The average size of zooplankton is decreasing


The great plankton change – good or bad for the Baltic Sea ecosystem?

The phytoplankton community of the Baltic Sea is changing. A trend towards more cyanobacteria and fewer diatoms affects the entire marine ecosystem. The big question for the marine scientists is: How?

Text: Henrik Hamrén

For an ecosystem where large eats small, the first link in the food chain - the food base - is particularly important. In the sea, the food base consists of phytoplankton. Without them, the ecosystem collapses.

In recent decades, climate change has led to a change in the composition of phytoplankton in the world's oceans in a way that is now worrying more and more marine scientists. A recently published British study is warning that the ongoing change could have serious consequences for many marine coastal environments. The study focuses on the northeastern Atlantic, where two important phytoplankton in particular - diatoms and dinoflagellates - have fallen sharply, while the number of small cyanobacteria has skyrocketed.

A similar change is also taking place in the Baltic Sea.

– The dominant trend is that the amount of filamentous cyanobacteria has increased quite a lot over the past 50 years while the diatoms has decreased, says Monika Winder, professor at the Department of Ecology, Environment and Plans Sciences at Stockholm University.

Poor food situation for zooplankton

The change is mainly driven by eutrophication and increased temperature, which lead to earlier algal blooms in the Baltic Sea.

– Spring bloom are dominated by diatoms which are good food for zooplankton. Nowadays it comes already in March-April when it is still cold and not many zooplankton in the water, says Monika Winder.

The summer bloom, which is dominated by cyanobacteria, also starts earlier in the year. It has also become more widespread and lasts longer. In general, cyanobacteria are poorer food for zooplankton than diatoms. Among other things, they lack essential fatty acids, omega-3 and other vital substances that the animals in the food chain can only get through food.

In addition, many zooplankton and benthic organisms have difficulty eating or digesting larger cyanobacteria. A new experimental study shows that three of the Baltic Sea's most common benthic organisms (the amphipod M. affinis, the clam L. balthica and the polychate Marenzelleria) prefer diatoms over cyanobacteria.

– During the summer, when the biomass of copepods and other zooplankton is at its largest, the sea is now dominated by cyanobacteria, which are relatively nutrient-poor food for them, says Monika Winder.

Science is fragmentary, sometimes contradictory

The trend towards more cyanobacteria and fewer diatoms is fundamental and affects the basis of all marine wildlife. But exactly what the effects are and which species will be affected – and how – is still unclear.

Extensive plankton research is underway around the world, and a steady stream of new studies is being produced. But the overall picture of what is happening in the sea is still fragmentary – and sometimes even contradictory.

One reason for that is that much of today's plankton research is based on laboratory experiments. They can provide theoretical answers to how different species react to certain foods. But the marine ecosystems of reality are much more complicated, and the complexity increases for each trophic level in the food chain. Therefore, a large part of plankton research so far revolves around different hypotheses.

Copepods form a large part of the Baltic Sea's zooplankton fauna, and are an important nutrient link between phytoplankton and fish. Foto: Azote

A basic hypothesis is that the increase in cyanobacteria in some way affects the next trophic level in the food chain: zooplankton.

– It is of course close at hand to assume it. We know, for example, that some copepod species produce fewer eggs and decrease in growth if you give them a lot of cyanobacteria, says Monika Winder.

Cyanobacteria are often associated with toxic algal blooms and other negative aspects. But there is also a lot of research that highlights positive effects. Last summer, for example, a study showed that the small picocyanobacteria Synechoccocus (0.2 - 2 micrometers) can be a good carbon source for certain species of copepods.

Other previous studies (such as this and this) have also shown positive effects of zooplankton eating Synechoccocus.

May be necessary for maintaining a large fish production

According to a compilation of several different studies, larger cyanobacteria can also indirectly benefit both the zooplankton community and benthic fauna in the Baltic Sea. The filamentous species of cyanobacteria that sometimes form surface blooms during the summer have the ability to fix nitrogen gas from the air. In this way, they add new nitrogen to the water and thus fertilize other phytoplankton, which in turn is good food for zooplankton.

– The traces from this specific process can be followed throughout the food web all the way up to fish, says Agnes Karlson, who is a researcher at the Department of Ecology, Environment and Plant Sciences at Stockholm University.

She will soon begin a study that will examine these connections more closely.

– It may turn out that cyanobacteria are even necessary for maintaining a large fish production, she says.

Zooplankton size is decreasing

The question is how all these different research findings should ultimately be valued in relation to each other – and to the ecosystem as a whole. Is the increase in cyanobacteria overall good or bad for marine animals? Does the ongoing change in the phytoplankton community lead to a net deficiency of certain nutrients higher up in the food web? Or can zooplankton and other benthic organisms compensate by finding other food?

– The variation in nature makes it difficult to say anything in general about how the increase in cyanobacteria affects the zooplankton community, says Elena Gorokhova, professor at the Department of Environmental Science and Analytical Chemistry at Stockholm University.

