Benthic fauna – the animals living on or in the seafloor – is an important tool for management. Since these animals are relatively long-lived and don’t move around too much, they can show accumulated effects of the environment and are therefore used in environmental monitoring, tells Eva Ehrnsten, Stockholm University Baltic Sea Centre, whose research is focused mainly at these small creatures. But the benthic animals themselves also affect the environment; by eating and mixing the sediments they impact the recycling of nutrients and carbon and, by extension, eutrophication and climate change. 

 – And they are also important as food for fish, particularly flatfish and cod, says Eva Ehrnsten.

The high inputs of nutrient to the Baltic Sea in the second half of the 20th century caused a drastic increase in phytoplankton. This has led to more food for the benthic animals, but also to an increased area of oxygen-depleted bottoms, where almost no animals can live. Eva Ehrnsten's modelling-based research shows that there has been a loss of benthic fauna in the deep areas, but an increase in the shallow ones.

 – Altogether, the biomass of benthic fauna has increased by about 50 percent from 1970 to about 1990 and stayed stable after that. Most of the increase has happened in the Baltic Proper.

 

Increase and decrease in species

In another project Ehrnsten and her colleagues looked at monitoring data for specific benthic species and noticed a large increase in the Baltic tellin (Macoma balthica).

– It can tolerate many different environments and have really been benefiting from the increase in food supply, says Eva Ehrnsten.

Another group of species that have increased is the Marenzelleria worms – the three species probably reached the Baltic Sea through ship traffic and were first found in the 1980s.

– Since then they have been spreading throughout the sea.

Among the species that have decreased is the amphipod Monoporeia affinis. It is sensitive to oxygen depletion, but has also decreased drastically in areas with good oxygen conditions, possibly because of toxins or changes in the quality of its food. The river mussels, who live in estuaries and bays and are sensitive to coastal construction and changes in forestry, have also decreased.

– The situation we have today is a very different composition of fauna compared to what we had in the 70s, concludes Eva Ehrnsten.

 

Effects on eutrophication and fish

Benthic fauna could both increase and decrease the release of nutrients from the seafloor. The latest, not yet published, study by Eva Ehrnsten and her colleagues suggest that the net effect is reduced eutrophication. The changes in benthic communities can also affect the fish stocks.

It’s possible that the increase in flounder that we have seen lately is related to more benthic food. Cod on the other hand eats crustaceans like Monoporeia that has been decreasing, and Saduria for what we don’t really know what has happened, says Eva Ehrnsten, adding that fisheries and reproductive conditions probably have larger impacts on the cod stocks.

Future simulations show that with current nutrient inputs, the benthic fauna will decrease, and even more so if the inputs are reduced according to the Baltic Sea Action Plan, while very high inputs will make the amount of fauna increase further. When the effects of climate change are added to the model, the decrease is pronounced and occurring even in the high-input scenario. 

– This steep decrease might look worrying, but actually this is more a return to the levels of the 1920s, says Ehrnsten.

Large-scale changes in the Baltic Sea food web

Not only the amount and composition of benthic animals have changed – the current Baltic Sea ecosystem is completely different today than 100 years ago, says Maciej Tomczak, the main author behind a study describing these large-scale changes in the central Baltic Sea. 
Is the Baltic Sea the same sea today as it was 100 years ago, he asks rhetorically to provide the answer:

– No, it's a completely different sea. It's a completely different system.

The increased nutrient loads have caused a regime shift from a low productive to a high productive system, and from an oxic to hypoxic state. The interaction between the benthic animals and the pelagic ones is not as strong as it used to be, and the controlling driver has changed from salinity to oxygen. 

– The eutrophication process in the Baltic Sea has not only consequences for phytoplankton and zooplankton – it reaches higher trophic levels and has broad consequences, says Maciej Tomczak.

When studying ecosystem changes it’s important to look at long time periods – not only, as is quite common, go back to the 1970s or perhaps to the 1950s, says Maciej Tomczak. Only by looking at long reconstructed time series, the changes in for example oxygen and salinity are fully noticeable.

– Also for nutrient inputs, phytoplankton biomass and fish stocks, such as cod and flounder, it’s important to go further back than a few decades to get a fuller picture.

Eighty years – four periods

In the new study, the researchers compiled number of historical data series to characterize how the food web changed. Statistical analyses showed that the main shift between the low and the high productive periods happening in the mid 1970s, and when the two periods were analysed separately, further structural changes were noticed. The eight decades 1925-2005 were finally divided into four periods.

– First we have the flounder peak period: the biomass of flounder is high, the one of cod is relatively low, the system is oxic and the fisheries mainly affect the flatfish communities. There are a lot of seals affecting the ecosystem from top-down. However, the basis – the phytoplankton production is low so the total food web biomass is relatively small, explains Maciej Tomczak.

In the middle of the 1930s, the flounder collapsed.

– This was probably the very first example of overfishing in the Baltic.

The following period is called the reference ecosystem by the researchers. The flounder biomass is low, but oxygen conditions are still good and nutrient inputs are increasing.

– It’s a relatively long period of time where none of the ecosystem compartments – the groups – are blooming, says Maciej Tomczak.

