Phosphorous and Nitrogen Science communication


No "silver bullet" to restore the Baltic Sea

I had the honor of joining a panel discussion organized by Centrum Balticum at the Turku School of Economics in Finland. The question posed to the panel was “Can geo-engineering save the Sea?” The general sentiment among the panelists was that there is no “silver bullet” to reverse eutrophication.

My fellow panelists included a professor of marine biology, a professor of maritime law, a representative of the Finnish Ministry of the Environment, a representative of HELCOM, and a representative from an environmental consulting firm. The discussion was moderated by a judge from the European Court of Justice.

The session kicked off with 6 questions and here I summarize the answers and discussion from my notes.

What are internal phosphorus reserves?

There are 2 natural pathways by which phosphorus is are removed from the sea – it is buried in the sediments or it is flushed out to the North Sea. In the past century, the amount of P in the water column and in the surface sediments has increased by about 2.5 times. This is faster than the natural pathways can remove the phosphorus, resulting in an accumulation. This accumulation is a consequence of discharging untreated or poorly treated sewage into the sea and the excessive use of fertilizers in the past.

What do internal phosphorus reserves mean for eutrophication in the Baltic Sea?

This accumulation of phosphorus contributes to the symptoms of eutrophication that we see today. In addition to fertilizing crops, phosphorus fertilizes algae in the sea, contributing to large algal blooms, reduced water clarity, and the large areas of bottom water with little or no oxygen (also called dead zones)

There is evidence however that the size of the phosphorus internal reserves has stabilized. In other words, it isn’t getting larger. This suggests that the accumulated pool of phosphorus could actually start to decrease – due to sedimentation and exchange with the N Sea. But it will take time (decades+) for the reserves to decrease because of their size.

Note: I personally don’t like the term “internal reserves”. What we are talking about is the accumulation of phosphorus in the sea: in the water column and the sediments. I prefer “storage pools”. I also prefer "restore the sea" over "save the sea". To me, "saving" is something you do when someone is drowning or trapped in a burning building.

What potential restoration methods have been suggested?

A number of sea-based restoration measures have been proposed to speed the sea’s recovery from eutrophication: oxygenation, removal of sediments, addition of chemicals and biological-measures. These measures have been widely used in lakes, but never at the scale of the open Baltic Sea.

  • Oxygenation, which would use large windmills to provide oxygen to deep, oxygen-free areas. In the presence of oxygen, there are certain chemical processes that could bind phosphorus in the sediments and reduce the amount of phosphorus in the water column.
  • Certain materials, like aluminum, will bind phosphorus in the sediments and remove it from the water column.  
  • Phosphorus-rich sediments can be removed by dredging, which could reduce the total amount of phosphorus that is in circulation (and which contributes to eutrophication)
  • Mussel farming has also been proposed as a restoration method. The idea is that the mussels would consume algae in the water column (which contains P), incorporate it into their bodies. Then, the mussels could be harvested and used as food or animal feed, removing phosphorus from the sea. Similarly, algae could be removed and used as bio-fuel or fertilizer.

How has HELCOM worked on internal nutrient reserves?

HELCOM has started working on this issue more recently. Last November, a joint HELCOM-EUSBSR (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. This workshop brought together more than 50 scientists and stakeholders to discuss the state of knowledge about phosphorus cycling in the sea.

In addition, the HELCOM PRESSURE group also started planning the implementation of the new commitment, with the aim to develop a risk assessment framework for the management of internal nutrient reserves.

What are the ecological risks of the methods?

The main risk is that our focus on one issue diverts our attention from the “big picture”. We must also consider the other factors that affect the Baltic Sea environment, such as climate change, contaminants, threats to biodiversity, over-fishing, marine litter, traffic, underwater noise.

What are the legal aspects of sea-based restoration methods?

There are no specific laws that address sea-based measures. The short answer is that if the measures work and don’t have negative effects, then they are okay from a legal perspective. But, it the measures create risk, then there are legal issues, primarily because of the precautionary principal.

Other discussion points that came up:

  • Sea-based measures are discussed in Sweden and Finland but not in other countries around the Baltic Sea. This is possibly because 1) Sweden and Finland bear the bulk of the consequences of eutrophication (e.g. extensive algal blooms) and 2) other, “low hanging fruit” measures have been implemented and there is a feeling that further reductions in land-based phosphorus loads are not cost-efficient at this point.
  • Scale matters when implementing geo-engineering measures. Geoengineering in the open sea would much more challenging and have more risk and uncertainty compared to small, enclosed bays where methods can be tested and monitored.
  • It is important to distinguish between the internal phosphorus load, which recycles “old”, nutrients, and the external load, which is the input of “new” nutrients from land. Another way to think of the internal load is that the sediments are less effective in retaining sediments compared to the past.
  • We should acknowledge that there have been dramatic reductions in external nitrogen and phosphorus loads to the sea. Current external loads are at about the same level as the 1950s. Not many areas of the world have seen this degree of success. Also, the sea is starting showing signs of improvement.
  • There is not consensus among the scientific community that oxygenation will bind phosphorus in the sediments to the extent that it would reverse eutrophication. This method has been used widely in lakes and there are numerous examples of where oxygenation did not improve the ecological situation. We need additional scientific research to understand the mechanisms for phosphorus removal in the sediments.  
  • The panelists agreed that that sea-based measures do not to replace actions on land to reduce external phosphorus loading. There was also agreement that we are still in an investigation phase, e.g. assessing risk, feasibility, potential benefits of geo-engineering, and that it is too soon to move forward on a large scale.
Michelle McCrackin

Michelle McCrackin