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

You might have seen posts and pictures of JOMEX on the Tvärminne Zoological Station Facebook page or on the news. But what is JOMEX and why is there this big raft with a white hut and multiple floating bags anchored in the middle of a coastal area?

JOMEX raft near Tvärminne Zoological Station in June 2019

JOMEX (Joint Mesocosm Experiments) is one of the research projects under AQUACOSM, a network of mesocosm facilities funded by EU. The purpose of AQUACOSM is to create conversation and cooperation between aquatic sciences. In the past there has been a gap between different aquatic sciences, for example between marine and freshwater sciences. However, in real life water systems are connected to one another and there are no boundaries separating them. Bringing these researchers together will improve the understanding of different water bodies and links between them.

Preparing the mesocosm bags

Another goal of AQUACOSM is to make aquatic research more comprehensive and comparable. The effects of certain stressors can vary widely among ecosystems due to spatial and temporal attributes of the environments. Therefore, to understand the actions and effects of different stressors, it is necessary to organize comparable experiments in disparate places. JOMEX, for example, will be organized in different types of water systems from Arctic to Mediterranean environments, and from freshwater to fully marine systems. The experiment in Tvärminne covers the brackish water environment in this joint research.

The aim of JOMEX is to study the responses of aquatic systems to a pulse of dissolved organic carbon (DOC).  In the future increased amounts of DOC will enter aquatic environments due to climate change and increased precipitation. Trenching of swamps also contributes the carbon load to water systems. DOC and brownification of water affect aquatic ecosystems by attenuating solar radiation and delivering nutrients to organisms to a greater extent. However, the exact impacts of brownification to aquatic ecosystems are still poorly known and for that reason this experiment is trying to clarify how vulnerable aquatic ecosystems are to the overload of carbon.

Adding DOC to the mesocsoms

JOMEX is a mesocosm experiment meaning that the experiment is performed outdoors but controlled. In our experiment there are 12 sample bags filled with 2000 liters of sea water. The bags are treated with DOC, nutrients or both. There are also 3 control bags with no additional treatments. Walls of the bags are made of plastic so the water in the bags isn’t in touch with ambient water.

JOMEX is carried out in cooperation with the Finnish Environment Institute SYKE. The people in SYKE have created an automatic measuring device called Aquabox. Aquabox automatically gauges many different abiotic and biotic factors, for example acidity and chlorophyll, tirelessly from the sample bags at regular intervals. The machine can be controlled remotely which enables massive amounts of data without requirement that someone is on the raft continually. Aquabox is not meant to be used just in this experiment and therefore JOMEX gives a perfect opportunity to test and develop the machine so it can be used broadly in aquatic research in the future. Hence, JOMEX has an infrastructural purpose in addition to the scientific clearance.

AquaBox – The automatic sampler

In addition to automatic measurements there is also manual sampling in the experiment. Every morning brave samplers go to the raft and take water samples for laboratory treatments and measuring. One reason for manual sampling and laboratory work is to make sure that the Aquabox is working well and properly.

Sampling the mesocosms

The experiment lasts two weeks but that’s just a tip of the iceberg. The programming and producing of Aquabox have taken about 2 years. At Tvärminne preparations for JOMEX started right after Christmas and practical works (e.g. building of the raft) began properly in May. So, as you can see, a lot of work has been required for this project.

Overall AQUACOSM and JOMEX will provide new ways to study the environmental responses to experimental manipulation. New technology used in the project will enable more ambitious and accurate use of mesocosms and cooperation with other facilities allows possibilities to more comprehensive and broader research. Recently AQUACOSM has applied for funding for the next mesocosm project – hopefully there will be more extensive and diverse projects like this!

Text by Roope Nykänen

Photos by Roope Nykänen, Alf Norkko, Joanna Norkko

JOMEX team at Tvärminne 2019

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.

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 →