Temporal and spatial variation of dissolved nutrients in the Baltic Sea in 2005

	Authors: Anniina Kiiltomäki, Tapani Stipa, Seppo Kaitala Finnish Institute of Marine Research

Key message

In 2005 the chlorophyll a values were lower than the Alg@line-reference (the average of the years 1993-2004) in Arkona and Bornholm Basins.

In the Gulf of Finland the chlorophyll-a concentration spring peak lasted longer in 2005 than in the Alg@line-reference. In the Eastern Gotland Basin the second chlorophyll-a concentration peak was higher than in the Alg@line-reference.

The winter surface water concentrations of dissolved inorganic phosphate were higher than the Alg@line-reference and the EUTRO-target values in all regions. In Arkona, Bornholm and Gotland Basins the phosphate values were higher than Alg@line-reference throughout the year 2005.

According to the EUTRO-target levels all examined open sea regions - Arkona Basin, Bornholm Basin, Eastern Gotland Basin, Northern Gotland Basin, Gulf of Finland and Bothnian Sea - are again classified as an eutrophication problem areas in the year 2005.

Results and assessment

Relevance of the indicator for describing developments in the environment

The Baltic Sea is strongly affected by seasonality: during the winter the water is rich in nutrients, but as long as the surface water stratification remains weak and the availability of light is limited, the phytoplankton biomass remains low. As the surface water stratifies and the amount of light increases, the biomass of phytoplankton increases massively during a short spring period. When the dissolved nitrogen is depleted from the surface water the algal biomass decreases significantly. The amount of phosphate left over varies between the years. When the sea water warms up during the summer, the blue-green algae become more common utilizing the surplus phosphate. The occasional upwellings of deeper, nutrient rich water can stimulate the algal growth.

The amount of nutrients together with temperature variation and the amount of light form the basis for phytoplankton succession. Nutrients as such indicate the level of eutrophication in a sea basin. Chlorophyll a concentration is a relative measure of phytoplankton biomass in the water. Since high nutrient concentrations increase the intensity and frequency of phytoplankton blooms, chlorophyll a can be used as an indicator of the eutrophication level in sea basin.

Policy relevance and policy references

Nutrients   and their ratios form the preconditions for harmful algal blooms as well as the overall eutrophication. The standard HELCOM COMBINE program samples the Baltic Sea several times a year. However, seasonal processes change the state of the Baltic Sea between the different COMBINE samplings in ways that need to be mapped with other methods. The combination of Alg@line monitoring and ecosystem models reveal the short-term fluctuations in the Baltic environment.

The indicator fact sheet of temporal and spatial variation of temperature and salinity in the Baltic Sea complete this indicator fact sheet.

The reference and target levels are in great importance when interpret the state of the sea. The comparison with averages of the years 1993-2004 (Alg@line-reference) represents deviation of this year from the present state of the Baltic Sea. The comparison with the preliminary target and reference levels by HELCOM EUTRO-report (EUTRO-target, EUTRO-reference levels) shows the deviation from the goals specified by HELCOM and the state of the Baltic Sea before 1950s.

Assessment: First sight -The conditions of the year 2005 compared to EUTRO-reference level, EUTRO-target level and Alg@line-reference values.

In 2005 the winter surface water concentrations of dissolved inorganic phosphate (DIP) were higher than both the EUTRO-target level and Alg@line-reference values in all regions, except in Bothnian Sea.

The winter surface water concentrations of the dissolved inorganic nitrogen (DIN) remained under EUTRO-target level and Alg@line-reference values in Arkona Basin and Bothnian Sea for the most of the time. In Bornholms Basin and Eastern Gotland Basin the winter concentrations were higher than EUTRO-target level, but lower than Alg@line-reference values. In Northern Gotland Basin and Gulf of Finland the winter DIN concentrations were higher than both EUTRO-target and Alg@line-reference.

The winter surface water DIN:DIP ratio was lower than Alg@line-reference in all regions except entrance of Gulf of Finland and remained within EUTRO-target levels in all regions where the levels were defined (Northern Gotland Basin, Gulf of Finland and Baltic Sea).

