygen conditions improve when the salinity in the Baltic Sea decreases. Mar. Pollut. Bull. 28:413–6.

HELCOM, 2010a. Ecosystem Health of the Baltic Sea 2003–2007: HELCOM Initial Holistic Assessment. Balt. Sea Environ. Proc. No. 122.

Kuosa, H. & Myrberg, K. 2009. Introduction to the Gulf of Finland Ecosystem. In Rintala, J.-M. & Myrberg, K. [Eds.]. Ministry of the Environment of Finland, Finland, pp. 21–5.

Leppäranta, M. and Myrberg, K., 2009. Physical Oceanography of the Baltic
Sea. Springer-Verlag. Berlin-Heidelberg-New York

Medvedev IP, Rabinovich AB and Kulikov EA (2016) Tides in Three Enclosed Basins: The Baltic, Black, and Caspian Seas. Front. Mar. Sci. 3:46. doi: 10.3389/fmars.2016.00046

Seifert, T., Kayser, B. and Tauber, F., 1995. Bathymetry data of the Baltic Sea. Baltic Sea Research Institute, Warnemünde.

Stewart, H.A. and Jamieson, A.J., 2019. The five deeps: The location and depth of the deepest place in each of the world’s oceans. Earth-Science Reviews197, p.102896.

Szaniawska, A. 2018. The Gulf of Riga. Springer, Cham.

Waern, M. 1952. Rocky-shore algae in the Öregrund archipelago. Uppsala universitet.

Zillén, L., Conley, D.J., Andrén, T., Andrén, E. & Björck, S. 2008. Past occurrences of hypoxia in the Baltic Sea and the role of climate variability, environmental change and human impact. Earth-Science Rev. 91:77–92.

The demise of Baltic Sea wrack?

Another post about climate change, but this time closer to home. Climate change has been identified as one of the largest contributors to environmental change within the Baltic Sea. It is predicted that Baltic waters will heat by some 2–4°C and become up to 50% fresher by the end of the twenty-first century (Meier 2015).

Coastlines of the Southern Baltic Sea

Coastline of the Southern Baltic Sea

But how will Baltic wracks fair under these new conditions?

Unfortunately the future doesn’t look good for bladderwrack. Research by Antti Takolander and colleges at the University of Helsinki indicated that warming waters and increased freshness pose real threats to bladderwrack. The once abundant brown seaweed may struggle to cope with these new environmental conditions and consequently be lost from much of the Baltic Sea.

Bladder wrack peeking up above the water 4

Bladderwrack – Brofjorden, Sweden

Climate change looks bad for bladderwrack, but is that the case for all wracks of the Baltic Sea?

It seems that the future may not be as bleak for all the Baltic Sea wracks after all. Narrow wrack, having evolved within the Baltic Sea from bladderwrack, can be found nowhere else in the world. As a unique seaweed to the Baltic you might expect that the dramatically changing conditions predicted pose an even greater threat to this sister of bladderwrack. Yet a study by Luca Rugiu and colleges at the University of Turku indicates that this is in fact not the case. By subjecting narrow wrack to higher temperatures and fresher conditions replicating predicted future conditions they assumed that narrow wrack would fair as poorly as bladderwrack; however their results were surprising. Though the predicted future conditions did lead to higher mortality, those individuals that did survive were larger and grew faster.

Bladderwrack (Fucus vesiculosus) and Narrow wrack (Fucus radicans) living side by side – SW Gulf of Bothnia (northern Baltic Sea) [From Pereyra et al., 2009; CC BY 2.0]

Does this mean that narrow wrack may benefit from climate change?

In short: Yes. Those narrow wrack individuals that can withstand the predicted future conditions will benefit from the increased growth and gain a competitive advantage over those that cannot. Since where each wrack can be found is largely affected by competition between the species the future looks even worse for bladderwrack. Both wracks frequently grow side by side and if narrow wrack profit and bladderwrack lose out from future climate conditions bladderwrack may be lost from these areas and instead replaced with narrow wrack.

The future looks bleak for bladderwrack; we may end up losing vast swathes of bladderwrack forest in the Baltic. Though there is the small silver-lining that narrow wrack appears somewhat tolerant to climate change. So the future may not be all bad. We won’t see the disappearance of all the wracks from the Baltic Sea; though we will see a very different Baltic then we see today.


Meier HEM (2015) The BACC II Author Team, Second Assessment of Climate Change for the Baltic Sea Basin, Regional Climate Studies, DOI 10.1007/978-3-319-16006-1_13

Takolander, A., Leskinen, E. and Cabeza, M., 2017. Synergistic effects of extreme temperature and low salinity on foundational macroalga Fucus vesiculosus in the northern Baltic Sea. Journal of Experimental Marine Biology and Ecology495, pp.110-118.

Rugiu, L., Manninen, I., Rothäusler, E. and Jormalainen, V., 2018. Tolerance to climate change of the clonally reproducing endemic Baltic seaweed, Fucus radicans: is phenotypic plasticity enough?. Journal of phycology54(6), pp.888-898.