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No. 223: A new Danish macroinvertebrate index for lakes

Wiberg-Larsen, P. & Rasmussen, J.J. 2016. A new Danish macroinvertebrate index for lakes - a method to assess ecological quality. Aarhus University, DCE – Danish Centre for Environment and Energy, 38 pp. Scientific Report from DCE – