Spatial distribution of the winter nutrient pool 

	Authors: Philip Axe & Pia Andersson, SMHI

Key message

Concentrations of DIN in the 2003 – 2004 offshore Winter Nutrient Pool were lower than the average for 1990 – 2001, though higher than the 1996 - 2004 average. Concentrations of DIP in the Central Baltic Proper were higher than both the 1990 - 2001 and 1996 -2004 averages. Decreases in the DIN:DIP ratio favour blooms of nitrogen-fixing plankton, such as cyanobacteria.

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. This suggests that the major source of DIN is land run-off.

Mean winter surface DIN (left) and standard deviation (right) based on each year’s gridded winter surface observations from 1996 – 2004 inclusive. Surface refers to the upper 0 – 10 m. Units are micro-moles/litre.

Highest DIP concentrations are also found in the Belt Sea and along the southern Baltic coast.

Mean winter surface DIP (left) and standard deviation (right) based on each year’s gridded winter surface observations from 1996 – 2004. Units are micromoles/litre.

The previous winter differs from the ‘average’ picture. DIN concentrations throughout the Baltic Proper, with the exception of the Swedish southeast coast, were considerably lower than normal. This can be attributed, at least in part, to the extremely dry conditions present over the Baltic Drainage Basin during 2003. This led to reduced river run-off, and also reduced atmospheric deposition. High concentrations apparent along the Swedish southeast coast are due to the inclusion of inshore measurements in the winter 2003-4 dataset. Swedish inshore measurements were not included when calculating the 1990 – 2001 means, which has led to a probable underestimate of DIN concentration along the Swedish east coast.

Phosphorus concentrations are higher than normal throughout the Baltic Proper and Gulf of Finland – with the exception of the Bornholm and Arkona Basins. The source of the phosphorus is most likely the deep waters of the Central Baltic Proper, where years of anoxia have released quantities of phosphorus into the bottom water. Winter storms have most likely been responsible for delivering this phosphorus to the surface waters. This excess of phosphorus may support severe cyanobacteria blooms during 2004.

Difference between winter 2003-4 nutrient concentrations (DIN: left; DIP: right), and the 1996-2004 means.

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)