To speed up recovery of the Baltic Sea from eutrophication, geoengineering solutions have been proposed to reduce the internal phosphorus load. These proposals include artificially oxygenating the bottom waters, adding chemicals to bind the phosphorus to the sediments, and removing phosphorus-rich sediments by for instance dredging.
Some argue that spending billions of euros on geo-engineering is needed to meet the rapidly approaching deadline to restore the Baltic Sea. While the HELCOM restoration deadline (Baltic Sea Action Plan) has good intentions, it is a political decision that does not take into account ecological recovery processes. It took decades for the Baltic Sea to become eutrophic – and it will take decades to recover.
Pilot experiments have demonstrated proof of concept for a number of geoengineering approaches. However, there are still many scientific uncertainties. Further testing is needed to assess the feasibility, costs, benefits, and risks of large-scale application to the Baltic Sea. But before more experiments are funded, one should ask if there are any important lessons to be learned from lakes, where geoengineering has been going on for decades.
I recently spent some time reviewing the peer-reviewed, scientific studies of lake restoration projects, and found that lake restoration in Europe and North America shows mixed results:
Keeping oxygen in the water generally requires continuous artificial oxygenation
In some cases, artificial oxygenation increased fish habitats and reduced phosphorus levels. But in others, 10 years of artificial oxygenation did not reduce phosphorus levels or maintain sufficient oxygen. I found no lakes that could “self-maintain” sufficient oxygen levels without reductions in external nutrient loads.
Chemicals have different effects in different lakes
The addition of chemicals to bind phosphorous in lake sediments generally works, but the degree of improvement in Secchi depth and phosphorus levels varied across lakes. It is not known why the results varied, but insufficient reductions in external loads could be a factor.
Not enough studies address dredging
When used with other restoration methods, dredging shallow (1-3 m deep) lakes generally increased water clarity and reduced phosphorus levels. But, I found no studies that assessed dredging alone or in deep lakes. As a result, it is hard to see how dredging could work in the Baltic Proper (>400 m deep).
Geoengineering is a not a substitute for reducing external nutrients
There is no consensus among scientists for when to use geoengineering in lakes and how to assess the risks. There does seem to be agreement, however, that geoengineering is not a panacea for saving eutrophic lakes.
There is a critical lack of follow up for restored lakes
Despite all the money that has been spent on restoring lakes, there is a lack of publicly available long-term monitoring data. As a result, it is difficult to assess the different methods.
As an inland sea, the Baltic shares many features with lakes; the main differences are salt and size. The long-term stability of phosphorus-binding chemicals is not known for salty environments like the Baltic. Then there is the issue of size. The Baltic Sea contains 22,000 km3 of water. By comparison, the largest lake in Sweden, Lake Vänern, contains 154 km3 of water. Even the dead zone in the Baltic Sea is huge, about the size of Denmark (42,000 km2).
There is potential for geoengineering when used in a targeted manner in small, enclosed bays, but we need to be skeptical of claims that geoengineering can measurably improve the sea as a whole. If the root causes of eutrophication are not addressed, there will be continual need to treat the symptoms of eutrophication.
The idea of geoengineering is politically attractive because it promises a quick-fix for the Baltic Sea, but, in reality, it is a serious distraction from the real problem – the external nutrient load.