Nitrogen in the Baltic Sea: Long-term trends, a budget and decadal time lags in responses to declining inputs

Abstract

The Baltic Sea Nitrogen (N) cycle has mainly changed due to increased N inputs from rivers and the atmosphere since the beginning of the 20th century. In order to better understand the complex N budget in the Baltic Sea we: 1) assembled a unique dataset from 255 stations (>390.000 observations) of water column total N (TN) concentrations to determine long-term changes, 2) constructed a simple TN budget, and 3) present possible scenarios for the time lag between a decrease in inputs and lower TN concentrations. We show that in most regions the water column TN pool increased from the 1970s up to the early 1990s where after it leveled off and in most cases have remained stable or started to decrease in the early 2000′s. The declines in TN were only generally evident in the nearshore areas (<12 nautical miles from the shoreline), which are more directly impacted by river inputs. The field data showed that the Baltic Sea water column TN pool has declined with 31 kiloton (kt) N y−1 in the period from 2011 until 2018, while the water column TN budget showed a decline of 48 kt N y−1. Using existing regional threshold concentrations for when the Baltic Sea is unaffected by eutrophication and their exceedance compared with regional TN levels found in our study, we show that a reduction of 28% compared with current levels is needed. This study reveals that all manageable TN inputs would need to be reduced by ̴ 50% before this reduction target (28%) for the water column can be reached within a ̴ 8 year period. However, as only 5% of the TN pool is contained in the water column, with the majority (95%) being in the top 0.2 m of the sediment, which exchanges N with the water column, the likely time lag is ̴ 20 times longer, with a minimum reduction of 20% leading to improvements in the open Baltic Sea after about 400 years. Our calculations show that recovery (“oligotrophication”) of the Baltic Sea is only possible for the water column within decadal timescales if the input reductions are considerably higher (e.g. 50%) than the current ambition of only 9%.