Press release 2013-02-19 at 10:00
The Gulf of Finland remained ice-free for a relatively long period in early winter, which resulted in a good oxygen situation in the Gulf. Of the eutrophication-causing nutrients, the distribution of phosphorus in the water column has changed. While the levels of phosphorus in the surface layer have decreased from those detected last year, they have slightly increased in deepwater. Similarly to last year's results, the concentration of nitrogen in surface water exceeds the long-term average.
These are the findings of the winter monitoring cruises of Aranda, research vessel of the Finnish Environment Institute SYKE, conducted jointly by the Marine Research Centre of SYKE and the Finnish Environment Institute. The cruises are part of the Combine monitoring programme of the Baltic Marine Environment Protection Commission HELCOM. Areas covered in the tours include the Gulf of Finland, the Archipelago Sea, the Bothnian Sea, the Bothnian Bay, the Åland Sea and the central parts of the main basin of the Baltic Sea, reaching until the southern tip of the Gotland Island. Since the ice cover has also remained thin in the north, for the first time over many winters Aranda was able to reach the most northern stations in the Bothnian Bay.
‘The mixing of waters caused by storm winds last winter is visible in the Gulf of Finland. In the Baltic Sea proper, the boundary (halocline) between saltier, heavier deep water and lighter, less salty surface water remains visible in the northern part of the sea basin, where it emerged in the previous winter (figure 1),’ explains cruise leader Juha Flinkman, Senior Scientist at the SYKE Marine Research Centre
Phosphorus levels remain low in the Gulf of Finland while nitrogen levels have increased
In comparison to the winters of 2010 and 2011, increased nitrogen concentrations, similar to those observed last winter, were detected in the proximity of the Finnish coastline and in the northern parts of the Gulf of Finland in general. This was caused by the heavy rainfall at the end of year, which led to an increase in the amounts of nitrogen carried into the sea by rivers (figure 2). The effect of heavy rains was also reflected in the decreased salt concentration in the surface layer.
The results of winter monitoring indicate that in the Gulf of Finland the phosphorus levels have remained low, close to those observed last winter, and are still lower than those detected in the winters of 2010 and 2011. In comparison to the preceding winter, the distribution of phosphorus has slightly changed, with a fall of 20 per cent in phosphorus levels in the surface water from last year, while clearly increased levels were detected in the deep water areas of the western Gulf of Finland.
‘The low phosphate level is largely attributable to efficient mixing of waters in the Gulf of Finland and good oxygen conditions, which have kept the internal nutrient load at a low level’, says Juha Flinkman (figures 3 and 4).
The good news is that the favourable oxygen situation on the sea floors of the Gulf of Finland prevents the onset of internal loading. More efficient removal of phosphorus from urban waste water originating from St. Petersburg is another factor explaining the lower phosphorus levels, especially in the eastern parts of the Gulf of Finland.
Blue-green algae blooms in the summer are influenced by the so-called surplus phosphorus, left over from the spring bloom of planktonic algae: the more surplus phosphorus there is, the greater is the likelihood of blooms. The amount of such surplus phosphorus can be estimated on the basis of phosphorus and nitrogen amounts in winter.
According to these calculations, the amount of surplus phosphorus would remain smaller across the Gulf of Finland in comparison to the amounts detected in 2012, which in turn had fallen significantly from those observed in 2010 and 2011 (figure 5). In practice, this would lead to less than normal amounts of algae blooms. However, the nutrient levels may change during the spring, depending on the weather conditions and mixing of waters. For this reason, SYKE will issue its forecast for algae blooms in the sea areas only in the beginning of June.
Thicker layer of oxygen-free water in the main basin of the Baltic Sea
The layer of oxygen-free water above the deep Baltic Sea floors still exists, and in places it has grown thicker. Oxygen-deficient water is now met already at a depth of 65 metres, and hydrogen sulphide at a depth of 90 metres. The levels of hydrogen sulphide have increased in the deep water areas of the main basin, in comparison to the preceding years (figure 6).
Located behind the threshold formed by the continuation of the Salpausselkä end moraines, the entire Gulf of Bothnia system, comprising the Åland Sea, the Bothnian Sea and the Bothnian Bay, are protected from the oxygen-free deep waters in the main basin of the Baltic Sea. In these areas, good oxygen conditions prevail, and no significant changes were detected from the previous year. Water in the Bothnian Sea, the Bothnian Bay and the Åland Sea is oxygen-rich until the bottom in all deep basins. When comparing to previous years, streaming of saltier, oxygen-deficient water into the area could hardly be detected (figure 7).
Further information
Tour Leader, Development Manager Juha Flinkman
SYKE's Marine Research Centre
firstname.lastname@ymparisto.fi
Tel. +358 40 750 3911
Chief Editor Aira Saloniemi
SYKE Communications, Baltic Sea issues
firstname.lastname@ymparisto.fi
Tel. +358 400 148 875