The winter surface water ratio of DIN:silicate was lower than Alg@line-reference in all regions except Gulf of Finland and remained much lower than EUTRO-reference in all regions where that was defined (Northern Gotland Basin, Gulf of Finland and Bothnian Sea).

The summer surface water chlorophyll a concentrations were widely fluctuating. The EUTRO-target level was exceeded in all regions, though in Arkona and Bornholms Basin the concentrations were very close to the target level.

	Click images to enlarge

Figure 1. The open sea winter concentrations of dissolved inorganic phosphate (PO₄ µmol l^-1) and dissolved inorganic nitrogen ( NO3+NO2+NH4) in different Baltic Sea regions in the year 2005 (Alg@line measurements = red stars, COMBINE SR5-location=magenta stars and BalEco-model mean and variance = magenta signs) compared to the measurements of the years 1993-2004 (Alg@line mean and variance = blue signs, COMBINE SR5-location mean and variance = green signs) and to EUTRO-reference and -target values (= black lines). Values are presented from two locations per sea region, shown in Fig. 16.

Baltic Sea regions: AB=Arkona Basin, BB=Bornholm Basin, GB_E=Eastern Gotland Basin, GB_N=Northern Gotland Basin, GoF=Gulf of Finnland and BS=Bothnian Sea.

Figure 2. Idem, but ratio of dissolved inorganic nitrogen and dissolved inorganic phosphate (left) and ratio of dissolved inorganic nitrogen and silicate (right).

Figure 3. Idem, but summer chlorophyll a concentrations.

Assessment: Baltic Proper (Alg@line observations)

Dissolved inorganic Phosphate (PO4)

In 2005 the range of annual surface water DIP concentration varied between 0.00–1.40 µmol l^-1. The highest winter nutrient concentrations were measured at the Bornholm Basin and in the Gulf of Finland. In the Arkona Basin and the Bornholm Basin the concentrations were exceptionally high during the whole year. On the beginning of the May the concentrations decreased to < 0.3 µmol l^-1 in the Northern Gotland Basin and in the Gulf of Finland.

The difference to the Alg@line-reference concentrations was significant. The DIP concentration was above the Alg@line-reference in all regions.

Dissolved inorganic nitrogen (NO3 + NO2 + NH4)

In 2005 the annual surface water concentration of DIN varied between 0.00-11.30 µmol l^-1. The highest early springlevels of DIN  concentrations were found in the Arkona Basin and at the entrance to and with in the Gulf of Finland. Concentrations decreased under 1 µmol l^-1 in the middle of April in the whole area. The decrease of the spring values was concurrent with the Alg@line-reference. Compared to the Alg@line-reference, on the autumn DIN concentrations started to rise earlier in the Gulf of Finland, but later in other regions.

Chlorophyll a

In 2005 the annual concentrations of chlorophyll a varied between 0.4-47.2 mg m^-3. The spring bloom started exceptionally almost at the same time in all regions, being strong and long at the entrance and within the Gulf of Finland and weakest at the Arkona Basin. The period of clear water was recorded most perfectly in the Arkona and Bornholm Basin. In the Eastern and Northern Gotland Basin the second increase in July was exceptionally high. The third increase started already in the middle of August in the Gulf of Finland and occurred from the middle to the end of October in the Gotland and Arkona Basin.

In 2005 the chlorophyll a concentrations were lower than on the Alg@line-reference in the Arkona and Bornholms Basin, but higher than on Alg@line-reference in other regions.

Silicate (SiO4)

In 2005 the annual surface water concentration of silicate varied between 1.3-19.9 µmol l^-1. The highest spring and winter levels of silicate concentrations were in the Arkona Basin. In the Arkona and Bornholm Basin the concentrations were higher than on  the Alg@line-reference throughout the year. The concentration in the Gotland Basin and on the Gulf of Finland followed instead the Alg@line-reference throughout the year.

