Spatial distribution of the winter nutrient pool

	Authors: Philip Axe & Pia Andersson, SMHI

Key message

Due to the poor weather during summer 2004, there was no major cyanobacteria bloom. As a result of this, levels of Dissolved Inorganic Phosphorus (DIP, or ortho-phosphate) remain extremely high throughout the Baltic Proper. This, combined with the relatively low dissolved inorganic nitrogen concentrations, means that the risk remains for severe cyanobacterial blooms.

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 bioavailable 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 may benefit, using 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.

The highest DIP concentrations are 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.

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.

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.

DIN concentrations remain below the 1993 - 2002 average throughout the Baltic. Part of this is due to better use of nitrogen-based fertilizers and is part of a longer term trend.

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 Baltic Proper. Unfortunately it is too early to see the eastward extent of the DIP pool. The DIP is exported from the Baltic through the Sound, giving higher concentrations even in the Baltic current in the southern Kattegat.

	Click image to enlarge!

Figure 4. Difference between winter 2004-5 nutrient concentrations (DIN: left; DIP: centre; Silicate: right), and the 1993-2002 means.

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 western parts of the Baltic Proper, particularly in the Arkona and Bornholm Basins. The picture is more complex in the Gulf of Bothnia. Lower rainfall along the Swedish side of the Bothnian Sea led to reduced run off, and so lower silicate levels. Increased rainfall in northern Norrland gave increased run-off and accounts for the higher silicate levels in the Bothnian Bay.

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)

For reference purposes, please cite this indicator fact sheet as follows:

[Author’s name(s)], [Year]. [Indicator Fact Sheet title]. HELCOM Indicator Fact Sheets 2005. Online. [Date Viewed], http://www.helcom.fi/environment2/ifs/en_GB/cover/. 

 Last updated 25.11.2005.