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.

Algae: What are they?

So in this blog I talk a lot about seaweeds, but what exactly are seaweeds? Seaweed is a common name used to refer to marine macroalgae, a type of large algae. So seaweeds are large algae that live in the sea. But this just seems to raise the bigger question: what are algae?

Most people probably know algae as the clumps of seaweed spoiling the beach or the green stuff that wraps up your sushi.


Seaweed washed up on the beach. Goleta. California. United States.

But algae can come in a vast array of shapes and sizes. The term algae actually encompasses a large range of organisms, from single celled individuals that are a similar size to bacteria, up to large multi-celled, plant-like organisms up to 53m in length in the case of giant kelp.

Diatoms, mostly Arachnoidiscus

Coloured electron microscopy of diatoms Arachnoidiscus

Kelp Forest

Kelp Forest. West coast of North America

Algae are not always alone, and can actually be found forming important symbiotic relationships with many marine animals, the most notable being that with corals, but also with jellyfish, sea anemones and even clams. These algae are provided shelter within the animal whilst in exchange providing nutrients to their animal host in a mutually beneficial relationship.

Fleet Of Jellyfish

Algae and Jellyfish symbiosis. Golden Jellyfish Mastigias sp. Jellyfish Lake. Palau

Algae are grouped together in a rather informal way, with many members not necessarily being closely related. These algae can come from multiple lineages, meaning that they are grouped together despite not sharing an immediate common ancestry. This explains why we see such a diverse range of characteristics within the group.

However though a diverse group, they do share one crucial similarity: they are photosynthetic. But then are algae plants? The answer is slightly contentious, but generally, we define them not to be. To be a plant you need a root or internal vascular systems and to produce seeds or flowers, which algae do not. Some algae can have similar looking structures, such as the vein-like midrib in bladderwrack, but these are not true vascular systems. Unlike plants, the body of an alga is relatively undifferentiated, whereby there is no division of labour within the algal body.

Bladder Wrack

Bladderwrack. St. Mary’s Island. NE coast of England.

So algae are a group of photosynthetic organisms that are corralled into a group because they do not fit well into the other more well-defined photosynthetic groups like plants, fungi and lichens. Hence it really isn’t unsurprising that they are so diverse.

The greatest anomaly within algae are the cyanobacteria. Cyanobacteria, also known as blue-green algae, are normally (but not always) included within algae and are probably most famous for forming a greeny sheen of scum on the surface of the water that stops you going in for a swim at the sea or lake side.


Cyanobacteria on the water’s surface

Cyanobacteria are actually very unlike all other algae, even residing within a different kingdom within the tree of life. Cyanobacteria reside within the kingdom of bacteria, alongside the familiar E. coli and Salmonella. Bacteria in general are primitive cells with a less complex structure and are always single-celled. These single cells can exist on their own or as several to many cells living in a colony, chain or filament.

Video: The Cyanobacteria: Oscillatoria and Gleocapsa

The Cyanobacteria: Oscillatoria and Gleocapsa.

All other algae have a complex cell structure, having many different cellular parts including those specialised for photosynthesis: the chloroplasts. Interestingly, cyanobacteria are considered the origin of chloroplasts, when a cyanobacterial cell was incorporated into the cell of an algal ancestor. Therefore, though cyanobacteria are not directly related to all other algae they are related to the photosynthetic part of them.

All these algae belong to the kingdom of protista. Of these complex algae, there are three major groups: Green, Red, and Brown. As you may guess they are named for their usual colour.

This evolutionary tree shows the hypothesised relationships among the six kingdoms. Each group branching off the tree can be thought of as a cluster of close relatives. Source: dalconenvironmental.com

The green group contains algal species within the scientific designation Chlorophyta, the red within Rhodophyta, and brown within Heterokonta, Haptophyta, Cryptophyta, and Alveolata. All algae within each of these groups share a set of characteristics that distinguishes them from those within another group, and thus are grouped accordingly.

Red and brown algae. Penguin Island. Western Australia.

The main characteristic difference between these groups are the types of pigment each contains. Just like plants, all these algae contain the green photosynthetic pigment chlorophyll; though the type of chlorophyll varies between them. In green algae we find chlorophyll a and b, which is the same as can be found in plants. Both red and brown algae also contain chlorophyll a, but the second is either chlorophyll d in red algae or chlorophyll c in brown algae. Green, red, and brown algae also contain other different accessory pigments that help chlorophyll in absorbing light. These pigments also contribute to the algae’s colour. 

Scenedesmus at 1000x - phase contrast

The green algae: Scenedesmus.

There are also a few other smaller groups of algae not contained within these three main groups. These are the freshwater Glaucocystophyta, the predominantly freshwater Euglenophyta, and the marine Chlorarachniophyta. They are all far smaller groups of solely single celled individuals with either 15, ~800 and 5 species respectively. Nevertheless the vast majority of algal species are grouped within the greens, reds and browns.  

Green algae are the most closely related algae to plants, sharing a common recent ancestor. They can be found in both marine and freshwater environments. Examples of green algae include sea lettuce (Ulva spp) and Dead man’s fingers (Codium fragile).