Ratio of dissolved inorganic nitrogen and dissolved inorganic phosphor (DIN:DIP)

In 2005 the DIN:DIP ratio exceeded the EUTRO-reference value (16 µmol l^-1) only once: In the Northern Gotland Basin in the beginning of the September. The lower EUTRO-target level (8 µmol l^-1) was exceeded in the Gulf of Finland before middle of March, momentarily in the middle of August and after end of October. In the other regions the same level was not exceeded under winter time. On the Arkona and Bornholm Basin the DIN:DIP ratio was lower than on Alg@line-reference on the whole year.

Ratio of dissolved inorganic nitrogen and silicate (DIN:SiO4)

In 2005 the DIN:silicate ratio did not exceed the EUTRO-reference value (1 µmol l^-1) during the whole year, but it was higher than Alg@line-reference during the spring on the Gulf of Finland. The highest ratios were found on the Gulf of Finland before April and after October. The ratio was lower than Alg@line-reference on other regions during the whole year 2005.

	Click images to enlarge

Figure 4. The annual variation of dissolved inorganic phosphate (PO₄ µmol l^-1) as a function of position and time along the ship route (see Figure 10). On the left is the year 2005 and on the right is the Alg@line-reference (average of the yrears 1993-2004) with 10 days interval. Cumulative day number is used as a temporal scale. The figures are based on red dots or triangles in case of west route of the Gotland.

Figure 5. Idem, but dissolved inorganic nitrogen (NO₃+NO₂ +NH₄ µmol l^-1).  

Figure 6. Idem, but chlorophyll a (mg m^-3).  

Figure 7. Idem, silicate (µmol l^-1).

Figure 8. Idem, but ratio of dissolved inorganic nitrogen and -phosphate ((NO₃+NO₂ +NH₄)/PO₄).  

Figure 9. Idem, but ratio of dissolved inorganic nitrogen and silicate ((NO₃+NO₂ +NH₄)/SiO₄).

Assessment: Bothnia Sea (BalEco™ model)

The DIP concentrations in the Bothnia Sea remained under 0.6 µmol l^-1 during the year 2005. The increase of phosphate concentrations started there at the same time as in the Baltic Proper (Alg@line transect), in the middle of the September. The concentrations of DIN were high on the early spring, but started to increase slowly aslike in the Baltic Proper and remained under 3 µmol l^-1 during the rest of the year. The chlorophyll a values remained low the whole year, but silicate values were high (as on the Southern Baltic Proper). The ratio of DIN:DIP was highest and exceed 16 µmol l^-1from the end of July to the end of October, but decreased under the EUTRO-reference level (16 µmol l^-1) at the beginning of November. The DIN:silicate ratio never exceeded 0.5µmol l^-1 (lower EUTRO-target level) during the year 2005 on the Bothnia Sea.

Figure 10. On the left: the annual variation of dissolved inorganic phosphate (PO₄ µmol l^-1) as a function of position and time along the model transect (see Figure 10) in the Gulf of Bothnia. On the right: the annual variation of dissolved inorganic nitrogen (NO₃+NO₂ +NH₄ µmol l^-1). Cumulative day number is used as a temporal scale. Note: figure is based on operational BalEco™ ecosystem model results. Period under raster needs further quality analysis of the model results.  

Figure 11. Idem, but chlorophyll a (mg m^-3) and silicate (µmol l^-1).  

 Figure 12. Idem, but ratio of dissolved inorganic nitrogen and dissolved inorganic phosphorus (left), ratio of dissolved inorganic nitrogen and silicate (right).

Assessment: depth-dependence on the Alg@line route (BalEco™ model)

The vertical structures of dissolved nutrients and silicate showed remarkable difference between the Eastern Gotland Basin and the entrance to the Gulf of Finland. The main difference between the locations was during the spring and autumn convection. The nutrient concentrations were higher on the Eastern Gotland Basin during the spring and in the Gulf of Finland in the end of the year. The silicate values were higher in the Eastern Gotland Basin during the whole year.

Figure 13. The annual variation of phosphate (PO₄ µmol l^-1) as a function of depth (m) and time. Left: Eastern Gotland Basin; Right: entrance to the Gulf of Finland (locations in the Fig.10). Cumulative day number is used as a temporal scale. Note: figure is based on operational BalEco™ ecosystem model results. 

