129I in the Baltic Proper and Bothnian Sea: application for estimation of water exchange and environmental impact

We report here new data and a mass balance model for 129I in the Baltic Proper and the Bothnian Sea covering the period from November–December 2009. The results showed that the general 129I concentrations in the Bothnian Sea were two-four folds lower than in the Baltic Proper for both surface and deep water. Water exchange between the two basins based on the 129I mass balance model suggests fluxes from the Baltic Proper to the Bothnian Sea and vice versa at 980 km3/y (600–1400 km3/y) and 1180 km3/y (780–1600 km3/y) respectively. Water retention time (residence time) in the Bothnian Sea was estimated at up to 4 years. Applying the 129I exchange model, an estimate of total phosphorus and nitrogen inflow from the Baltic Proper to the Bothnian Sea indicates values of 20 ± 7 × 103 tons/y and 300 ± 50 × 103 tons/y respectively. The values for the outflow from the Bothnian Sea to the Baltic Proper hold 12 ± 3 × 103 tons/y for total phosphorus and 283 ± 55 × 103 tons/y for total nitrogen. These data and application of 129I as a tracer of water masses provide information on small scale salinity changes which are vital for accurate understanding of the Baltic Sea ecosystems evolution through time. -- Highlights: ► Presenting new data on 129I in the Baltic Proper and the Bothnian Sea. ► A model based on 129I was developed to estimate water exchange. ► An estimate of phosphorus and nitrogen exchange comparable with earlier estimates. ► Application of 129I as a tracer of water masses highlights minor salinity changes