Ferromanganese concretion bottoms as habitats and a renewable resource in the Baltic Sea
Authors: Kotilainen A T , Kaskela A M , Alanen U , Kiviluoto S , Kontula T , Kostamo K , Kurvinen L , Sahla M , Sippo A , Virtanen E 
Published: September 2017

Ferromanganese (FeMn) concretions, also called FeMn nodules or polymetallic nodules, are iron-manganese-rich mineral precipitates on the seabed. Those concretions are found in deep waters (e.g. the Pacific Ocean) but also in shallow seas like in the Baltic Sea.
FeMn concretions are formed on the seabed by natural geochemical processes, catalyzed by micro-organisms, e.g. archaea and bacteria. These supposedly renewable resources, FeMn deposits, have well known economic importance. They contain the high concentrations of iron, manganese, phosphorous but also
rare earth elements and environmental pollutants. However their role e.g. in the internal loading of nutrients and their ecological significance is still relatively poorly known.
Here we present FeMn concretion data from the Finnish waters of the Baltic Sea. The data has been collected over the past decades in the national marine geological mapping program of the Geological Survey of Finland (GTK) (1984-2016) and in the Finnish Inventory Program for the Underwater Marine
Environment (VELMU) (2005-2015). The data include nodule observations from nearly 300 sediment samples (e.g. box corer and van Veen grab samples), 7000 video observations/points and over 70 scuba dives.
Different types of FeMn -concretions (e.g. spheroidal, discoidal, irregular and crusts/plates) were found at the seafloor with water depths between 1-75 metres. Also the size of the concretions varied from small millimetre scale spheroidal nodules to large metre scale plates. FeMn concretions
occurred in all studied sea areas, from the eastern Gulf of Finland, up north to the Bothnian Bay. However, concretion fields/sites that form a HELCOM HUB-class "Baltic bottoms with at least 90% coverage of FeMn concretions" type habitat occurred only in the Gulf of Finland and the Archipelago
Sea. Sediment sample data show that FeMn concretions (1-3 cm thick layer) often cover silty clay (glacial and postglacial) seabed substrates.
FeMn concretions increase the microscale seabed heterogeneity (geodiversity) by forming relatively hard substrate on the predominantly soft seafloor sediment. Concretions provide a 3D environment and hard physical structures, and support benthic communities, probably increasing biodiversity. In
addition, concretion fields and crusts can protect the seafloor from erosion by near-bottom currents. It is likely that these mineral deposits occurring in the Baltic Sea form an EU Habitats Directive habitat 'Biogenic reef', of which a representative part should be protected according to EU
legislation.
Ferromanganese concretions could also provide future mineral potential. However, the sustainable utilization of these marine resources and the need for marine environment protection pose great challenges for the development of ecosystem-based maritime spatial planning processes and efficient Blue
Growth.
This work is a part of the SmartSea project that is funded by the Strategic Research Council of the Academy of Finland, grant No: 292 985, and the assessment of marine habitat types (LuTU-Itämeri) that will be carried out 2016-2017 in a group of national experts from the universities, research
institutes and authorities.