Summary

 

This report contains an analysis of the environmental state of Limfjorden from 1985 through 2003. The environmental state is described from concentrations of nitrogen (total nitrogen and inorganic nitrogen during winter and summer), phosphorus (total phosphorus and inorganic phosphorus during winter and summer), transparency of the water (Secchi depth early summer and late summer), chlorophyll concentration (early summer and late summer), distribution of oxygen depletion and the depth limit of sea grass. Conditions are described for seven main compartments of the fjord and Halkær Bredning. The year-to-year variation in the environmental conditions has been analysed in relation to a number of external factors with emphasis on the significance of nutrient loading from nitrogen and phosphorous. In addition climate factors such as air temperature, wind and irradiance are included. The salinity of the fjord has also been included as an indicator of the degree of water exchange with the North Sea. All relations have been analysed by means of linear models between each parameter that describes the environmental state and all external parameters. Time lag back to the year before has been analysed systematically.

The background for the report is a need to quantify relations between supplies of nutrients and the environmental state of the fjord based on existing data and to utilise these relations to predict the environmental state at reduced loads. The established models have therefore been applied in calculations of the state of the fjord in different scenarios for reductions in loadings.

The results show that the environment of the fjord is slowly recovering because of cuts in nitrogen and phosphorus loads. The phosphorus supply has been reduced with 869 tonnes/year or approximately 69% from 1985 to 1992 and has hereafter been constant when year-to-year variations in freshwater runoff are being taken into account. The nitrogen supplies have been reduced with about 4,000 tonnes/year, approximately 20%, when normalising to runoff. This reduction began in 1985 and has continued to 2003. A number of years with elevated runoff from 1999 to 2002 has resulted in the actual cut in loadings being less and therefore the positive effects on the environmental state of the fjord have been delayed.

The analyses show that concentrations of both nitrogen and phosphorus are decreasing in the fjord, and relationships between nutrient supplies and concentrations in the fjord are documented. The largest effect is seen for concentrations of inorganic nutrients. The concentrations of total nitrogen (TP) and total phosphorus (TP) have decreased less and this is probably due to the presence of large pools of nutrients in the sediment. These nutrient pools cause a significant time lag from the reduction in loading until the full effect is seen on the concentrations in the fjord. The analyses show that this time lag can be estimated to between 4 and 8 years.

Along with the decreased concentrations of nitrogen and phosphorus the water transparency has increased and the amount of planktonic algae has decreased. The Secchi depth has increased 8% on average for the seven main compartments of the fjord, while the chlorophyll concentration has decreased 18%. In addition, direct relationships were found between these improvements and the cuts in nutrient loadings.

Two environmental parameters were found not to have improved during the study period: distribution of oxygen depletion and the depth limit of sea grass. On the contrary, the distribution of sea grass has decreased, and the area that is affected by oxygen depletion has increased through the period.

The analyses show, that the key factor determining the extent of oxygen depletion is the weather during the summer months July through September, where high temperatures and irradiance coupled with low wind cause severe oxygen depletion. In this way, the temperature alone can explain 62% of the year-to-year variation in the extent of the area that is affected by concentrations below 4 mg O2 per litre. The difference between the observed area with oxygen depletion and the expected area, when related to temperature, can be interpreted as an expression of oxygen consumption of the sediment and this shows an increasing trend from 1989 to 1997, where after it decreases. This indicates that the area of oxygen depletion is decreasing, and that the latest years with severe oxygen depletion is mainly due to the very warm summers. However, there are other elements in the analyses indicating that the area of oxygen depletion can be expected to initially increase during reductions in loadings, and that a consistent reduction of the extent of oxygen depletion cannot be expected until after several years, when the sediment pools of organic matter have been reduced significantly. Because of the uncertainty regarding the interpretation of these analyses, no scenario calculations have been made for oxygen depletion.