Sea lettuce - Ulva sp

Sea lettuce, Ulva spp. Bluefish Point. Manly. Australia

Red algae are mostly marine, and can be found living at greater depths than green and brown algae. This is because they can absorb blue light, which penetrates deeper than other light waves do, and thus they are still able to photosynthesise where other algae cannot.

Some red algae can even be reef builders, similar to that of the corals. Coralline algae are encased within calcareous deposits and form spiky underwater carpets known as maerl beds. These maerl beds are ecologically important, but unfortunately due to their fragility and slow growth are also under threat.

Fal and Helford Maerl Beds

Maerl. Fal and Helford Maerl Beds. UK.

And finally brown algae. As you may guess from the larger number of subgroups included within the main group, brown algae are rather loosely gathered together. Many do not share common recent ancestors. They are mostly marine and also include the largest of the individual algae. Yes that is the kelps, and also bladderwrack and its relatives belong in this group too. But brown algae are not only large, there are also small single celled ones too, including the diatoms. Diatoms are little cells encased in glasshouses, and despite their small size play an important ecological role on a global scale.

Coscinodiscus (light micrograph)

The diatom: Coscinodiscus.

So to conclude, algae are the misfits of the tree of life, they do not quite fit anywhere else so are instead lumped together in one big group. There are many differences within the group, but there are also similarities as well. Diversity of algae is vast, from tiny to giant, from simple to complex, but all need light to live. And the number of algae isn’t small either. Currently we have described 50,000 species of algae (Guiry & Guiry, 2020). That’s a lot of algae!


Guiry, M.D. & Guiry, G.M. 2020. AlgaeBase. World-wide electronic publication, National University of Ireland, Galway. https://www.algaebase.org; searched on 28 August 2020.

Free-living Bladderwrack – Why should we care?

To answer this question it helps to look at the bigger picture: Why should we care about the natural environment at all? Though the reasons are many, one of the most obvious is that their functioning directly affects our society. This idea is the basis for the ecosystem services concept.

Ecosystems Services – What are they and why do we need them?

Ecosystem services are the benefits provided by the natural environment to society. They are the foundation of human well-being. These services are numerous and highly varied depending on the ecosystem. In forests and woodlands the production of wood as a raw material for the use in manufacturing is one such service. On a smaller scale, individual species can also provide important services, such as bees acting as pollinators for agricultural plants. These services are invaluable to human existence, and often come with no accompanying monetary cost. Being freely available, many are undervalued and consequently protection for the environments that provide them is frequently limited.

Though ecosystem services are often taken for granted, they are hugely important. Imagine if the continued use of pesticides, most famously neonicatoids, led to the extinction of numerous bee species. Many fruit and seed producing crops would be left unpollinated, leading to crop failure and the consequential food shortages within shops and supermarkets. Hence the ecosystem services we take for granted can have monumental effects on society and our quality of life. It therefore seems necessary to provide protections for these environments so that they can continue to provide the services we rely on.

So now we know what Ecosystem Services are – How does this relate to bladderwrack?

As an underwater environment that many people rarely, if ever, see bladderwrack forests are a hugely underrated environment in terms of their value and the ecosystem services they provide. However bladderwrack forests can be considered similar to giant kelp forests, which are some of the most productive habitats on earth.

Bladderwrack forests are highly productive environments storing large quantities of carbon. Because some of this carbon is sequestered bladderwrack can be considered to provide a service in reducing CO2 within the atmosphere and thus help our society with mitigating climate change.

This is not the only services these underwater forests provide. As an ecosystem engineer; a creature that modifies its environment; bladderwrack also provides the additional benefits of food and shelter for a myriad of different plants and animals, all of which themselves contribute to the ecosystem services provided by this habitat. Notably bladderwrack plays an important part in food production by providing nursery and feeding habitat for juvenile fish of commercial importance including cod, pike and perch. By supporting populations of these important fish species they also provide valuable recreational services including recreational fishing, boating and SCUBA diving.

Plaice resting next to an attached bladderwrack stand

An entirely different service that bladderwrack provides influences people’s health by reducing their contact with harmful environments. Within the Baltic Sea, the enrichment of water bodies with excessive nutrients has led to widespread eutrophication and resulting nuisance blooms of cyanobacteria. These blooms can be detrimental for human health and are monitored by the Finnish Environmental Institute SYKE. Importantly though, bladderwrack forests can act as filters against high nutrient inputs from terrestrial sources, providing a service in reducing excessive load of nutrients and consequently benefiting human health and well-being.

These are just a few of the ecosystem services this fascinating habitat provides, though there are numerous others that have not been listed here. We can therefore conclude that both bladderwrack and the associated community of plants and animals are important for the Baltic Sea ecosystem and many of the services we require.

What about free-living bladderwrack and the associated animal community?

