Bladderwrack is back in the inner archipelago – old herbarium material provides new knowledge
Improved wastewater treatment has led to better water quality in the Stockholm archipelago, and a new study shows that bladderwrack has now returned to the inner part of Trälhavet, north of the city. The study also shows that archived herbarium material can be an important tool for detecting environmental changes, says researcher Lena Kautsky.
During the 20th century, increased eutrophication of the Baltic Sea led to the disappearance of bladderwrack (Fucus vesiculosus) from many coastal areas, including large parts of the Stockholm archipelago. One example is Trälhavet north of Stockholm, where dive surveys conducted by Inge Lennmark in the late 1960s and 1990, respectively, clearly showed how the distribution of bladderwrack changed. The area is affected by nutrient inputs from Lake Mälaren and the river Åkerströmmen, and decades of eutrophication led to the disappearance of bladderwrack from the inner parts of the archipelago in 1990.
"Since then, the treatment of nitrogen and phosphorus in wastewater has improved, nutrient levels in the water have dropped and visibility has improved", says researcher Lena Kautsky. "We wanted to find out how this had affected bladderwrack – had it come back?"
Herbarium material from Lennmark's studies is stored at the Museum of Evolution at Uppsala University. But the museum also has even older material: bladderwrack thalli collected by fisheries superintendent Rudolf Lundberg between 1885 and 1886 with a benthic dredge. Lundberg's descriptions of the surveys show where the samples were taken and that bladderwrack grew even further into the archipelago at the end of the 19th century than it did in 1968.
New investigations show improvements
In 2001, a thesis was conducted in the same archipelago area. It showed some improvement since 1990, but bladderwrack was still only found far out in the archipelago. In 2017 and 2020, Lena Kautsky and her colleagues Ellen Schagerström and Susanne Qvarfordt returned to Trälhavet for new surveys. With all this new and old data in hand, they can now conclude in a new study: bladderwrack is back, and in a big way.
"Its distribution is now at the same level as at the end of the 19th century,” says Lena Kautsky. "It is probably now found as far into the archipelago as possible – further in, the salinity is too low for it to thrive regardless of nutrient levels and visibility."
The most recently studied bladderwrack plants are larger than those collected previously, which Lena Kautsky sees as a sign that the environment has been favourable ever since they were small seedlings. The situation in Trälhavet is probably not unique either, but representative of many other areas along the Baltic coast where nitrogen and phosphorus emissions have decreased in recent decades.
"These results show that when discharges are reduced and water quality improves, bladderwrack will be able come back on its own without restauration efforts. But it takes time," emphasises Lena Kautsky.
Herbarium records tell us about other species
The historical herbarium material provides more information than the presence of bladderwrack in a particular site; the presence and coverage of epibionts also give valuable insights on how the environment has changed over the years.
"An herbarium sheet is like an archive, and together they tell a story," says Lena Kautsky.
About half of the bladderwrack thalli from 1885 and 1886 were covered with filamentous algae, indicating that there were probably already a lot of nutrients in the water due to Stockholm's rapidly growing population and lack of sewage treatment. Also, the introduced species Cordylophora caspia, a hydroid, and the barnacle Amphibalanus improvisus occurred in very small amounts.
"Herbarium sheets can provide evidence that a species was present at the site earlier than you might have thought, and before anyone started looking for it", notes Lena Kautsky.
Epibiotic cover on bladderwrack (11314 Kb)
Also in 1968, filamentous algae dominate the cover together with small amounts of filtering species, mainly Einhornia crustulenta (a bryozoan).
In 1990, the epibiotic coverage was instead dominated by the filter feeders Einhornia crustulenta and Amphibalanus improvisus.
"Visibility was low and the water was very turbid, so there were lots of particles for these filter feeders to eat", says Lena Kautsky.
As the nutrient content decreases and the water clarity improves, the epibiotic coverage decreases during the first decades of the 21st century. The most recently collected bladderwrack have a low cover and a high diversity of species of both algae and filtering animals.
Lena Kautsky believes that archived herbarium material can be an underutilised source of knowledge.
"There is so much knowledge to be gained from past collections of herbaria in our museums. For example, it is possible to analyse the content of environmental toxins, stable isotopes or, as in our study, the presence of different epibiotic species. At the same time, it is important that new collections are made, which document how things look today."
Text: Lisa Bergqvist
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Last updated: February 3, 2025
Source: Baltic Sea Centre