– In the laboratory experiments, zooplankton eat what we put in the jar. Some only get cyanobacteria while others only get diatoms, then we study how they react. But in nature it looks different. There they can choose between thousands of different phytoplankton and other organisms. If something is missing, they can in many cases compensate by eating something else, she says.

In addition, the plankton composition and the amount of cyanobacteria vary between different areas in the Baltic Sea, which makes it difficult to draw conclusions that apply to the entire sea.

– The only general trend we can see in almost the entire Baltic Sea is that the average size of zooplankton is decreasing, says Elena Gorokhova.

Why is that?

– We do not know for sure. Climate change can contribute, as the body size of many invertebrates, including zooplankton, decreases at higher temperatures. But my interpretation is that it is mainly related to the fisheries.

How do you mean?

– In the Baltic Sea, cod has declined sharply, which means that there is more sprat and other plankton-eating fish, such as sprat. The fish prefer large zooplankton and take them first. If they can choose, they would rather eat a large copepod than hundreds of smaller organisms.

Blue mussels are losing weight

Blue mussel in the Baltic Sea have halved their average weight since the 1990s, partly due to changes in the phytoplankton fauna.

It's not just zooplankton that eat phytoplankton. A Swedish study was recently published which shows that blue mussels in the Baltic Sea have halved their average weight since the 1990s. Using chemical analyzes and environmental monitoring data from the Askö Laboratory, the researchers found a certain connection between the decline of mussels and changes in the phytoplankton community.

– The mussel filters and eats different kinds of plankton. These plankton give different chemical imprints in the mussel. We looked specifically at trace isotopes of nitrogen and carbon in the mussels, and saw quite clear changes over time, says Agnes Karlsson.

The analyzes showed not only a connection to the changed plankton composition but also to an increased influx of particles from land.

– These particles consist of organic material from forests and land, decaying plant parts and other things, that flow into the sea via rivers and streams, says Agnes Karlsson.

And they are not good food for the blue mussel?

– Some may be edible even for the mussel, but they are not as nutritious as a fresh phytoplankton, she says.

According to most forecasts, climate change will increase precipitation in the Baltic Sea region. This also increases the supply of particles from land – which can further impair the food base for mussels and perhaps also other benthic organisms.

Looks like the mussels can adapt

To understand more about the importance of summer cyanobacterial blooms for blue mussel weight loss, Agnes Karlsson and her colleagues recently conducted an experiment in which mussels were fed with different plankton compositions for 50 days. Unlike similar experimental studies, they used field samples of actual summer and spring blooms, which contain not only cyanobacteria but also a lot of other phytoplankton, small heterotrophic organisms and small zooplankton.

The results show that the summer blooms might be better food for the mussel than expected.

– Together, they seem to be a pretty good food for the mussel, says Camilla Lienart, researcher at the Department of Ecology, Environment and Plans Sciences at Stockholm University and author of the study.

The mussels that received spring bloom samples with a lot of diatoms took up all the essential fatty acids that are missing in the summer blooms. But on the other hand, it turned out that the mussels that got summer bloom samples could take advantage of shorter fatty acids and in some way elongate them to build themselves the longer essential fatty acids, Camilla Lienart explains.

– This is preliminary data and more analyzes need to be done. But when it comes to fatty acids, summer blooms seem to have other benefits that diatoms lack. And it looks like the mussels can adapt, she says.

Favorable for some species, worse for others

From the mussel's perspective, this could mean that the spring and summer blooms complement each other during the year. And in that case, it may not be the cyanobacteria that are the biggest problem, but instead the fact that the spring blooms with diatoms decrease.

– Yes, that may very well be the case. Of course, large cyano blooms can lead to other problems, but the reduced spring bloom is an important aspect that I think we talk too little about, says Camilla Lienart.

What is the best phytoplankton composition for the Baltic Sea? More diatoms and fewer cyanobacteria?

– It depends entirely on what we define as good, and for whom. The current situation may be favorable for some species but worse for others. I think it is important to look at the entire ecosystem and remember that many species can adapt to new conditions, says Camilla Lienart.

So, maybe there is no optimal food base for the sea?

– That thing with optimal food… it is not something I really believe in, not in this context anyway. I believe that diversity and balance are the most important keys to a functioning ecosystem.

Facts: Phytoplankton

Microalgae, or phytoplankton, are microscopic plants that are found in all aquatic environments and form the basis of the food chain. There are probably up to 10,000 different species of phytoplankton in the Baltic Sea. The three predominant varieties are diatoms, dinoflagellates and cyanobacteria.

As phytoplankton grows, large parts of the nutrients in the water are converted to plankton biomass. It becomes food for the rest of the animals in the food chain, from zooplankton and mussels further up to fish, seabirds and seals.

Cyanobacteria, also called blue-green algae, are not really algae but a kind of bacteria. These include the poisonous Nodularia spumigena, which is sometimes warned about in the Baltic Sea during the summer.


Henrik Hamrén

science journalist