Horizontal variation of dissolved nutrients in the Baltic Sea in 2002

	Authors: Anniina Kiiltomäki, Eija Rantajärvi, Tapani Stipa, FIMR

 Key message

• In 2002, the phosphate values were higher throughout the year than in the reference year 2001.
• The highest winter nutrient values were measured at the entrance and in the Gulf of Finland.
• In the Gotland and Bornholm basins and in the Gulf of Finland an increase in phosphate values in
  the turn of June/July was probably due to upwellings.

The temporal and spatial variation of dissolved nutrients (phosphate, nitrate and nitrite) was studied in
2002 for this water quality report. The temperature and chlorophyll a values were viewed as additional
parameters. The Alg@line SOOP data were used, the amount of samples along one voyage from
Helsinki to Travemünde was 24 per week.

The values from the year 2001 are used as a reference data.

Figure 1. Route of the merchant ship Finnpartner with Alg@line sampling (Travemünde-Helsinki). In the text the Baltic Sea is divided to following sub-areas: the Arkona Basin (longitude 14.5°-17°), Bornholm Basin (14.5°-17°), Gotland Basin (17°-21°), Northern Baltic Proper (21°-23°) and the Gulf of Finland (23°-24.97°).

Results and assessment

Relevance of the indicator for describing developments in the environment

Nutrients as such indicate the level of eutrophication in a sea basin. The amount of nutrients (together with the amount of light and the variation in temperature) form the basis for phytoplankton succession. 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.

Assessment

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 gets warmer during summer, the blue-green algae become more common. The occassional upwellings of deeper, nutrient rich water can stimulate the algal growth.

Phosphate (PO4; µmol l^-1)

In 2002 the annual concentration range of phosphate varied 0.01-1.4. The highest winter nutrient concentrations were measured at the entrance and in the Gulf of Finland (0.8 -1.4). From the middle of March to the early beginning of May the concentrations decreased < 0.4 in the whole study area.

Exceptionally, in the Gotland and Bornholm basins, clearly elevated concentrations were measured from the middle of February (0.8-1.0) until the end of April (0.5-0.7). In the same area an increase was detected again in the end June to the beginning of July and a weaker one in the Gulf of Finland as an exception compared to other areas. That was probably due to the upwellings.

There is a difference between the years 2002 and 2001 in the phosphate minimum values. In 2001 the phosphate was depleted in middle of June in all areas, in 2002 the decline in phosphate level occurs notably later.

Figure 2. The annual variation of phosphate (PO4 µmol l^-1 ) as a function of position and time along the ship route (see Figure 1.), for years 2002 and 2001. The water sampling locations are indicated with red points, and with triangles when the west route was used. Cumulative day number and months are used as a temporal scale.

Nitrate and nitrite (NO3 + NO2; µmol l^-1)

In 2002 the annual concentration of dissolved nitrogen varied 0-11.2. The highest winter levels of
nitrogen concentrations were found in the Arkona Basin, at the entrance and in the Gulf of Finland
(6.5-11.2). In the Gotland and Bornholm Basins the winter concentrations increased only up to 3 – 5.5.
Concentrations decreased < 1 in the middle of April almost in the whole study area. Exceptionally, in
the Arkona Basin the concentration decreased < 1 already in the middle of March, but increased again
strongly towards the end of April.

In 2002 nitrate and nitrite concentrations decreased later < 1 than in the reference year 2001 

Figure 3. The annual variation of nitrate and nitrite (NO3+NO2 µmol l^-1) as a function of position and time along the ship route, for years 2002 and 2001.

Temperature

In 2002 the temperature variation was from 2 ºC to 23 ºC, and three periods of rapid changes were detected. Temperature started to increase in the middle of March reaching its highest values in the middle of August. The first period of rapid temperature change was detected in the turn of June/July: the amount of decrease was 1 - 4 °C in the Gulf of Finland, and > 2 °C around the longitudes 18.5° and 16.5°. An other rapid change occurred in the same longitudes, 18.5° and 16.5°, in the end of July: a decrease of 3-6°C. The third period of rapid change was in the middle of October, in the Western Gotland Basin. Simultaneously, the temperature decreased also in the Northern Baltic Proper. The rapid changes of temperature were most probably due to upwellings.

In 2002, compared to 2001, the surface layer started warm up earlier but more slowly and stayed warm longer in the autumn. 

Figure 4. The annual variation of temperature as a function of position and time along the ship route, for years 2002 and 2001.

Chlorophyll a (mg m^-3)

In 2002 the annual concentrations of chlorophyll a varied 0.4-39. The high spring blooms values were first detected in the Arkona Basin (7-10), then in the Northern Baltic Proper and in the Gulf of Finland (12-22, max 39), and weakler ones in the Gotland Basin (2-12). From the end of April to the beginning of the June the concentrations decreased to 1-3. The period of the clear water was short and an increase in concentrations (3-10) were recorded from the middle of June to the middle of August especially in the Gulf of Finland and in the Northern Baltic Proper. In the same areas in autumn a slight increase was detected again.

Figure 5. The annual variation of chlorophyll a (mg m^-3) as a function of position and time along the ship route, for years 2002 and 2001.

Metadata

The temporal and spatial variation of dissolved nutrients (phosphate, nitrate and nitrite) was studied in 2002 for this water quality report. The temperature and chlorophyll a values were viewed as additional parameters. The Alg@line SOOP data were used, the amount of samples along one voyage from Helsinki to Travemünde was 24 per week.

Technical information

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

2. Description of data:
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

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

4. Temporal coverage: 2001-2002.

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 a few hours to a couple of days depending on the sampling site

8. Reliability, accuracy, precision, robustness (at data level):
Measurement uncertainty: Chl a: 0.5 mg m^-3 if the concentration < 5.0 mg m^-3, 1.0 g m^-3 if the concentration > 5.0 mg m^-3.
Measurement uncertainty: temperature 0.1 °C
Measurement uncertainty: Phosphate 30-20 %
Measurement uncertainty: Nitrate and nitrite 20-14%

9. Further work required (for data level and indicator level): -
Acknowledgement: This work has been partly funded by the Nordic Council of Ministers
(NoComments project)