Temporal development of winter nutrient concentrations in Baltic Sea surface waters

Key message

In most of the investigated areas DIN and DIP concentrations increased from the 1970s and peaked in the mid/end 1980s and started then to decrease (figure 2). Exceptions are the Gulf of Finland and the Baltic Proper where an increase in DIP concentrations has been observed during recent years.

Earlier measurements are rare, but extremely important for the description of the overall eutrophication development and the assignment of background concentrations as can be shown for the eastern Gotland Basin (figure 3).

The ratio between ambient DIN and DIP concentrations can be used to indicate whether N or P is the limiting nutrient. Based on this information, the Bothnian Bay is mostly phosphorus limited, the Bothnian Sea can be seen as a transition area whereas the other open water areas, especially the Baltic Proper, are most likely nitrogen limited.

Figure 2. Winter means (December-March) of DIN and DIP in surface water (0-10m). Error bars show the 95% confidence limits of the means. Solid curves are 5-year moving averages. BB = Bothnian Bay; BS = Bothnian Sea; GF = Gulf of Finland; GR = Gulf of Riga; BP = Baltic Proper; DS =Danish Straits (HELCOM, 2009)

Assessment

A detailed description of the observed trends in figure 2 can be found in Chapter 2.2 of the HELCOM integrated thematic assessment on eutrophication (HELCOM 2009).

For the eastern Gotland Basin measurements reach back to the 1950s (figure 3). Phosphate concentrations of around 0.20-0.25 µmol/l as found in this period are assumed to represent a period relatively unimpacted by anthropogenic activities disturbing the ecological balance of the open Baltic Sea. After the remarkable increase of phosphate in the 1960s and 1970s concentrations of this nutrient remained on a high level with strong fluctuations as a result of mainly internal processes. Major Baltic Inflows (MBIs) are of great importance in this respect. After the MBIs of 1975/76, 1983 and 1993 lower phosphate concentrations in the subsequent years were measured whereas a comparable decrease after the MBI of 2003 could not be observed, indicating that the vertical transport through the permanent halocline is not sufficiently understood. The historicity of the inflow events and the seasons of MBIs as well as the intensity and depth of vertical mixing must be considered (Nausch et al. 2008; Reissmann et al. 2009).

The intense eutrophication during the 1960s and 1970s is also reflected as high nitrate (+nitrite) concentrations (figure 3). However, the number of nitrate values from the 1960s is low. Therefore, these values should be used with some caution with respect to background concentrations. Recently, nitrate background values of 2.0 – 2.5 µmol/l seem to be most reliable. From the mid 1970s onwards, nitrate concentrations fluctuate on a high level. MBIs before 1992 had only little influence on the nitrate concentrations in the surface layer because the layer below the halocline was relatively well supplied with oxygen favouring the development of a huge nitrate pool between the halocline and the redoxcline which could be mixed upwards. During the stagnation period after the MBIs 1993/1994 unfavourable oxygen conditions below the halocline caused denitrification and only a small nitrate maximum remained. Thus vertical mixing transported only low amounts of nitrate into the surface layer and the winter concentrations of DIN were low over a longer period of time (Nausch et al. 2008). Also the depth of the halocline influences vertical mixing processes.

The relation between DIN and DIP can be used to indicate whether N or P is the limiting nutrient. According to Redfield et al. (1963) phytoplankton incorporates carbon, nitrogen and phosphorus in a molar ratio of C:N:P = 106:16:1. N:P ratios higher than 16 suggest P limitation whereas lower values are standing for N limitation. Figure 4 summarizes the trends for the DIN:DIP ratio for six basins of the Baltic Sea (c.f. HELCOM, 2009).

In the Bothnian Bay, the overall excess of N relative to P indicates potential P limitation.

In the Bothnian Sea a shift from potential P to potential N limitation is observed.

In the Gulf of Finland, the different trends of DIN and DIP resulted in an increase of the ration from 7 in the 1970s to 15 in the 1980s and returned to about 7 recently.

In the open Baltic Proper, DIN:DIP ratios remained stable over decades between 7 and 9. The above described opposite trends for DIN and DIP in the winter layer resulted in further decreasing N:P ratios resulting in a stronger N limitation of the spring bloom. On the other hand, low N:P ratios favour bloom of cyanobacteria in summer.

In the open Danish Straits waters N:P ratios are somewhat below 16 with a decreasing trend during recent years.

Coastal waters generally show higher N:P ratios, mainly caused by the high N input from the catchment area.

Figure 3. Mean phosphate and nitrate+nitrite concentrations in the mixed winter surface layer (0-10m) pooling five stations in the eastern Gotland Basin from 1958 to date (from Feistel et al. 2008).

Figure 4. Winter DIN:DIP ratios in six basins of the Baltic Sea (HELCOM, 2009)   

References

HELCOM 2009. Eutrophication in the Baltic Sea – An integrated thematic assessment of the effects of nutrient enrichment in the Baltic Sea region. Helsinki Commission. Baltic Sea Environment Proceeding No. 115B.

Feistel, R., Nausch, G. and N. Wasmund (Eds.) (2008): State and evolution of the Baltic Sea, 1952-2005. – John Wiley & Sons, Inc. Hoboken, New Jersey, 1-703.

Nausch, G., Nehring, D. and K. Nagel (2008): Nutrient concentrations, trends and their relation to eutrophication. In: Feistel, R., Nausch, G., Wasmund, N. (Eds.) (2008): State and evolution of the Baltic Sea, 1952-2005. – John Wiley & Sons, Inc. Hoboken, New Jersey, 337-366.

Redfield, A.C., Ketchum, B.H. and F.A. Richards (1963): The influence of organisms on the composition of seawater. – In Hill, M.N. (Ed.): The Sea, Vol. II, Wiley, New York, 26-77.

Reissmann, J., Burchard, H., Feistel, R., Hagen, E., Lass, H.U., Mohrholz, V., Nausch, G., Umlauf, L. and U. Wieczorek (2009): Vertical mixing in the Baltic Sea and consequences for eutrophication – A review. – Progress in Oceanography 82, 47-80.

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