Molluscs and crustaceans reveal the state of the Baltic Sea — a century of monitoring also helps to predict the future

Twice a year, in spring and autumn, researchers arrive to dig in the seabed off Tvärminne. The creatures living in the sludge can tell us a lot about the state of the Baltic Sea.

There is a reason research coordinator Laura Kauppi has taken the precaution of wearing rainwear: it’s a messy business! The daring photographer, hoping to get a close-up, gets a face full of seawater. Kauppi is using a hose to clean a scoop containing the latest sample in an unusually lengthy research project.

In the spring of 1926, Professor Sven Segerstråle started monitoring the quantity of bottom-dwelling animals, or benthos, in the bay in front of the Tvärminne Zoological Station in Hanko. Segerstråle missed a few years, not least because of the Second World War. However, the samples have been collected in the same locations every spring and autumn since 1964.

That is why, on this beautiful day in May, we are standing on the deck of the research vessel Augusta watching as the scoop of the crane brings a sample to the surface and spits out 0.15 square metres of sludge smelling of rotten eggs into a large black container.

Kauppi, along with Tvärminne Station interns Milla Sigg and Eveliina Piispanen, once again scrape and hose down the scoop until it’s completely empty. To ensure that the sample is comparable with earlier ones, it is important to get every last bit into the sample container.

“Benthos are a great indicator for the state of the ecosystem. Some species are more sensitive to pollution and eutrophication than others. The number of distinct species and their relative amounts are an effective way of measuring the state of the environment more generally as well,” says Kauppi.


The return of the clam

Long-term tracking over nearly a century is rare even on an international scale. The results can be used for modelling and predicting the future.

The time series was born out of the passion of a professor, but in the 1960s was picked up by the Finnish Institute of Marine Research. Today the monitoring and sample analysis is conducted by the University of Helsinki’s Tvärminne Zoological Station.

Professor Segerstråle was especially interested in the Baltic clam and its population distribution. Ironically, he only got to witness the decline of the population. However, by the end of the 1980s something unexpected was going on: the clams returned. Today the population is as large as it was in the 1920s.

Laura Kauppi has been researching the population dynamics of the Baltic clam. According to her, the rise in average temperature and an increase in nutrient quantities have influenced the return of the clams. Neither of these factors are good for the environment.

“However, the clams utilize the nutrients and that way cleanse the sea. Had they not returned, the sea would be in a worse condition,” says Kauppi.

Stable Augusta

During our trip, the research vessel Augusta is skippered by the Senior Technician, Göran Lundberg. The vessel was acquired with the help of donations and was specifically designed for the needs of the Tvärminne research station. It is a catamaran that can also operate in shallow waters. 

Augusta is used for all kinds of marine research. The only limit is the imagination of the researchers, notes Lundberg.

Augusta is twice as fast and double the width of our old vessel. It doesn’t tilt when the crane is in use, which enables more precise sample collection. We have yet to come up with anything negative to say about it,” says Lundberg.

And sure enough, the vessel stays completely still as Lundberg’s colleague, Jostein Solbakken, a research technician, operates the large red crane. The crane hoists five scoops of sludge into the containers from a depth of 20 metres.

In two days’ time, Kauppi and her colleagues will make their way to the other sampling location, where the samples are collected from a depth of 35 metres.

Life at the bottom

The deeper you go, the colder it gets. The temperature has a significant effect on the lives on clams: they live longer, grow slower, and reproduce less frequently in the deeper regions than in shallower waters.

In addition to the samples collected from the seabed, samples of seawater are also collected and measured for their temperature, salinity, and oxygen concentration. When this data is studied in tandem with the quantities of benthos, conclusions can be drawn.

“The rise in temperature and the increase in nutrients have shortened the lifespan of clams. Even in the deeper waters, they no longer live to be 30 years old like they did in Segerstråle’s day,” states Kauppi.

Eutrophication and the warming of the sea may have played a part in the clams’ return to the Baltic Sea, but you can have too much of a good thing. Hot summers and an overabundance of nutrients directly correlate with deoxygenation, which is lethal to clams.

A few years ago, after intense heatwaves in Tvärminne, Kauppi found only a few living clams in the deeper sampling site. The shallower sampling site, however, did not suffer from deoxygenation at all during the monitoring period.

The unpleasant smell of the samples is caused by hydrogen sulphide, which is created when dissolving nutrients consume oxygen near the bottom and it doesn’t get replaced because of temperature differences between the surface and the bottom water layers. Nevertheless, deoxygenated sediment is not too abundant among the samples.

“The benthos appear to be doing relatively well here,” Kauppi concludes.


The sensitive Monoporeia

Tiny crustaceans scurry around in one of the sample boxes. Monoporeia affinis, a type of amphipod, was a dominant species in Tvärminne until the late 1980s. Then it practically vanished. However, over the last decade the species has slowly begun to return. 

