Temporal and spatial variation of dissolved nutrients in the Baltic Sea in 2004 

	Authors: Anniina Kiiltomäki, Tapani Stipa, Seppo Kaitala

Key message

The dissolved inorganic nitrogen were lower in all regions except at the entrance to and with the Gulf of Finland throughout the year 2004 when compared to the reference (the average of the years 1993-2003).

In 2004 also the chlorophyll a values were lower than the reference in all regions except at the entrance to and with the Gulf of Finland.

In the Gulf of Finland the chlorophyll a concentrations peaks were higher than in the reference.

The phosphate values were higher than the reference throughout the year 2004 in all regions except in the Arkona Basin. Upwellings have influenced phosphate values in the Bornholm and Gotland Basins from the middle to the end of September.

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 and temperature variation (together with 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 a 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 between the COMBINE efforts 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.

Assessment: Baltic Proper (Alg@line observations)

Phosphate (PO4)

In 2004 the annual concentration range of phosphate varied between 0.00 – 1.65 µmol l-1. The highest winter nutrient concentrations were measured in two regions: at the entrance to and with the Gulf of Finland, and in the southern Gotland Basin. From the early beginning of April to the beginning of July the concentrations decreased to < 0.3 µmol l-1 in the whole study area. In the Arkona Basin the concentrations were exceptionally low during the whole year.

The difference to the reference concentrations was large. The phosphate concentration was above the reference in all regions, except the Arkona Basin, in 2004. The decrease of concentrations were slower on the spring and the increase towards the winter started earlier. In the Arkona Basin, the concentration was lower than the reference throughout the year.

Dissolved inorganic nitrogen (NO3 + NO2 + NH4)

In 2004 the annual concentration of dissolved inorganic nitrogen (DIN) varied between 0.00-10.47 µmol l-1. The highest early spring levels of DIN concentrations were found in the Arkona Basin and at the entrance to and with the Gulf of Finland. In the Bornholm and Gotland Basin the spring concentrations increased only to 2.5– 3.5 µmol l-1, but there was no information before February 20. Concentrations decreased under 1 µmol l-1 in the middle of April in the whole area. Sporadically higher values were observed a few times during the year in different locations, probably due the NH4 increase.

Compared to the reference, in 2004 the DIN concentrations started to decrease earlier on the spring. The late autumn increase of DIN started as in mean values. In the Arkona, Bornholm and Gotland Basin the values were lower than respective reference values.

Temperature

In 2004 the temperature varied between 0°C and 23°C, and four periods of rapid changes were detected,(see Fig. 3). Temperature started to increase in the middle of March reaching its highest values in the middle of August. The first period of rapid changes was detected in the middle of June: the amount of decrease was 0-4°C . The warming stopped for about 20 days, and 17°C was reached later than on 10 years mean.

The highest temperature occurred exactly at the same time as in reference, but temperature started to decrease later in 2004. Generally water was warmer in 2004 than reference temperature, but the warmest period was shorter than normally.

Chlorophyll a

In 2004 the annual concentrations of chlorophyll a varied between 0.39-40.87 mg m-3. The spring bloom was first detected in the Gulf of Finland, then in the Arkona Basin and about 20 days later on the rest of the area. The spring bloom was weakest in the Bornholm Basin and in south Gotland Basin, and highest in the Gulf of Finland. From the end of April to the beginning of the June the concentrations decreased to a level of 1-3 mg m-3. The period of clear water was recorded most perfectly in the Bornholm Basin. In other regions the chlorophyll a concentrations were relatively low during the whole year. The second increase in the middle of July was exceptionally weak. In the Gulf of Finland there was high third increase in the middle of the August.

In 2004 the chlorophyll a concentrations were lower than on the reference in the all regions except in the GoF, where the concentrations were higher than average.

Figure 1. The annual variation of 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 2004 and on the right is the reference (10 year average 1993-2003) 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 2. Idem, but dissolved inorganic nitrogen (NO3+NO2 +NH4 µmol l-1).

Figure 3. Idem, but temperature (°C).

Figure 4. Idem, but chlorophyll a (mg m-3).

Assessment: Gulf of Bothnia (BalEco™ model)

The phosphate concentrations in the Gulf of Bothnia remained under 0.6 µmol l-1 during the year 2004. The increase of phosphate concentrations started there at the same time as in the Baltic Proper (Alg@line transect), in the beginning of the September. The concentrations of dissolved inorganic nitrogen remained over value 3.0 µmol l-1 longer in the Gulf of Bothnia than in the Baltic Proper. The concentrations decreased there under 0.1 µmol l-1 in the turn of May/June,  about one and a half month later than in the Baltic Proper. The concentrations started to increase already in the middle of September. The temperature evolved identically in all areas, but remained lower in the Gulf of Bothnia.

Figure 5. On the left: the annual variation of 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.

Figure 6. Idem, but temperature (°C).

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

The vertical structures of dissolved nutrients and temperature showed remarkable difference between the Gotland Basin and the entrance to the Gulf of Finland. The main difference between the locations was during the autumn convection. In the spring the nutrient concentrations were higher in the Gulf of Finland.

Figure 7. The annual variation of phosphate (PO4 µmol l-1) as a function of depth (m) and time. Left: Gotland deep (57.5 N, 20.0 E); Right: entrance to the Gulf of Finland (59.00 N, 22.00 E). Cumulative day number is used as a temporal scale. Note: figure is based on operational BalEco™ ecosystem model results.

Figure 8. Idem, but DIN µmol l-1.

Figure 9. Idem, but temperature (°C).

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:

Observations:

Original unit of measure: nutrients µmol l -1
Original unit of measure: Chl a mg m-3
Original unit of measure: temperature °C
Original purpose of the data: Phytoplankton monitoring of FIMR, Alg@line project

BalEco™ model:

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

3. Geographical coverage: Gulf of Finland, the Baltic Proper.

Figure 10. 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; Gotland Basin, 13-18; Northern Baltic Proper and 19-24; Gulf of Finland.

4. Temporal coverage:
Reference: 1993-2003
Analysis: 2004

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: Temperature 0.1°C
Measurement uncertainty: Phosphate 20-30 %, Nitrate 14-20% and Nitrite 12-45%, Ammonium 20-60%

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

The accuracy of the forecast model BalEco™, compared against the Alg@line observations in 2004, is illustrated in Figure 11. 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 the year 2004. The ME of phosphate varied between 0-0.2 µmol l -1 on the whole area. The accuracy of dissolved inorganic nitrogen increased towards north-east, probably because of the late autumn values. The ME in temperature was negative on the whole study area, but varied more on the east from 18°E. The forecast temperature was 1-2°C lower in the mean compared to the Alg@line temperature; part of this difference is due to model bias. 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 11. The ME MAE, RMSE of PO4 and DIN on each water sample location; 6h forecast. The legendbox 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 2005. Online. [Date Viewed], http://www.helcom.fi/environment2/ifs/en_GB/cover/. 

 Last updated 25 Nov 2005.