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No. 206: Streams 2015

Thodsen, H., Windolf, J., Rasmussen, J., Bøgestrand, J., Larsen, S.E., Tornbjerg, H., Ovesen, N.B., Kjeldgaard, A. & Wiberg-Larsen, P. 2016. Vandløb 2015. NOVANA. Aarhus Universitet, DCE – Nationalt Center for Miljø og Energi, 68 s. - Videnskabelig rapport fra DCE - Nationalt Center for Miljø og Energi nr. 206. http://dce2.au.dk/pub/SR206.pdf




This year’s report treats the two topics “ecological state of selected streams” and “nitrogen and phosphorus runoff to Danish coastal waters”. All data presented are collected within the framework of the National Monitoring Program for the Aquatic Environment and Nature (NOVANA).


Ecological state


The ecological state in streams is described using different biological quality elements: macroinvertebrates, fish and macrophytes. The Danish Stream Fauna Index (DVFI) – a biotic macroinvertebrate index – has been in use for more than two decades, whereas the indices for fish (DFFVa and DFFVø) and macrophytes (DVPI) have only been employed for a few years. Thus, assessment of the ecological state for a longer period is only permitted using DVFI, which covers, however, only a limited network of 250 stream sampling sites. At these sites, ecological state has significantly improved since 1994. Thus, the percentage of sites with faunal classes (the categorical index values of DVFI, seven in all) 1, 2 and 3 (representing “bad” to “poor” ecological state) decreased from 22-26% in the period 1994-1998 to 5-11% in 2010-2015. The percentage of sites with faunal class 4 (“moderate” status) declined from 45-58% to 27-37%. Accordingly, the number of sites with faunal classes 5, 6 or 7 (i.e. “good” to “high” ecological status) has increased from 19% in 1994 to 56-65% in 2011-2015. Especially the percentages of faunal classes 6 and 7 have increased markedly. Based on the two fish indices, the proportion of sites with at least “good” ecological status was 15-38% during the period 2004-2015, and 59-71% of these exhibited at least “good” ecological status according to the DVPI as well during the same period.


Generally, the ecological state was poorer in the small watercourses than in the large watercourses. Based on DVFI and DFFV, ecological quality increases with increasing watercourse size. Also water chemical parameters (nitrogen and phosphorous compounds, pesticides and a few metals such as zinc) occur in the highest concentrations in small watercourses. The physical state of watercourses as determined form the Danish Physical Index (DFI) did not differ significantly between stream size categories, however. DFI has been stable during the period 2004-2015 in which 40-41% of the sites had minimum “good” physical status and 24-25% “bad” or “poor” status. There was no significant change in the calculated values of DVPI or DFFV during the period 2004-2015.


The primary reason for the significant improvement in DVFI is more efficient urban wastewater treatment and reduced pollution with organic matter from fish farms. There is evidence that poor physical conditions, due to canalisation and intensive management (weed cutting and dredging), are the main environmental problem in Danish watercourses today. This report documents that physical conditions have not improved at the study sites during the last ten years. However, improvements in water quality (or physical conditions) are not always immediately reflected in increased faunal class values and thus improved ecological state. Species indicative of “good” or “high” status often need considerable time to colonise the stream reaches where the habitats have been improved, especially if they have to disperse over land between stream networks. The physical state, elevated nutrient levels and pollution with xenobiotic substances and metals are expectedly among the reasons for the lack of improvement of fish and plant indices.


Nitrogen and phosphorus


The concentrations of nitrogen and phosphorus in streams have markedly declined since 1989. The main reason is reduced leaching from cropped areas, resulting in an average reduction of approximately 43% for nitrogen due to several general regulations of farming practices (e.g. fertilization norms, seasonal regulation of manure application, catch crops), whereas an approximate 40 % reduction of phosphorus is due to improved treatment of domestic and industrial wastewater. However, the concentrations of nitrogen and phosphorus in the streams are still, respectively, approximately four and two times higher than in unimpacted (reference) streams.


Similar reductions of total nitrogen and phosphorus loading from land to Danish coastal waters have been calculated for the period 1990-2015. The reductions are, in fact, even higher, i.e. 44 and 61% percent for nitrogen and phosphorus, respectively, if calculated as discharge-weighted mean concentrations to take interannual climatic differences into account. Considering the diffuse runoff of nitrogen alone, including leaching from cropped as well as uncropped areas and the contribution of wastewater from scattered dwellings, the reduction is approximately 36%.


In 2015, total nutrient loading from land was estimated to about 78,000 tons nitrogen and 3,100 tons phosphorus. In most months, these amounts were significantly lower than the average for the period 1990-2014. However, loading was high in January, November and December. This should be seen in the context that the total water runoff was 31% higher in 2015 than the yearly average for the period 1990-2014 and that 2015 is the second wettest year since recording started 1874.


As it is impossible to measure all water transported to the sea, there is some statistical uncertainty regarding the estimated transport values as these must be based on measurements of discharge and nutrient concentrations combined with model estimates for ungauged catchments.