Workreport 2016-65



Studies on the Aquatic Environment at Olkiluoto and Reference Area: 4. Olkiluoto Open Sea Area in 2013


Kallio, K., Kankaanpää, H., Kauppila, P., Nurmi, M.



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The working report focuses on the Olkiluoto open-sea area, where a two-day measurement campaign was carried out in September 2013. The main objectives for the study were 1) to provide input such as concentration ratios (water-to-biota) for the needs of surface modelling in the biosphere assessment and 2) to increase knowledge of the Olkiluoto open-sea area and its ecosystem. Sampling was carried out aboard RV Muikku at four stations each with a different surface sediment type. Among others, 73 elements were determined in the seawater, phytoplankton, zooplankton, macrozoobenthos, and sediment. Species compositions and biomasses were determined for the biota sampled. The discussion focuses mainly on the key elements (Ag, C, Cl, Cs, I, Mo, Nb, Ni, Pb, Pd, Se, Sn, and Sr) specified in the biosphere assessment.

Water quality was approximately the same across the four stations. In the open sea, concentrations of Cl, Cs, I, Mo, Ni, and Sr in the seawater were, on average, higher than those in the coastal sea (measured in August–September 2013 in connection with Posiva Oy’s routine monitoring). An exception to the pattern was Pb, levels of which were slightly lower in the open sea. Total C concentrations in the two areas were about the same. Open-sea phytoplankton and zooplankton samples revealed normal brackish-water community structures showing little spatial variation. Also, the macrozoobenthos communities were typical for the Bothnian Sea, varying from one survey location to another. Macoma balthica and Saduria entomon made up the majority of the macrozoobenthos biomass, and invasive Marenzelleria spp. occurred very abundantly everywhere. Zooplankton contained more Cl, Mo, Pb, Ni, Se, and Sn than the other biota did, while there were higher concentrations of Ag, Cs, Nb, and Pd in Saduria entomon and (especially) of Sr in all macrozoobenthos material. Phytoplankton showed the highest levels only for I.

The seabed samples generally consisted of low-accumulation-rate or erosional bottom‑type material. For the most part, the available seabed substrate map data correlated well with the ground-truthing data. Cl and Sr were the most abundant elements in sediment pore water and leachates. In comparison to seawater, the all-stations average concentrations of the key elements in sediment pore water and extracts were generally higher, but concentrations of Pd remained below the limit of quantification. There was high between‑station variability in element concentrations in the sediment.

Concentration ratios for the key elements exhibited quite similar order between plankton and macrozoobenthos, with Pb, Ag, and Se having among the highest and Cl, Pd, and Mo among the lowest. For macrozoobenthos, the concentration ratios generally deviated greatly from those reported earlier. The deviations are at least partly explained by differences in sampling material (composite samples vs. single species) and weight normalisation (wet or dry).


Olkiluoto, open sea, Bothnian Sea, biosphere assessment, plankton, macrozoobenthos, surface sediment, key elements, concentration ratios.


Studies on the Aquatic Environment at Olkiluoto and Reference Area: 4. Olkiluoto Open Sea Area in 2013 (pdf) (6 MB)


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