Author Archives: Mats O Westerbom

Lush undersea gardens

If you take a dive under the surface, you might be surprised to see a lush and vibrant undersea meadow consisting of a diverse range of aquatic plants. As the Baltic Sea is brackish, the biota consists of both marine and freshwater species. This is especially true for aquatic plants, and they are often found growing intermixed, marine seagrasses and freshwater plants side by side.

These aquatic plants form valuable habitats because of many reasons. They produce oxygen, store nutrients and carbon, provide a living environment for many small animals and clarify the surrounding waters. Aquatic plants are found on soft sediments, so where there is enough sunlight reaching the bottom and sandy, muddy or gravelly sediments, there are usually plants around as well. When taking a closer look at these grasses, pondweeds and milfoils, you might see that the different species look quite different. Some of them are tall with ribbon-like leaves and short roots, while others are small with needle or thread-like leaves and long roots. These plant characteristics or traits underlie the effect that these species have on for example, primary production but we do not really know which traits influence production the most. As a plant meadow or community can consist of various species, all with slightly different traits, the functional diversity (i.e. the diversity of traits) can be high.

Different plant species thrive in different environments. Some prefer clearer waters and wave-swept sandy bottoms found in the wind-exposed outer archipelago, whereas others like the calmer waters and muddy sediments found in the sheltered inner archipelago. Therefore, the species and trait composition of plant communities can vary quite a bit depending on if you dive into an undersea meadow in the outer or inner archipelago.

In a recent study, researchers explored which plant traits are important for primary production and whether potential relationships between traits and production change when moving from the outer to the inner archipelago. They visited 30 sites along a wave exposure gradient and used SCUBA diving to sample plant communities and measure different plant traits. The researchers found that one specific trait, plant height, was important for primary production across the archipelago (outer to inner archipelago). This means that no matter what species were present in a community, the taller individuals or species influenced the production to a greater extent. Plants need sunlight to grow and if they grow taller they can access more light and consequently photosynthesize more efficiently.

On the other hand, the study also revealed that the relationship between other traits than plant height and primary production changed in communities when moving from the outer to the inner archipelago. This means that the underlying biological mechanisms that support primary production can vary in different plant meadows and finding out which specific traits are important for production is not always clear-cut.

Not only are lush and vivid undersea meadows beautiful, but these functionally diverse plant communities also play a vital role in the functioning of coastal shallow habitats. With climate change on the rise, it has become ever more so important to recognize that diverse marine habitats such as undersea meadows (whether tall or short!) are important in maintaining ecosystem functioning, increasing the stability of coastal ecosystems and ultimately, contributing to a healthy Baltic Sea.

Read more about the study here: Gustafsson C & Norkko A (2019) Quantifying the importance of functional traits for primary production in aquatic plant communities. J Ecol 107:154-166 

Text by Camilla Gustafsson
Photos by Mats Westerbom

Never underestimate the power of small ones


As you could read from the previous Emerging Stories from the Sea -blog post, oceans and seas are vital for our atmosphere, because they produce over 50% of the oxygen that we breathe. In the marine world, there is one specific group of organisms that is responsible for almost half of this oxygen production, they are diatoms. Diatoms are microscopic algae that rarely exceed the length of 0.1 mm, but are abundant especially during the spring period.

Every diatom species has its own characteristic features. Photo by Leena Virta.


Spring is the prime time for diatoms in the northern hemisphere. In the Baltic Sea, the abundance of diatoms explodes as soon as the amount of light increases and the warming of surface waters stabilizes the vertical water layers preventing planktonic diatoms from sinking to deeper and darker waters where light is too low to allow photosynthesis. The period is called the diatom spring bloom. It lasts only for a month, but during this time diatoms form blooms that are so extensive that they can be seen from space. When diatoms photosynthesize, they produce oxygen and transfer carbon dioxide from the atmosphere and store it in the bottoms of the sea. Without this service by diatoms, climate warming would take off to a whole new level.

When nutrients are depleted from the water mass, the spring bloom ends and the biomass of diatoms starts to decline. The story of diatoms is not, however, over here – they just shift from the open waters to the microphytobenthos. Microphytobenthos are species that live in or on the sediment of the bottom or are attached to submerged substrates, such as rocks or underwater vegetation. Here diatoms are food for invertebrates, which are eaten by e.g. fish that in turn may end up on the menu of larger predators, such as us humans. This food web is like a building: if we are the penthouse, diatoms form a major part of the foundation of the building. If the foundations becomes unstable, also the penthouse comes crashing down.

Although diatoms are a favorite dish for many grazers, they are not defenseless. They have developed different strategies to avoid being eaten: some species stay low and rely on the shelter of higher species; some attach themselves firmly to the substrate; and some others move up and down in the sediment while evading grazers and simultaneously searching for light and nutrients.

There are tens of thousands of diatom species in the world, and under a microscope every one of those species looks different. Even in the Baltic Sea, which in general is characterized by low species richness because of the challenging salinity conditions, a single sediment sample may contain over a hundred different diatom species, each of them decorated with unique and beautiful siliceous structures. So even though the microphytobenthic layer looks like a boring green mat, it is full of diatom personalities with different characteristics but with an equal importance for the ecosystem functioning. The lesson of this story: never underestimate the power of the small ones!


Taking diatom samples from stones in the archipelago of Tvärminne. Photo by Alf Norkko.

Text by Leena Virta.

From the Blue Planet to the Green Baltic Sea

The ocean is a vast place, covering over 70% of the world’s surface. Oceans provide us with many valuable resources and services. They are intimately bound to all of us, far more than what we might initially think. Even when we practically do nothing, we are tightly connected to the ocean since the oxygen in every second breath we take comes from the ocean – organisms living in the ocean produce more than 50% of the oxygen in our atmosphere. If this service fails, imagine then how unpleasant it would be if you would be able to breathe only every second time you feel you need to! Oceans also stabilize the climate. They are the biggest absorbents of heat. Without the oceanic buffer; global temperatures on land would be far greater than what they are. This heat stabilizing system is now under threat as global oceans are becoming warmer due to human action. The Intergovernmental Panel of Climate Change (IPCC) predicts that by 2100 the ocean temperature may increase by 1 – 4 ℃. This increase is huge, even at the lower range, as the average temperature in ocean surface waters is only 17 ℃. Imagine next how uncomfortable you are when your body temperature increase with only 1-2 degrees when you have a harmless flue! Proportionately, the change in the ocean is much bigger and the consequences are worse. We are currently hearing the first coughs of the “Patient Ocean” and we know that after cough comes fever. The health of the ocean is not a trivial issue!

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In this blog, we publish every month a short story including photographs of the Baltic Sea. Our aim is to introduce and bring the the Baltic Sea closer to people who are interested in the sea. Our aim is to show the seascapes and the species to the crowd and combine photographs with old or fresh research information. This project is carried out in cooperation with Svenska kulturfonden and Tvärminne Zoological Station. Continue reading