No. 165: Streams 2014

Wiberg-Larsen, P., Windolf, J., Bøgestrand, J., Larsen, S.E., Tornbjerg, H., Ovesen, N.B., Nielsen, A., Kronvang, B., & Kjeldgaard, A. 2015. Vandløb 2014. NOVANA. Aarhus Universitet, DCE - Nationalt Center for Miljø og Energi, 54 s. - Videnskabelig rapport fra DCE - Nationalt Center for Miljø og Energi nr. 165. dce2.au.dk/pub/SR165.pdf

Summary

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

Ecological state

The ecological state in streams is described using different so-called biological quality elements: benthic macroinvertebrates, fish and macrophytes. Whereas the Danish Stream Fauna Index (DSFI) –  a biotic macroinvertebrate index – has been used for more than two decades, the index for fish and macrophytes has only been used for a couple of years. Thus, it is only possible to assess ecological state using DSFI for a longer period and only within a limited network including 250 stream sampling sites.

Focusing on this network of sites, the ecological state has improved significantly since 1994. Thus, the percentage of sites with faunal classes (the categorical index values of DSFI, 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 the period 2010-2014, whereas the percentage of sites with faunal class 4 (moderate state) declined from 45-58 to 26-37. Accordingly, the part of sites with faunal classes 5, 6 or 7 (i.e. good to high ecological status) increased from 19% in 1994 to nearly 65% in 2014. Especially the percentage of faunal classes 6 or 7 increased markedly.

The primary reason for this significant progress is an improvement in the efficiency of urban waste water treatment plants and a reduction of organic pollution from fish farms (located in Jutland). However, it is not possible to improve the ecological state just by reducing discharges of organic pollution (measured as BOD5) if the physical (i.e. hydro-morphological) condition of the streams is not able to support macroinvertebrates indicative of good and high ecological quality. There is evidence that poor physical conditions, due to canalization and intensive management (weed cutting and dredging), are the main reason for streams not fulfilling, at least, the criteria for good ecological status in the Water Framework Directive. The report documents that the physical conditions at the studied sites have not improved during the last ten years. Improvements in water quality (or physical conditions) are, however, not immediately reflected in increased faunal class values and, thus, enhanced ecological status. Species indicative of good or high status often need considerable time to colonise the stream reaches with improved habitats, especially if they have to disperse overland between stream systems.

The network of 250 stream sites (see above) is unfortunately not representative of Danish streams in general. Thus, small streams, constituting about 2/3 of all Danish streams, are significantly under-represented in the network. Further, the small streams generally exhibit a poorer ecological condition than the larger ones due to a relatively higher human impact. However, the studied network does reflect the general trend in Danish streams that ecological status has improved considerably during the last twenty years.

Nitrogen and phosphorus

The levels of nitrogen and phosphorus in streams have declined markedly since 1989. The main reason is reduced leaching from cropped areas resulting in a mean reduction of approximately 45% for nitrogen due to several general regulations of farming practices (e.g. fertilization norms, seasonal regulation of fertilization, after-crops), whereas an approximately 31% reduction in phosphorus is due to improved waste water treatment in the cities and industry. However, the concentrations of nitrogen and phosphorus in the streams are still 3-4 and 2-3 times, respectively, as high as in un-impacted (reference) streams.

Similar reductions in the total runoff of nitrogen and phosphorus from land to Danish coastal waters have been estimated for the period 1990-2014. The reductions are, in fact, even higher, namely 50 and 63% for nitrogen and phosphorus, respectively, if calculated as discharge-weighted mean concentrations in order to take into consideration climatic differences between years. For the so-called diffuse runoff of nitrogen alone, including leaching from cropped as well as uncropped areas and the contribution of wastewater from scattered dwellings outside the cities, the reduction is approximately 43%.

In 2014, the total runoff from land was estimated to about 63,000 tons of nitrogen and 2,600 tons of phosphorus, these amounts being significantly higher than the average for the period 1990-2013 in nearly all months. This should be seen in the context that the total runoff of water was 7% higher in 2014 than the yearly average during 1990-2013.

As it is impossible to measure ‘each and every drop of water’ transported to the sea, there is, of course, a certain statistical uncertainty of the estimated transports as these have to be based on both measurements of discharge and concentrations combined with models for catchments without measurements.