Results

According to the revised HELCOM nutrient reduction scheme adopted in the 2013 HELCOM Ministerial Declaration (HELCOM 2013a) reduction requirements were set for nitrogen inputs to the Baltic Proper, Gulf of Finland and Kattegat and for phosphorus inputs to Baltic Proper, Gulf of Finland and Gulf of Riga. Of these reduction requirements, only the one on nitrogen inputs to the Kattegat has been met according to the average normalized annual inputs during 2010-2012 (Figure 1). However, since the reference period (1997-2003), significant decreases in inputs can be noted for nitrogen and phosphorus inputs to the Baltic Proper and Kattegat as well as the Baltic Sea as a whole, and further for nitrogen inputs to Bothnian Sea and Danish Straits (Figure 2 and Table 2). There is rather high uncertainty in the data for waterborne inputs to the Gulf of Riga and to a significant part of the waterborne inputs to Gulf of Finland due to gaps in reported data, which might explain the lack of change in inputs to these basins.

 

Table 1. Maximum allowable annual inputs of (a) nitrogen and (b) phosphorus per sub-basin, the average normalized annual input during 2010-2012, and the remaining reduction needed to reach MAI. Inputs 2010-2012 in percentages of MAI and classification of achieving MAI with 2010-2012 inputs are also given. Colours in the classification column are as follows: green=MAI fulfilled, yellow=taking into account statistical uncertainty of input data it is not possible to determine whether MAI was fulfilled, and red=MAI not fulfilled and inputs increasing. (Units in columns 2-4: tonnes per year).

Table 1a.table 1a.jpg 

Table 1b.table 1b.jpg  

*As adopted by the 2013 HELCOM Copenhagen Ministerial Meeting (HELCOM 2013a)

 

Table 2. Changes in nitrogen and phosphorus inputs to the different Baltic Sea sub-basin since the reference period 1997-2003. Inputs are calculated as average normalized inputs during 2010-2012. Changes that are statistically significant are indicated in bold.

Sub-basin

Changes in norm. N input in 2010-2012 compared with the reference period (%)Changes in norm. P input in 2010-2012 compared with the reference period (%)
Bothnian Bay-1.15.6
Bothnian Sea-8.2-8.9
Baltic Proper-12.7-20.0
Gulf of Finland0.3-13.7
Gulf of Riga3.20.5
Danish Straits-18.9-5.4
Kattegat-19.1-8.3
Baltic Sea-9.4-13.6


 

Trends

Figure 2_nitrogen.jpgFigure 2_phosphorus.jpg

Figure 2. Achieved reductions (in %) in average annual inputs of nitrogen (left) and phosphorus (right) during 2010-2012 compared with the reference period 1997-2003. The average annual inputs in 2010-2012 and in the reference period were calculated using normalized annual inputs. The arrows indicate decreasing (↓) and increasing (↑) inputs, with darker colours indicating statistically significant changes (e.g. Baltic Proper nitrogen bold arrow ↓ and -12.7% indicates that average annual nitrogen inputs in 2010-2012 were significantly lower (12.7%) than during the reference period). The results are also given in Table 2.

 

Normalized inputs are used for the riverine and the atmospheric inputs to reduce the impact of interannual variability in the inputs caused by weather conditions. This allows a trend analysis of more comparable inputs, making it easier to detect trends and also to detect effects of taken measures in the catchment areas, which would otherwise be hidden in the naturally large annual variation in river flow.

Trend analyses show that total inputs of nitrogen to the Baltic Sea from 1995 to 2012 decreased with statistical significance (18%). The reduction in nitrogen inputs was between 6-35% in the different sub-basins (Table 3) and statistically significant in all except for the Bothnian Bay, Gulf of Finland and Gulf of Riga (Figure 3). Total phosphorus inputs to the Baltic Sea also decreased with statistically significance (23%). The reduction was statistically significant in the sub-basins Bothnian Sea, Baltic Proper, Danish Straits and Kattegat.