Then occurred the shift to the high productive period, where eutrophication significantly increased the biomass of the system.

– At the same time, in the 1980s, there were really good oxygen and salinity conditions because of inflows from the North Sea. And especially cod was supported by both benthic and pelagic pathways.

But also the cod period came to an end.

– Cod collapsed because of no inflows and high fishing pressures. Then followed the still very productive, but now sprat dominated period, when interactions between cod, sprat and Pseudocalanus zooplankton plays a major role and keeps the system at the current state, says Maciej Tomczak.

 

Important for management

Knowledge about the large ecosystem changes is important for the management of the Baltic Sea, means Maciej Tomczak.
– As a reference system in the Baltic Sea, we usually refer to the 1980s, which was a perfect state for fish. But that is not perfect for management, because it could never happen again. So, we have to think: should we go back in the past and refer our environmental policy to what has happened before or are we setting a target to where we want to be?

If you were to send a wish list to the Santa Claus of management – what would be on it, asks moderator Charles Berkow.

– In both of these studies, and many others, we use the long monitoring series to see what happens in the ecosystem. I really hope that we can continue with these long time series and to do them in a comparable way, says Eva Ehrnsten.

– Use the integrated approach to manage the Baltic Sea and don’t manage eutrophication separately, fisheries separately and climate change separately… And follow the science advice for management – now it’s often lost somewhere, says Maciej Tomczak.

Text and photo: Lisa Bergqvist

 

Answers to questions from the audience

To what extent can ecological regime shifts in the Baltic be anticipated or even actively managed through human intervention? Which organisation would be responsible for such an approach?

Eva Ehrnsten: We can never forecast the future, but we can make scenarios to try to understand the range of possible futures. For management, it is important to monitor and analyse different parts or level of the system in concert to find early warning signals of possible regime shifts. HELCOM has an important coordinating role here.

Maciej Tomczak:  We can’t manage regime-shifts, but we can manage the pressures i.e. climate, nutrients input and fisheries. Thru that we can actively manage ecosystem state. With current knowledge we can’t predict regime-shifts and ecosystem thresholds, but with system understanding and integrated strategic management plans for build on simulation scenarios we can be prepare and act accordingly. 

Would it be helpful (or even possible) to aggregate data on the history, current status and future forecasts of the Baltic food web into some kind of open access decision tool for researchers, fisheries, authorities and other stakeholders?

Eva: We cannot make one general tool for everything, tools need to be built for specific types of problems or decisions. An example of a successful tool is the Baltic Nest System with the BALTSEM model, that was used to calculate the nutrient reduction targets in the Baltic Sea Action Plan.

Maciej: Such a tool exists, built at the co-creation process at number of project to support decisions, but also designed to be used by stakeholders. You can look at the MareFrame output or BlueWebs that give examples of what is available: mareframe.github.io/dsf/baltic-sea or Uusitalo et al: Integrating diverse model results into decision support for good environmental status and blue growth.

Eva, did I understand correct that when you added climate change to the model the effect from eutrophication (of dead sea floor) diminished?

Eva: The extent of hypoxic areas or “dead bottoms” is determined by a suite of physical and chemical properties which may all change with climate change, and therefore there is no simple answer to your question. What we saw in the simulations is that the amount of nutrient inputs is much more important than effects of climate change: with increased nutrient inputs the hypoxic area increases, and with decreased inputs it decreases. The main reason we get less fauna with climate change is that as the water gets warmer, a larger proportion of the phytoplankton (algae) gets degraded and recycled in the water column and a smaller proportion sinks to the bottom as food for the fauna. For a more thorough explanation, please see our open access paper: Ehrnsten et al 2020: The meagre future of benthic fauna in a coastal sea – benthic responses to recovery from eutrophication in a changing climate. 

Eva, do you see any possibility to include in your future simulations that new species might settle, which might be adapted to reduced supply with organic material?

Eva: This is an interesting question. At the moment, our model works with functional groups of fauna rather that species. However, if a newcomer would have different functional traits, for example if it eats a different part of the sediment or has a slower growth rate compared to the existing species, we could add this to the model as a new functional group and see how that changes the system. 

Maciej: What has happened after 2005?

Maciej: We use time period 1925-2005 for consistency of time series data. For what happened after 2005, it’s better to use the observed data. Similar analysis at the integrated level can be found in the report of ICES/HELCOM Working Group on Integrated Ecosystem Assessment of The Baltic Sea.

My question is how we can best manage the lag between when we see a shift and would need immediate management, and the much delayed response by management bodies? Is it possible to reduce the response time?

Maciej: The response time at the ecosystem level is usually long, except regime shift which may happened quite rapidly. But environmental management need to have integrated strategic plans for ecosystem to act when necessary.

Is there any specific kind of monitoring data that is missing to understand the ecosystem better? I.e. data on certain levels in the food web?

Maciej: Mysids are an important component in food webs, for example as food for herring, that we lack data on.

Eva: Mysids are an example of a food-web component that is neither completely benthic nor pelagic and therefore tends to “fall between the cracks” in a world where we compartmentalize science and management instead of looking at the system as a whole.

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