Summary
The marine ecosystem in the Danish estuary Limfjorden is influenced by variations in climate, hydrography, nutrient inputs and large manmade physical modifications. Marine monitoring in the years 1978-2003 has provided insight into the biological effects of nutrient load and inter-annual variations in climate. The goal of this project was to evaluate the factors that have influenced the marine ecosystem in Limfjorden in the past 100 years.
Climatic conditions that drive the hydrographic conditions in Limfjorden are described by geostrophic wind (1874-1987), observed wind (1988-2003), the NAO-index (1874-2003) and discharge from 3 rivers (1918-2003). Hydrographic conditions are described by daily observations of temperature and salinity at two bridges (Oddesund (1897-1973) and Vilsund (1941-2003)). Nitrogen input to Limfjorden was estimated from yearly values of national agricultural nitrogen surplus and the magnitude of the human population in the catchment area in the years 1900-1983. Post 1983, nitrogen input is based on observations.
The yearly average temperature varies with ±1.5° C, and has an increasing trend of 0.08° C per year. These variations are, however, small in comparison with the annual temperature range. In the beginning of the 20th century, mean annual salinity increased by 2 at the Oddesund Bridge. During this time, the estuary opening towards the North Sea was modified as a consequence of the establishment of Thyborøn Channel. Since 1925, mean annual salinity has varied by ±2.5 at the Vilsund Bridge. The salinity observations at Vilsund further document that the degree of stratification in the summer months varies between years. Stratification was in the 1940s stronger than in the following years, and only in 1996 a period of equally strong stratification was observed. Consequently, periods of strong stratification have occurred more frequently than within the past 30 years. The strength of stratification in part governs the estuary sensitivity towards oxygen depletion, and these observations indicate that periods of greater sensitivity have occurred.
The temporal evolution of the marine ecosystem in Limfjorden is described by yearly observations of bottom fauna and fish. In the years 1910-1952, the Danish Biological Station has determined species composition and biomass of bottom fauna in 10 different embayments within the estuary, and the time series have been continued by the local authorities as part of the national and regional monitoring programme in the years 1978-2003. The evolution of fish populations is based on commercial catch statistics in the years 1900-2003 and on trial fisheries conducted by the Danish Institute for Fisheries Research in the years 1980-2003.
In the years 1900-1960, the magnitude of bottom biomass as well as the fish population in Limfjorden increased. Annual values of bottom fauna biomass have been related to mean annual salinity, temperature, NAO-index and nitrogen input, and significant, positive relations were found with mean annual salinity and temperature as well as with nitrogen input. In the years 1910-1952 (corresponding to the years where bottom fauna was observed) temperature and sali¬nity increased by 5-10% of the annual mean value (0.8° C and 1.5 respectively), whereas nitrogen input doubled. The organisms found in Limfjorden are adapted to highly variable hydrographic conditions and because the changes are small, relative to the change in nutrient load, we consider nutrient load the driving factor of the increase in biomass. Fish populations were related to the same factors as bottom fauna and an additional effect of the NAO-index 1, 2 or 3 years prior to the observation was tested for the years 1900-1960, but a significant and positive relation was found only for nitrogen input.
In the years 1961-2003 the increasing trend in bottom fauna biomass and fish populations was reversed. In these years the population of bottom dwelling fish was strongly reduced and when the monitoring of bottom fauna was reinitiated in 1978, biomass had been reduced relative to the level in 1952. Around 1980, both bottom fauna biomass and fish populations were the lowest in the entire period, whereas nitrogen loads were estimated at their highest level. In this period, biomass and fish populations were also compared to annual values of salinity, temperature, NAO-index and nitrogen input. A significant and negative relationship was found for nitrogen input and bottom fauna biomass if the year 1996 was excluded from the relation. The year 1996 differs from the predominant trend because both biomass and nitrogen input were low. The low biomass was a likely consequence of low biomass the previous year and an extended period of low oxygen conditions that arose because stratification was particularly strong that year. No significant relationships were found for the remaining variables.
A significant and negative relationship was also found for nitrogen input and fish populations in the years 1900-2003. This trend may be strengthened by the general high level of the winter NAO-index in the 1970s because this has a documented effect on the recruiting success of fish in the North Sea. The local fishing pressure is unlikely to have been sufficiently large to cause the negative development. No significant relationships were found for the remaining variables.
The analyses in this study suggest that the magnitude of nitrogen input to Limfjorden is of critical importance for the development of bottom fauna and fish populations. The increase in nitrogen input in the years 1900-1960 fertilised the system and increased productivity. After 1960, the negative consequences of the nutrient input such as increased frequency of oxygen depletion events had become so large that neither bottom fauna nor fish can be sustained at previous levels. In spite of large reductions in nutrient inputs of 69% of phosphorous and 18% of nitrogen since the mid 1980s these populations are not at the levels found prior to 1960. Our results suggest a threshold value for bottom fauna and fish populations corresponding to the nutrient load level in 1950-1955. This level has been estimated to approximately 10,000 tons per year.
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