The Monoporeia is sensitive to eutrophication. Its return might signal that the nutrient quantities have changed to a more advantageous level. The strain on the Baltic Sea has lessened, although the amount of nutrients is still considered high, according to Kauppi.

“The return of the Monoporeia is, without a doubt, good news.”

Although the changes in the sea directly affect the benthos, it is not always easy to determine the cause. What benefits one species might harm another.


Who’s eating who?

Sample sifting is hard work. The mud is rinsed through two sieves, the first of which has a mesh size of one millimetre and the second half a millimetre.

Tvärminne’s trainees, Milla Sigg and Eveliina Piispanen, take on the task of sifting. They are both third-year students, Sigg majoring in biology and Piispanen in environmental sciences. They started their training at the station a week ago and have already managed to study the benthos with a microscope. Even so, the creatures are hard to identify with the naked eye. 

Most of the benthos are very small. Sigg and Piispanen spray the mud through the sieves, pick out the small animals with tweezers, and place them in a glass jar. The clams have a diameter of one centimetre at best, the Monoporeia not even that. These sea creatures wouldn’t make much of a meal.

Putting the creatures in containers proves to be surprisingly challenging:

“Help! They are eating each other!” Piispanen yelps.

An amphipod and a polychaete are going at each other. Piispanen is forced to use tweezers to separate the two roughhousing beasties and place them on opposite sides of the tin.

Once the benthos have been put in the jars, formaldehyde is poured in, which kills and preserves the creatures. Later a petri dish containing the sample is put under a microscope, which is used to count, measure, and weigh the individual members of each species.

The fine material caught in the second sift is also put in a jar. If you wanted to keep a closer eye on the state of the environment, these samples should also be analysed. Unfortunately, the researchers haven’t received sufficient resources for this endeavour. This means that samples dating back to 2017 are still awaiting analysis.

An invasive burrower

Laura Kauppi has spent ten years working in Tvärminne. She has utilized the data collected from benthos in her doctoral thesis, as well as in her subsequent research.

In her dissertation, Kauppi focused on an invasive species of polychaete called Marenzelleria spp. This species was first seen in the Baltic Sea in the early 1990s. After that, its numbers began to grow. The Marenzelleria was the dominant species in Tvärminne between 2006 and 2016.

“You used to pluck loads them from the sifts,” says Kauppi.

The Marenzelleria is no longer the dominant species. A balance between it and other species, such as the Baltic clam, has been found.

“Introducing the Marenzelleria into the ecosystem of the Baltic Sea has had a complex impact. It can be useful in oxygenating the seabed, as it burrows in the mud. This also enhances the process of decay. On the other hand, the burrowing can also release toxic hydrogen sulphite, and other polluting agents,” states Kauppi.

Kauppi picks up a polychaete and places it on the palm of her glove. It´s a worm, roughly one-inch long, very thin and wriggling. Soon it will be in the glass jar, together with the other benthos, thus forming a part of a century-long narrative of the Baltic Sea. One worm’s loss is science’s gain.


The article has been published in Finnish in the 6/2022 issue of the Yliopisto magazine. It was translated by Miila Karjalainen, Max Lainema, Nea Nieminen, Miro Palokallio, Noora Suominen, Janita Tuomainen and Atte Virtanen under the supervision of John Calton, lecturer in English.


On the surface and in depth

In 1902, 120 years ago, Professor of Zoology, Johan Axel Palmén, established Finland’s first biological research station in Tvärminne.

Palmén believed that students would find it more interesting to study different species in a natural setting rather than in museums and lecture halls. He found a suitable location in Tvärminne, a village near Hanko on the south-west tip of the Finnish coastline and bought a croft there to be used as the premises of the zoological station.

Palmén bequeathed the research station to the University of Helsinki, and today it is a part of the Faculty of Biological and Environmental Sciences.

The research station is especially known for its longitudinal research projects, the longest of which, the Storfjärden benthos study, was started in 1926 and is still ongoing. Likewise, the measuring of the seawater temperature every ten days dates back to this time.

The researchers have also been monitoring the common eider population since 1990, and the bay mussel since 1997.

Last year the research station initiated a research project on the effects of coastal water diversity on climate change. In the project, biologists and climatologists aim to investigate whether the Baltic Sea works to store carbon or produce it.

“There has been plenty of research on what the effects of the climate on the sea are. But in this project we are looking at things the other way round: how the state of the sea affects the kind of emissions that end up in the air,” states research coordinator Joanna Norkko.

In August, a course on scientific diving took place in Tvärminne. In Finland, there has not been training in scientific diving in ten years, and there is beginning to be a shortage of qualified divers. Although technology is a great help in marine research, it is no substitute for researchers getting in the water to collect samples and study the sea.