Figure 14. Idem, but DIN (µmol l^-1). 

Figure 15. Idem, but silicate (µmol l^-1). 

Acknowledgements

This work is partly founded by the Nordic Council of Ministers (No Comments and BANSAI projects).

Metadata

Technical information

1. Source: Finnish Institute of Marine Research, contact persons Anniina Kiiltomäki and Tapani Stipa.

2. Description of data:

Reference and target -levels:

EUTRO-reference levels for the open Baltic sea are from before 1950s. EUTRO-target levels are based on the EUTRO -reference levels. For more details see EUTRO-report by HELCOM.

Observations:

Original unit of measure: nutrients, silicate µmol l ^-1

Original unit of measure: Chl a mg m^-3

Original purpose of the data: Phytoplankton monitoring of FIMR, Alga@line project

BalEco™ model:

Operational ecosystem model running at the Finnish Institute of Marine Research.The analysis is based on the +6 hour forecast t ime series of the model. FIMR HELCOM COMBINE observations have been assimilated into the model, but the model results are independent of the Alg@line time series.

3. Geographical coverage: Alg@line: Gulf of Finland, the Baltic Proper. BalEco: Bothnian Sea, Baltic Proper

Figure 16. The Alg@line sampling locations on the route of M/S Finnpartner between Travemünde- Helsinki and the BalEco™ model transect in the Bothnian Sea. The Alg@line sampling locations are on following regions of the Baltic Sea: 1-2; Arkona Basin, 3-5 Bornholm Basin, 6-12; Eastern Gotland Basin, 13-18; Northern Gotland Basin and 19-24; Gulf of Finland. The vertical analysis location are BY5 and LL12. The locations of the first sight -figures are: 3,4=AB; 5,6=BB; 11,12=GB_E; 14,15=GB_N; 18,19=GoF; SR5 and latitude 62^o at the same longitude=BS .

4. Temporal coverage:

Alg@line-reference: 1993-2004, Analysis: 2005,

Winter period: January to 15th of March, Summer period: June to September

5. Methodology and frequency of data collection:

Automated flow-through sampling system on merchant ships, sampling depth ca. 5 m, Weekly biweekly monthly sampling during the period January- December.

6. Methodology of data manipulation: None.

Quality information

7. Strength and weakness (at data level):

Strength: Medium temporal and spatial sampling frequency. Weakness: storage time of samples from hours to a couple of days depending on the sampling site

8. a) Reliability, accuracy, precision, robustness (at data level) of Alg@line:

Measurement uncertainty: Chl a: 0.5 mg m^-3 if the concentration < 5.0 mg m^-3, 1.0 mg m^-3 if the concentration > 5.0 mg m^-3.

Measurement uncertainty: Phosphate 20-30 %, Nitrate 14-20% and Nitrite 12-45%, Ammonium 20-60%, Silicate 15-50%

8. b) Reliability, accuracy, precision, robustness of BalEco™ model:

The accuracy of the forecast model BalEco™, compared against the Alg@line observations in 2005, is illustrated in Figure 17. The statistical indicators mean error (ME), mean absolute error (MAE) and root mean square error (RMSE) were calculated for every Alg@line water sample location along the route of the M/S Finnpartner. The time period was 1.6-31.12.2005. The ecosystem model has currently limitations in the description of nutrient field development in the winter and in anoxic conditions on the process level.

Figure 17. The ME MAE, RMSE of DIP (left up)_, DIN (right up), SiO4 (left down) and Chla (right down) on each water sample location; 6h forecast. The legend-box shows the values calculated to the entire area.

9. Further work required (for data level and indicator level):
Comments from HELCOM MONAS for further development of the indicator report are welcome.

For reference purposes, please cite this indicator fact sheet as follows:

[Author’s name(s)], [Year]. [Indicator Fact Sheet title]. HELCOM Indicator Fact Sheets 2006. Online. [Date Viewed], http://www.helcom.fi/environment2/ifs/en_GB/cover/.
 

Last updated 29.9.2006.