Eutrophication is a major threat to the Baltic Sea ecosystem, affecting marine animals, plants, and habitats and limiting opportunities for people to enjoy the sea. While there have been reductions in inputs of nitrogen and phosphorus from land, the sea remains eutrophic. The slow recovery has been blamed on the so-called internal load – the movement of phosphorus between the sediments and water column.
As a result, a number of geo-engineering measures have been proposed to mitigate the internal load and speed recovery of the Baltic Sea. However, there are still uncertainties about the fluxes and dynamics of phosphorus in the sea and their contribution to eutrophication.
Researchers discussed scientific knowledge of phosphorus cycling
In November 2017, more than 50 scientists and stakeholders – including several researchers from the Baltic Sea Centre – convened at a workshop in Gothenburg to discuss these uncertainties. A joint HELCOM and the EU Strategy for the Baltic Sea Region workshop was organized by the Swedish Agency for Marine and Water Management and the Finnish Ministry of Environment. The aim of the workshop was to highlight the scientific knowledge base of phosphorus cycling in the sea. The participants also discussed the environmental risks of geo-engineering as well as and relevant political and legal aspects.
The discussion at the workshop revealed multiple gaps in scientific understanding of phosphorus dynamics in the Baltic Sea and how proposed sea-based measures could affect the phosphorus cycle. These knowledge gaps prevent the safe implementation of large-scale measures in the open sea.
"We don’t know enough about phosphorus cycles in the Baltic Sea environment to predict whether geo-engineering could work on a large scale” says Bo Gustafson, Head of Baltic Nest Institute at Stockholm University’s Baltic Sea Centre.
Geo-engingeering methods could be tested on a small-scale
The workshop concluded that there could be potential for geo-engineering measures to manage internal phosphorus reserves to speed the recovery of the sea. However, it was not clear where and what type of measures could be possible. Geo-engineering methods could be pilot-tested in enclosed bays in coastal areas and lakes to increase understanding of long term-effectiveness of sea-based measures.
“In Björnöfjärden, the external phosphorus load from land was reduced and sediments were treated with aluminium to bind phosphorus. As a result, we have seen reduced phosphorus concentrations in the water column and improved ecological status in several aspects, just within a few years. There is potential to use this restoration method in enclosed bays.” says Linda Kumblad, Researcher at Baltic Eye at Stockholm University’s Baltic Sea Centre.
The workshop further concluded that a systematic, ecosystem-based analysis should be used when considering sea-based measures and the potential for adverse environmental effects. Any actions taken in the sea to address the internal phosphorus load should not cause additional pressure on the Baltic Sea or lead to further deterioration.
Long-term solutions are land-based
The long-term solution to eutrophication in the sea is the reduction of external nutrient inputs. Even though recovery from eutrophication takes time we are starting to see signs of recovery in the sea. Model results show that significant improvements are expected from the nutrient input reductions already achieved so far. There was discussion at the workshop that HELCOM targets for nitrogen seem to be achievable with currently proposed measures in the country’s river-basin management plans and programmes of measures. However, achieving phosphorus reduction targets could be more challenging and new measures are needed to reduce phosphorus inputs to the sea from agriculture.
“We estimate that better recycling of phosphorus from livestock manure could reduce fertiliser imports by up to 200 000 tons in the Baltic Sea region” says Annika Svanbäck, Researcher at Baltic Eye at Stockholm University’s Baltic Sea Centre. “This could reduce phosphorus surpluses in agriculture – and reduce the risk of leakage to the sea.”
Text: Annika Svanbäck, Bo Gustafsson and Michelle McCrackin