Photo: Andre Maslennikov /Azote.

Most people agree that eutrophication is the biggest threat to the Baltic Sea ecosystem. But opinions differ on what the solutions are and which measures society should take to address eutrophication. To clarify the position of Stockholm University's Baltic Sea Center, we compiled our stance on the questions that often arise in this debate:

Today, more phosphorus is supplied to the sea from the seabed than from land. Should we not address that source first, instead of investing in efforts on land?

No, reducing nutrient inputs from land is still the most important measure. It is true that the so-called internal load, i.e. flux of phosphorus from bottom sediments to the water column, is now greater than the supply from land. But combatting eutrophication requires that we address the causes rather than the symptoms and the main cause of eutrophication is the excessive input of nutrients that are supplied to from land to the sea. The internal load is a consequence – a symptom  – and sediments do not release "new" phosphorus, but recycles old emissions from land.

Does it really matter what you call things? Aren’t actions more important?

Absolutely. But serious and effective measures require that we know against what we are taking action. This is especially true when resources are limited and spending must be prioritized.  Over the past century, large pools of phosphorus have accumulated both on land and in the sea. The phosphorus pool on land (over 40 million tonnes) is about 20 times greater than the amount stored in Baltic Sea sediments. In addition, the land pool continues to grow by nearly 400,000 tonnes each year - which increases the risk of nutrient leakage to the sea.

What should be done about the internal load?

If nutrient inputs from land decrease, the internal load will also gradually decrease, as phosphorus becomes buried in the deep layer of sediments, from where it can no longer leak. One could say that this layer is the “final resting place” for phosphorous. In the Baltic Proper, about 17,000 tonnes of phosphorus are permanently buried in sediments. At the same time, inputs from land have been reduced to less than 17,000 tonnes annually during the last 15 years. Hence, the total amount of phosphorus in the Baltic Proper is gradually decreasing.

Phosphorus inputs from land have been halved since the 1980s, but the sea is still eutrophic. Doesn’t this show that land-based measures don’t work?

No, it shows that the sea responds slowly. The largest reduction in phosphorus inputs occurred only in the last decade. The ecological reality -- of not just the Baltic Sea, but all eutrophic water bodies -- is that it will take time to recover. In Kattegatt and Belt Sea, where nutrient reductions occurred 20 years earlier than the rest of the Baltic Sea, the symptoms of eutrophication have decreased significantly in recent years.

When can we hope to see positive effects in the rest of the Baltic Sea?

The Baltic Sea is on the verge of recovery. The powerful measures of recent decades are already producing visible improvements in some areas. But recovery takes time. It is naive to believe that we can transform the entire Baltic Sea, with its 22,000 cubic kilometers (!) of water, in just ten years. The important thing is that we are making progress. For example, in recent years, sewage treatment in the major cities of St. Petersburg, Warsaw, and Kaliningrad has been expanded, and nutrient inputs to the Gulf of Finland have been halved. If we continue these efforts, and do even more, the sea will recover even faster.

There are suggestions to use “geo-engineering” to speed up the recovery of the sea. Doesn’t it make sense to use these technologies?  

No, not if it means less resources for land-based measures. The geo-engineering proposals include artificial oxygenation of the sea’s bottom waters, chemical precipitation of phosphorus, and large-scale dredging and removal of sediments. At present, none of these methods are realistic alternatives to actions on land.

Why not?

There not enough knowledge to assess the effects, consequences, and risks of geo-engineering in the Baltic Sea. While these techniques have long been used in lakes, the results are mixed. We simply do not know enough about how these methods will work on a much larger scale, in the sea. This doesn’t mean that we shouldn’t discuss and further investigate the potential for these methods to combat eutrophication. But these efforts cannot shift the focus from the successful land-based work that is currently in progress and must continue.

During the Baltic Sea Festival, Tina Elfwing (director of the Stockholm University Baltic Sea Center), Sven Blomqvist (Lecturer of Systems Ecology at Stockholm University) and Bengt Simonsson (Research and Marketing Manager at Teknikmarknad) met to discuss how to combat eutrophication in the Baltic Sea.

The conversation took place in Stockholm on August 23rd and can be seen here (in swedish):

Conversation in Radiohuset in Stockholm August 23rd 2017