Phytoplankton spring bloom

	Authors: Vivi Fleming and Seppo Kaitala, FIMR

Key message

The spring bloom in 2003 was in all areas the most intense since 1992.

However, no rising trend can be detected from 1992 to 2003.

Results and assessment

Figure 1. Development of the spring bloom intensity from 1992 to 2003 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 highlighted.

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 level 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 Gulf of Finland

The spring bloom intensity index in 2003 was higher than the previous year and the long time average (1992-2002, Figure 1). The bloom started in early march, 23 days earlier than the previous year. It had two peaks, of which the first one was not completely covered by the data (Figure 2). The second peak rose nearly to 30µg/l, which is higher than in 2002 and in average, but lower than in 2001.

Figure 2. The seven-day running average chlorophyll a curve in 2003 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 broken line. Also the peak and length of bloom are presented.

The spring bloom estimate for the northern Baltic Proper

The spring bloom intensity index in 2003 was higher than the previous year and the long time average (Figure 1). The bloom started in mid-March, thirteen days earlier than the previous year. The length of the bloom was higher than the previous year and the average, although the last few weeks of the bloom were barely over the threshold (Figure 3).

Figure 3. The seven-day running average chlorophyll a curve in 2003 in the 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 broken line. Also the peak and length of bloom are presented.

The spring bloom estimate for the Arkona Sea

Even though the bloom started earlier than data collection, according to the estimate it lasted longer and was considerably more intense than the previous year and the long time average (Figure 1). The bloom consisted of two peaks, of which the first was higher and outside the data range; the peak was at least 14µg/l , which is more than average (Figure 4).

Figure 4. The seven-day running average chlorophyll a curve in 2003 in the Arkona Basin (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 broken line. Also the peak and length of bloom are presented.

Metadata

Table 1. The spring bloom estimates for Arkona Basin (AB), the Northern Baltic Proper (NB) and the Western Gulf of Fiinland (GOF) from 1992 to 2003. 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.

 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
1992-2003.

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, 3 and 4). Also the length, peak and mean chlorophyll a level during the bloom are calculated.

Special cases:
Figure 5: If the chlorophyll a value sinks 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.
Figure 6: If the spring bloom does not exceed the threshold, the intensity index is zero, as well as the length, peak and mean values.
Figure 7: 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. In Table 1 these years are marked with an asterisk.

Figure 5. The seven-day running average chlorophyll a curve (green), with spring bloom intensity index highlighted (pink area) in a case when spring bloom consists of two periods.

Figure 6. The seven-day running average chlorophyll a curve (green) in a case when spring bloom chlorophyll a level does not exceed the threshold.

Figure 7. The seven-day running average chlorophyll a curve (green), with spring bloom intensity index highlighted (pink ‘area’) in a case when 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. 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):
Sophisticated statistical analysis using the spring bloom intensity index as parameter.

Last update 29 September 2003