Since free-living bladderwrack fulfils a similar ecological niche to the attached form, albeit generally on soft bottoms rather than rocky substrates, we surmise that it provides similar ecosystem services as well. Both forms support a similar animal community living around and on the seaweed, but the free-living form also supports an additional community living within the sediment below the algal mats. It is likely that this community will provide additional services that benefit us, however what these services are is difficult to tell unless we have a greater understanding of the associated animal communities of the free-living form. Hence this is where our study comes in. In one of our projects we are interested in identifying the animals on and below the surface of the sediment, and how these communities vary from those of bare, soft bottoms. To find out how important these creatures are to our society we will delve below the surface of this barely studied habitat.

Free-living bladderwrack forest

The curious case of free-living Fucus: what is it and where does it come from?

Bladderwrack (Fucus vesiculosus) is a brown algae commonly found within many parts of the Baltic Sea. It forms structurally complex habitats at depths of 0.5-7m, providing shelter and food for many marine invertebrates and fish. It is one of the major foundation species in the Baltic Sea coastal zone. Generally, bladderwrack is considered a rocky shore organism, being most notability found growing attached to rocks, boulders and pebbles. However, interestingly an unattached form can also be found.

A meadow of free-living bladderwrack, resulting in 100% coverage. Image taken in the Askö area (Sweden)

These unattached individuals form free-living populations, that can be quite extensive (10-100m2) occurring year after year at the same sites. They have been observed since the late 19th century (Kjellman, 1890) and are generally described as pieces torn from attached populations and deposited in sheltered locations, with no ecological significance. However with modern molecular techniques; including microsatellites and DNA barcoding; we aim to test this theory.


The origins of life?

Firstly we aim to test this long held idea that the free-living populations are solely supplied by the surrounding attached populations. To put it simply, do they rely on supplies of torn off pieces to start and replenish a population or are they fully or partially self-sustaining through their own means? This really is a question of ‘can they reproduce?’, and if so ‘how do they do it?’.

The processes involved in forming and maintaining free-living bladderwrack populations

We surmise that the founding members of any free-living population are supplied by pieces from attached populations, as has been suggested since their first documentation. However this is where the ideas diverge, rather than assuming any replenishment to the population are from supplies of material from the nearby attached population, we view that these free-living populations have some level of self-sustainability.

How do they do this? The current idea is through fragmentation, a method of asexual reproduction where new, smaller, genetically identical individuals are formed through breakage from the main individual. If you ever get your hands on a free-living bladderwrack individual, you will see how easily one individual becomes many with just the simplest of handling. Through splitting into many individuals that continue to grow and eventually break apart once more, soft bottoms can quickly become dominated by many genetically identical plants.

Two distinct morph types from different free-living populations around the Askö area (Sweden)

The level at which this asexual reproduction occurs will be defined by the amount of genetic variation within the population. If populations contain only a few genetically different individuals then we can assume that fragmentation plays a large role in maintaining these free-living populations. If we observe the reverse; many genetically different individuals; then it is likely that either attached populations are largely responsible for supplying these populations, or that the free-living plants can themselves reproduce sexually. The latter seems improbable, in part because few free-living individuals have been observed to form sexual structures known as receptacles.


Population connectivity?

Now that we have established the possible mechanisms for forming, maintaining and regenerating free-living populations, we can consider the dispersal of a normally immobile seaweed. It is frequently observed that broken off pieces from attached individuals can be transported by currents over great distances; and since reattachment is incredibly rare; either these free-floating pieces eventually sink becoming loose-lying pieces which eventually decay or they contribute material to free-living populations.

However the question is, are free-living individuals equally as mobile? Can free-living individuals migrate between patches, and do distances and other abiotic factors affect their dispersal?

We currently have little idea as to answering this question, but it seems likely that distance and geological features will be the major influences on the dispersal potential. By identifying the genetic variation; or the level of relatedness; between and among populations we will hope to answer this intriguing question.


Ecologically important?

Thirdly, we have little idea of the importance of free-living populations. What function and ecosystem services do they provide in the coastal zone? Through inhabiting soft bottoms, that are normally uninhabitable by the attached form of bladderwrack, they can provide a complex habitat that would normally not be found on this substrate type. Consequently this habitat can support a vast variety of plants and animals that would otherwise not be found in that location. Through environmental surveys and the monitoring of biological measurements we aim to identify the important functions and services that are provided by the free-living populations.


The importance of this study

Now comes the most important question: Why do we need to study these questions in the first place? As an integral part of the Baltic Sea ecosystem, free-living bladderwrack is considered an important biotope at risk of damage. As such they are listed on HELCOM Red List of biotopes and habitats as endangered (HELCOM, 2013). This means that policy makers and conservationists need to implement methods to best protect these populations. Without adequate knowledge, including the genetic diversity, of these populations successful management is doubtful. As such, if we wish to maintain the health of these populations and consequently that of the Baltic Sea, we need all the research we can collect.



HELCOM (2013) Red List of Baltic Sea underwater biotopes, habitats and biotope complexes. Baltic Sea Environmental Proceedings No. 138.

Kjellman FR (1890) Handbok i Skandinaviens hafsalgflora. I. Fucoidae., Stockholm