The depth limit of sea grass has decreased about 50% during the period, and now the depth limit is only 2 m in most parts of the fjord. The decline has come evenly throughout the period and in all areas, however, there is a slight improvement from 2001 to 2003. Low salini¬ty and wind and high irradiance are factors that have a profound negative effect on the extension of sea grass. The determining factor for sea grass is thought to be the extent of oxygen depletion in the fjord, and therefore the reduction of the depth limit must be seen in relation to the presence of oxygen depletion. As for oxygen, no scenarios for the depth limit of sea grass were made.

The climate has a significant influence on all parameters, and in many cases there is a time lag so that weather conditions the year before are determining the state of the fjord the following year. The importance of the weather varies between the parameters, which is seen in the specific models. The most profound effect is that high temperatures during summer and fall the year before cause lower concentrations of nutrients and generally improved conditions in the fjord the following year. Another important effect is, as mentioned previously, that high temperatures and irradiance and low wind during the summer cause more oxygen depletion and affect the sea grass negatively. There is also the effect of the coupling between precipitation and the depletion of nutrients. This effect is not included in the models, because these are based on the nutrient loadings directly, but in Chapter 6 a close relationship between runoff and supply of both nitrogen and phosphorus to the fjord is shown.

The environmental state of the fjord, and the relation to external parameters, varies between the different areas of the fjord. The most problematic conditions are found in the southern parts, Skive Fjord and Lovns Bredning, where there is extensive oxygen depletion and the depth limit of sea grass is the lowest. The conditions in these areas also deviate from the rest of the fjord by having very high concentrations of phosphorus and chlorophyll in the water column in the late summer. This is because phosphorous, in particular, is released from the sediment to the water column during anoxia. One target in these areas should therefore be improvements of conditions, so that oxygen depletion only occurs rarely and that the seasonal distribution of phosphorus and chlorophyll will follow the pattern seen in the remaining part of the fjord. It will probably affect the state of the whole fjord in a positive way, if the sediment in these areas ceases to act as a nutrient source because of extensive oxygen depletion. The conditions in Nissum Bredning and Kås Bredning are less affected by external supplies than the rest of the fjord due to the proximity of the North Sea.

The scenario calculations show that the environmental state of the fjord will be improved by a reduction of nutrient loadings. The improvements are limited at a reduction from the present values of approximately 400 tonnes phosphorus and 18,300 tonnes nitrogen per year to 360 tonnes/year and 15,000/year respectively for phosphorus and nitrogen (Scenario 3). Also the most extensive scenario (Scenario 1) with supplies of 360 tonnes/year and 12,000 tonnes/year respectively, is thought to be insufficient to improve the ecological conditions of the fjord. The report includes a scenario based on an improvement of the Secchi depth with 25% in relation to the mean situation from 1985 to 2003, because 25% seems to be a minimum to improve the ecological state of the fjord significantly. These calculations show that the loadings must be reduced to about 300 tonnes phosphorus and 9,300 tonnes nitrogen per year. As a further criterion to evaluate the state of the fjord, summer concentrations of inorganic nitrogen have been used. Here calculations show the nitrogen loading must be reduced to 10,500 tonnes/year in order to bring concentrations below 2,0 µmol N per litre in the areas Nissum, Kås, Løgstør and Thisted Bredning. A concentration of 2,0 µmol N per litre is normally considered to be limiting for algae growth. In the other areas, the calculations show that the concentrations will be higher, between 7 and 10 µmol N per litre, at this level of loading. The situation today is that these areas are so affected by the reoccurrence of oxygen depletion events that it is difficult to predict how they will respond to a reduced nutrient load, when the extent of oxygen depletion is reduced significantly.

In addition to the above-mentioned scenarios, which are calculated for years with a ‘normal climate’, scenario calculations for Løgstør Bredning with variable climate forcing have also been made. The effects are visualized from reduced loadings in years with a particular favourable or unfavourable weather condition. Similar calculations can be made for the other areas of the fjord from data that are included in this report.

Full report in pdf-format (3,700 kB).