Nutrient removal by biomass accumulation on artificial substrata in the northern Baltic Sea

We studied the potential of utilizing non-seeded artificial substrata in growing macrofauna and macroalgae to remove excess nutrients from seawater in the northern Baltic Sea. Succession, species composition, and biomass on two different substratum types were studied. Organic carbon (C), nitrogen (N), and phosphorus (P) as well as heavy metal (As, Cd, Cu, Pb, and Zn) contents were measured from the algal species and the invertebrate fraction. The invertebrate fraction formed more than 94% of the total biomass and consisted of blue mussels (Mytilus trossulus), bay barnacles (Amphibalanus improvisus), and hydroids (Cordylophora caspia and Gonothyraea loveni). Major algal species consisted of green algae Ulva spp. and Cladophora glomerata, red algae Ceramium tenuicorne and Polysiphonia fibrillosa, and filamentous brown algae Pylaiella littoralis and Ectocarpus siliculosus. The highest biomasses were measured at the end of the experiment, after 14.5 months of exposure in the sea, when the total biomass was on average 2.4 kg dry weight (DW) m−2 substratum, containing approximately 50 g N and 5 g P. The heavy metal concentrations were site and species dependent and on average lower in the invertebrate fraction compared with the macroalgal fraction. In some cases especially, Cd may limit biomass use as fertilizers. According to these results, annual N and P emissions from small-sized point sources could be reduced by using artificial substrata to remove sessile organisms, but this would require extensive cultivations. Furthermore, the utilization of the marine macroalgal and invertebrate biomass in biogas production requires adding more carbon-rich feedstock to obtain optimal results.