Phytoplankton spring bloom biomass in the Gulf of Finland, Northern Baltic Proper and Arkona Basin in 2006

	Vivi Fleming and Seppo Kaitala

Key message

The spring bloom in the Gulf of Finland was less than half the size of last years bloom

The spring bloom was negligable in the Arkona Basin

No rising trend can be detected from 1992 to 2006 in the Gulf of Finland, the northern Baltic Proper or the Arkona Basin

Fig. 1:   Development of the spring bloom intensity from 1992 to 2006 in Arkona Basin (AB), the northern Baltic Proper (NB) and the western Gulf of Finland (GOF). The ship route is shown in the map as broken line and study areas are shadowed. Click image to enlarge.

Results and assessment

Environmental context

Chlorophyll a concentration is a relative measure of phytoplankton biomass in the water. Since high nutrient concentrations increase phytoplankton growth and subsequently the intensity and frequency of blooms, chlorophyll a can be used as an indicator of the eutrophication in a sea basin.

The intensity of the spring bloom reflects the scale of the nutrient reserves. The spring bloom species of diatoms and dinoflagellates consume most of the phosphorus and nitrogen nutrients that were built up in the water mass during the previous winter.

Policy relevance

The availability of nutrients regulates the primary production and biomass of planktonic algae and the nutrient ratio of the main nutrients nitrogen and phosphorus largely determines which species can proliferate. The eutrophication of the Baltic Sea is still accelerated by the diffuse loading of nutrients from the whole drainage area from scattered housing, agriculture, aquaculture and from traffic.

The spring bloom estimate for the Western Gulf of Finland 

The spring bloom intensity index in 2006 was lower than in the previous year and below the ten-year average (1996-2005) (Fig. 1). Both the bloom peak and length were lower than last year and the ten-year average (Fig. 2, Appendix 1). In addition, the bloom started very late, on the second week of April.

Fig. 2: The seven-day running average chlorophyll a curve in 2006 in the western Gulf of Finland (green). Pink ‘area’ illustrates the intensity index of the spring bloom, the spring bloom threshold is shown with a solid black line. Also the peak and length of bloom are presented.

The spring bloom estimate for the northern Baltic Proper

The 2006 spring bloom intensity index was slightly lower than the previous year and the long time average (Fig. 1, Fig. 3). Generally the bloom begins later in the northern Baltic Proper than the Gulf of Finland, but this was not the case in 2006 (Appendix 1).

Fig. 3: The seven-day running average chlorophyll a curve in 2006 in the northern Baltic Proper (green). Pink ‘area’ illustrates the intensity of the spring bloom, which is calculated as the intensity index; the spring bloom threshold is shown with a solid black line. Also the peak and length of bloom are presented. Click image to enlarge.

The spring bloom estimate for the Arkona Sea

The chlorophyll a seven day running average –curve did not rise above the threshold concentration of 5 µg/l in the Arkona Sea sub region. However, a slight rise in the chlorophyll a level reaching about 4 µg/l could be detected from the middle of March to the middle of April (Fig. 4, Appendix 1).

Fig. 4: The seven-day running average chlorophyll a curve in 2006 in the Arkona Basin (green). The intensity index is presented, and the spring bloom threshold is shown with a solid black line. Click image to enlarge.

Metadata

Technical information

1. Source:

Finnish Institute of Marine Research, contact persons Vivi Fleming and Seppo Kaitala.

2. Description of data:

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

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

3. Geographical coverage:

Gulf of Finland, Archipelago and Åland Sea, the Baltic Proper.

4. Temporal coverage:

1992-2006.

5. Methodology and frequency of data collection:

Automated flow-through sampling system on merchant ships, sampling depth ca. 5 m, fluorescence measurements every 100 to 300 meters during the period February/March to October/November. The fluorescence is calibrated to chlorophyll a using the weekly chlorophyll a samples. Interpolation of calibration constant is used in order to calibrate measurements also for the transects not taking water samples.

6. Methodology of data manipulation:

The spring bloom is estimated setting a chlorophyll a threshold level of 5 µg/l for the beginning and end of the bloom. The intensity index is calculated by integrating the ‘area’ under the chlorophyll seven-day average curve (pink area in figures 2 to 5). Also the length, peak and mean chlorophyll a level during the bloom are calculated.

Special cases:

If the chlorophyll a value decreases below the threshold in the middle of the bloom, the bloom is regarded to consist of two separate periods. Thus the intensity index and the length of the bloom is calculated using only the periods exceeding the threshold (Fig. 5a).

If the spring bloom does not exceed the threshold, the intensity index is zero, as well as the length, peak and mean values (Fig 5b).

If the data does not cover the beginning of the bloom (data collection has started too late), the calculated intensity index as well as length and mean values are smaller than in reality (Fig 5c). In appendix 1 these years are marked with an asterisk.

Fig. 5: The seven-day running average chlorophyll a curve (green), with spring bloom intensity index highlighted (pink area). a) A case where the spring bloom consists of two periods b) a case where spring bloom chlorophyll a level does not exceed the threshold c) a case where the data does not cover the beginning of the bloom.

Quality information

7. Strength and weakness (at data level):

Strength: Very high both temporal and spatial sampling frequency. Weaknesses: 1. The flow through chlorophyll a is not as accurate as measured directly from water samples, due to the fluorescence calibration method. 2. During some years data collection starts too late to cover the beginning of the spring bloom in the Southern Baltic; this is inevitable since sometimes the Gulf of Finland is still covered with ice at the start of the Southern Baltic spring bloom.

8. Reliability, accuracy, precision, robustness (at data level):

Measurement uncertainty: Chl a: 1 µg/l l if the concentration < 5.0 µg/l, 2.0 µg/l if the concentration > 10.0 µg/l.

9. Further work required (for data level and indicator level):

During some years the data collection started too late to cover the beginning of the spring bloom, due to ice in the Northern Baltic. In further development of the method one could take into account the missing data, possibly by approximating a starting date using information of the progress and beginning of bloom during previous years.

Appendix 1

The spring bloom estimates for Arkona Basin (AB), the northern Baltic Proper (NB) and the western Gulf of Finland (GOF) from 1992 to 2006. Estimates: spring bloom intensity index, mean chl a during bloom, length of bloom, highest peak of the bloom and the starting day of the bloom. Years in which data have not covered the beginning of the bloom are marked with an asterisk.

Arkona Basin

Northern Baltic Proper

Western Gulf of Finland

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 8.9.2006