Fig 3_BB_N.jpgFig 3_BB_P.jpg
Fig 3_BS_N.jpgFig 3_BS_P.jpg
Fig 3_BP_N.jpgFig 3_BP_P.jpg
Fig 3_GF_N.jpgFig 3_GF_P.jpg
Fig 3_GR_N.jpgFig 3_GR_P.jpg
Fig 3_DS_N.jpgFig 3_DS_P.jpg
Fig 3_KT_N.jpgFig 3_KT_P.jpg
Fig 3_BAS_N.jpgFig 3_BAS_P.jpg

Figure 3. Actual total (air- and waterborne (riverine + direct point source discharges)) annual inputs of nitrogen and phosphorus to the different sub-basins and the Baltic Sea as a whole during 1995-2012 (tonnes). The normalized annual inputs of nitrogen and phosphorus are given as a black line. The trend line for normalized total nitrogen and phosphorus input is given as a green line to indicate possible trend (solid line = a statistically significant trend; dotted line = not statistically significant trend). The MAI as adopted by the 2013 HELCOM Copenhagen Ministerial Meeting (HELCOM 2013a) is shown as the bold dotted red line. Actual input data can be accessed here.

 

In general, the highest percentage in reduction of total waterborne nitrogen and phosphorus inputs was found for the sub-basins were the proportion of nutrients inputs from wastewater was high in the mid-1990s.​


Table 3. Percentage of change in atmospheric and waterborne normalized inputs of nitrogen and phosphorus to the Baltic Sea sub-basins and total inputs to the Baltic Sea from 1995 to 2012 based on a statistical trend analysis. Only results where the trend is statistically significant are shown. For more information see the PLC-5.5 report, e.g. tables 5.4a and b (HELCOM 2015).

Sub-basinChange in airborne N inputs since 1995 (%)Change in waterborne N inputs since 1995 (%)Change in total N inputs since 1995 (%)Change in total P inputs since 1995 (%)
Bothnian Bay-23---
Bothnian Sea-26--13-28
Baltic Proper-27-24-22-30
Gulf of Finland-22--6-
Gulf of Riga-25---
Danish Straits-28-41-35-21
Kattegat-30-26-26-17
Total Baltic Sea-27-18-18-23

 

The total nutrient input to the Baltic Sea can vary significantly depending on whether it is a wet or a dry year. For example, 2010 was a very wet year in the southern part of the Baltic Sea catchment area, hence the actual, i.e. not normalized, nutrient input figures are very high for e.g. Baltic Proper (Figure 3) and relatively high to the Baltic Sea. Additionally, atmospheric deposition was also rather high in 2010.

In 2012, flow was 15% higher than the average for 1995-2012, in particular to Bothnian Bay, Bothnian Sea and Gulf of Finland. The total input of nitrogen was about 872,000 tonnes, of which about 27% was from atmospheric deposition. The total phosphorus inputs to the Baltic Sea in 2012 were about 30,700 tonnes of which only about 7% was via atmospheric deposition (Table 4, Figure 4). Compared with MAI, inputs in 2012 were 10% higher for nitrogen and 42% higher for phosphorus.

 

Table 4. Water flow as well as actual waterborne and airborne inputs of phosphorus and nitrogen to the Baltic Sea sub-basins in 2012. Average flow 1995-2012 is shown for comparison.

Sub-basin Ave. flow 1995-2012 (m3/s) Flow
(m
3/s)
Nitrogen (t) Phosphorus (t)
WaterborneAirborneTotal WaterborneAirborneTotal
Bothnian Bay 3,412 4,45971,7259,70081,4253,5491813,730
Bothnian Sea 3,006 3,66163,02026,90089,9202,6573943,051
​Baltic Proper​3,584​3,266​214,333​131,500​345,833​10,882​1,046​11,928
Gulf of Finland 3,454 4,131118,21115,300133,5115,6881505,838
Gulf of Riga 1,087 1,28187,68211,30098,9822,886932,979
Danish Straits 226 22732,31120,60052,9111,4551051,560
Kattegat 1,100 1,22749,26619,80069,0661,5391181,657
Total Baltic Sea
15,869 18,252636,548235,100871,64828,6562,08730,743

 

Figure 4a.jpgFigure 4b.jpg

Figure 4. The total actual inputs of water- and airborne nitrogen (left) and phosphorus (right) from HELCOM countries to the Baltic Sea in 2012.