HELSINKI COMMISSION HELCOM MONAS INDICATOR WORKSHOP 1/2004 Monitoring and Assessment Group First Meeting Helsinki, Finland, May 2004

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1 HELSINKI COMMISSION HELCOM MONAS INDICATOR WORKSHOP 1/2004 Monitoring and Assessment Group First Meeting Helsinki, Finland, May 2004 Agenda Item 2 HELCOM indicators Document code: 2/16 Date: Submitted by: Philip Axe & Pia Andersson, SMHI, Sweden PROPOSAL FOR HELCOM INDICATOR FACT SHEET 2004: SPATIAL DISTRIBUTION OF NUTRIENTS The Meeting is invited to consider the attached proposal for HELCOM Indicator Fact Sheet Note by Secretariat: FOR REASONS OF ECONOMY, THE DELEGATES ARE KINDLY REQUESTED TO BRING THEIR OWN COPIES OF THE DOCUMENTS TO THE MEETING Page 1 of 5

2 1 Title of indicator: Spatial Distribution of Nutrients Figure 1. Mean winter surface DIN (left) and standard deviation (right) based on each year s gridded winter surface observations from inclusive. Winter was taken to be December January in the Kattegat, and December February in the remainder of the Baltic. Surface refers to the upper 0 10 m. Units are micromoles/litre. While the high concentrations in the inner Gulf of Finland and along the Polish and German coasts seem real, the high values along the Kaliningrad Lithuania coast, and in the Gulf of Riga look like gridding artefacts. Nearshore concentrations around the Swedish coast may be underestimated due to a lack of data. This will be checked Figure 2. Mean winter surface DIP (left) and standard deviation (right) based on each year s gridded winter surface observations from Units are micromoles/litre. It is likely that the high values along the Kaliningrad Lithuania coast, and in the Gulf of Riga are gridding artefacts. Page 2 of 5

3 Figure 3. Difference between winter nutrient concentrations (DIN: left; DIP: right), and the means. Of note are the high DIN values along the southeastern Swedish coast, and the high DIP values in the central Baltic Proper, and Århus Bight. The high coastal values may be misleading, and could be due to a lack of coastal data in the calculated means. This will be corrected in future versions. The high offshore DIP values are in an area well covered with observations so can be considered reliable. 2 Key message: Concentrations of DIN in the offshore Winter Nutrient Pool were lower than the average for Concentrations of DIP in the Central Baltic Proper are higher. This decrease in the DIN:DIP ratio favours blooms of nitrogen-fixing plankton. 3 Results and assessments: 3.1 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 prevented 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 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. Page 3 of 5

4 Mapping the ratio of winter DIN:DIP may serve as a warning for areas where cyanobacteria blooms are likely. 3.2 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 Future versions of this report will also describe the spatial distribution of nutrient concentrations in the bottom water and also space-temporal variations in the distribution of hydrochemical variables below the halocline (Helcom COMBINE Manual Annex C). 3.3 Assessment Concentrations of DIN are highest in the coastal waters of the Belt Sea, along the German and Polish coasts, and in the inner Gulf of Finland. This suggests that the major source of DIN is land run-off. The high variability along the Danish to Polish coasts indicates the separation between coastal and offshore water. Highest DIP concentrations are also found in the Belt Sea and along the southern Baltic coast. High variability was found along the Swedish west coast, South Skåne, and also around Gotland. This is most likely due to intermittent periods with high phosphorus concentration, caused by wind-driven upwelling of the phosphorus rich bottom water. 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 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 dataset. Swedish inshore measurements were not included when calculating the 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 References Helcom COMBINE Manual (Annex C), December Data: This study has made use of data provided by the following institutes: - Swedish Meteorological and Hydrological Institute (SMHI) - Umeå Marine Research Centre (UMF) Page 4 of 5

5 - Finnish Institute for Marine Research (FIMR) - International Council for the Exploitation of the Sea (ICES) - Danish Environmental Research Institute (DMU) Page 5 of 5