An important reason for slow recovery of the Baltic Sea’s state of eutrophication is the large nutrient store in the sediment and water. Phosphorus does not bind effectively at the bottom sediment in poor oxygen conditions, and large amounts of phosphorus are gathered in the deep layers of water. It has been estimated that the phosphorus storage of the deep layers (over 60 meters) of the Baltic Sea is 10 times as big as the annual total loading of phosphorus. Compared to a biologically usable loading of phosphorus the storage is remarkably larger still. Phosphorus from the deep water is carried to the productive surface layer among other things because of winter storms, which causes widespread blue-green algae blooming. Algae blooming on the other hand binds large amounts of gassy molecular nitrogen on the ecosystem circulation of the Baltic Sea.
Despite the great importance of the internal processes, the external loading of nutrients in the catching area of the Baltic Sea must be decreased to achieve the objective of a good water status in the sea. Need for reduction applies to both scattered and point loading. Reduced scattered loading and simultaneously intensified agricultural production can lead to conflict situations, which slow down or prohibit achievement of the goals. There are, in other words, many factors of uncertainty connected to the goals to decrease the scattered loading. It has also been suggested that even if it was possible to reduce the loading, it takes decades for the Baltic Sea to recover.
In addition to planning and implementation of the measures to reduce the loading, it is of current interest to experiment the possibilities to decrease the eutrophication level by targeting a part of the measures to the sea itself. These measures can be divided into those removing nutrients from the sea (e.g. dredging, fishing) or those preventing the release of them (e.g. adding of chemical compounds) and measures slowing down the circulation of nutrients (e.g. recovery of food webs).
This measure provides an explanation to which recovery methods can be used – or have already been used – to control the eutrophication of the sea. In the study, the theoretical impact mechanisms will be demonstrated as well as what kind of research has been done with these methods in the Baltic Sea. The study will also evaluate the effect of the measures and their suitability for different scales of space in the Finnish territorial waters.
The study will produce up-to-date information for decision makers who reflect, in addition to means to decrease the scattered loading, the handling of internal processes to recover the state of the Baltic Sea. The study also provides information about the possible utilisation of different methods of compensation procedure and problems connected to this procedure. An optimal protection of the Baltic Sea can in the future even include measures targeted to internal mechanisms, but there must be evidence-based information before large-scale implementation.
Suomen ympäristökeskus (SYKE) / Merikeskus