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Spatial distribution of the winter nutrient pool
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Key message
The excess ortho-phosphate (dissolved inorganic phosphorus) available in surface waters at the end of 2004, coupled to the extremely calm summer of 2005, led to an intense and widespread cyanobacteria bloom in the central Baltic Proper. Sedimentation of these cyanobacteria, as well as transport by currents, has reduced the concentration of dissolved inorganic phosphorus in the central Baltic Proper. Levels remain extremely high in the Arkona Basin, and are high even in the Gulfs of Finland and Bothnia. This remaining excess phosphorus was the cause of the even-more widespread cyanobacteria blooms of summer 2006.
Results and assessments
Relevance of the indicator for describing developments in the environment Dissolved Inorganic Phosphorus (DIP) is essential for phytoplankton development. While rivers deliver phosphorus to the Baltic, most of this phosphorus is chemically bound to particles, and is not directly available for biological use. Large amounts of DIP enter the Baltic with inflows of salt water, and phosphorus is also released from bottom sediments during periods of anoxia. Deep water DIP can become bio-available if it is transported to the surface waters, but this transport is hampered by the permanent stratification. Dissolved Inorganic Nitrogen (DIN) is composed of nitrate, nitrite and ammonium compounds, which are also required by phytoplankton. While DIN concentrations are much higher than DIP in surface waters, marine phytoplankton require 15 - 16 times as much DIN as DIP, often causing a lack of DIN to limit phytoplankton activity. Where DIN is used up, bacteria that can fix nitrogen from the air can still flourish, making use of the remaining DIP, and causing blooms. Cyanobacteria exhibit this behaviour. Nitrogen is cycled within the water column and sediment, while ‘fresh’ nitrogen is also supplied, directly or via rivers, by agricultural run-off and sewage discharges, and also through atmospheric deposition. Eutrophication is the supply of excessive amounts of nutrients. The spatial distribution of the primary bio-available nutrients (surface waters, during winter) highlights problem areas, and shows the availability of nutrients for the spring bloom. Changes in the spatial distribution may indicate changes in the hydrography, or the effect of remedial works. Mapping the ratio of winter DIN:DIP may serve as a warning for areas where cyanobacteria blooms are likely. Some cyanobacteria are toxic.
Policy relevance and policy references
The Helcom COMBINE programme uses nutrient data to help quantify the effects of anthropogenic activities. This Indicator Report contributes to the programme’s requirement for information on:
- the winter pool of nutrients
- the supply of nutrients and nutrient limitation in coastal waters
Assessment
Concentrations of DIN are highest in coastal waters from the southern Belt Sea to the inner Gulf of Finland. Levels are also high in the Bothnian Bay. This is unsurprising as the major source of DIN to the Baltic is land run-off. Variability in winter DIN (indicated by the standard deviation plot in Figure 1) is generally due to variability in the land run-off, so is highest near sources of DIN.
Figure 1. Mean winter surface DIN (left) and standard deviation (right) based on each year’s gridded winter surface observations from 1993 - 2002 inclusive. Surface refers to the upper 0 - 10 m. Units are micro-moles/litre. Click image to enlarge.
The highest DIP concentrations are usually also found in the Belt Sea, the southern Baltic coast and the inner Gulf of Finland, though levels are also significant along the Swedish east coast and in the Kattegat. Lowest levels are found offshore, in particular in the Gulf of Bothnia. This is because while some DIP originates from land sources, a large reservoir also exists in the deep water of the Baltic, which can come to the surface during upwelling events.
Figure 2. Mean winter surface DIP (left) and standard deviation (right) based on each year’s gridded winter surface observations from 1993 - 2002. Units are micromoles/litre. Click image to enlarge.
Highest silicate concentrations are found in the Bothnian Bay. The great rivers of surrounding this bay transport large amounts of silicate released through natural processes.
Figure 3. Mean winter surface silicate concentrations (left) and variability (right) based on each year's gridded HELCOM/ICES observations from 1993 - 2002. Units are micro-moles per litre. Click image to enlarge.
Figure 4. Difference between winter 2005-6 nutrient concentrations (DIN: left; DIP: centre; Silicate: right), and the 1993-2002 means. Click image to enlarge.
DIN concentrations remain below the 1993 - 2002 average, with the exception of the central part of the Gulf of Finland. This widespread decrease can be attributed to the political decisions taken to limit nitrogen discharges to the Baltic. Dissolved inorganic phosphorus concentrations were considerably higher than normal particularly in the Bornholm and Arkona basins. The high concentrations appeared to extend up the Finnish side of the Gulf of Bothnia as far as the Bothnian Bay, and across much of the northern and southern Baltic Proper. The DIP is exported from the Baltic through the Sound, giving higher concentrations even in the southern Kattegat. This distribution of silicate is similar to that of DIP. Large amounts were brought to the surface layer during 2003-4, and this accounts for the high levels in the Arkona and Bornholm Basins. In the Gulf of Bothnia, lower rainfall along the Swedish side of the Bothnian Sea led to reduced run off, which resulted in lower silicate levels. Increased rainfall in northern Norrland gave increased run-off and accounts for the higher silicate levels in the Bothnian Bay. Over large areas of the Baltic, silicate levels were lower than the 1993-2002 mean. This may be due to decreased run-off during the previous year.
References
Helcom COMBINE Manual (Annex C), http://www.helcom.fi/Monas/CombineManual2/CombineHome.htm, December 2003. Data This study used data collected under the HELCOM COMBINE programme, and archived for HELCOM by ICES (http://www.ices.dk), supplemented with data collected by SMHI and FIMR for winter 2005/6.
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: 29.9.2006.