Aarhus University Seal / Aarhus Universitets segl

No. 353: Streams 2018. NOVANA

Thodsen, H., Tornbjerg, H., Rasmussen, J.J., Bøgestrand, J., Larsen, S.E., Ovesen, N.B., Blicher-Mathiesen, G., Kjeldgaard, A. & Windolf, J. 2019. Vandløb 2018. NOVANA. Undertitel. Aarhus Universitet, DCE – Nationalt Center for Miljø og Energi, 72 s. - Videnskabelig rapport nr. 353.  http://dce2.au.dk/pub/SR353.pdf



This year’s report presents topics on the nitrogen and phosphorus runoff to Danish coastal waters. All field data were collected in accordance with the National Monitoring Program for the Aquatic Environment and Nature (NOVANA). 

Nitrogen and phosphorus load to the sea

The concentrations of nitrogen and phosphorus in streams have been markedly reduced since 1989. The main reason is reduced leaching from cropped areas, resulting in a mean reduction of approximately 43% for nitrogen, whereas an approximate 40% reduction in phosphorus is found, mainly due to improved treatment of residential- and industrial wastewater. However, concentrations of nitrogen and phosphorus in the streams are still approximately 4 and 2 times higher, respectively, than in un-impacted streams.

Reduced loads of nitrogen and phosphorus from land to Danish coastal waters are calculated for the period 1990-2018. The reductions are 51% and 72% for nitrogen and phosphorus, respectively, if calculated as discharge weighted mean annual concentrations. Considering the diffuse runoff of nitrogen alone, including leaching from cropped as well as uncropped areas and contribution of wastewater from scattered dwellings, the reduction is approximately 38%.

In 2018, the total load from land was estimated to about 50 000 tons of nitrogen and 1600 tons of phosphorus. For most months, these amounts were significantly lower than the average for the period 1990-2018. The normalised diffuse nitrogen load was calculated to 50 000 ton N compared to 52 000 ton N in 2017. The normalised mean diffuse load for the presiding five years (2013 – 2017) is 48 000 ton N, ranging between 47 000 and 52 000 ton N. The annual runoff of water was 292 mm compared to 323 mm as the average from 1990-2017.

Since it is impossible to measure the total load of water and nutrients to the sea, the estimated loads are accompanied by uncertainty, as these loads need to be based on both measurements of discharge and nutrient concentrations combined with model estimates for ungauged catchments. 

In this year’s report, presented time series of total nitrogen loads have a dark background covering the period 2007-2014, as total nitrogen analyses from this period are under investigation for being analysed using a wrong method, supposedly underestimating the concentration.

Ecological status

Ecological quality in streams is assessed based on a suite of ecological quality elements: macroinvertebrates, fish, and aquatic plants. The Danish Stream Fauna Index (DSFI) is used to assess the ecological status based on macroinvertebrate community composition, while the Danish Stream Plant Index (DSPI) and Danish Fish Index for Streams (DFIS) assess the ecological status based on community composition of aquatic plants and fish, respectively. DSFI has been used for more than two decades, while DSPI and DFIS have been used during the previous two monitoring cycles. The temporal development of ecological status in Danish streams can only be assessed using DSFI on a limited part of the NOVANA stream stations (250) that are sampled every second year (every year in previous monitoring cycles). This report only contains DSFI measurements on these 250 stream stations. Overall, the ecological status steadily increased from 1994 till approximately 2012, especially the relative amount of faunal classes 6 and 7 has increased in this period. However, the positive development in ecological status now appears stagnant, as the relative distribution of faunal classes among stream stations has been stable during the past 5-7 years.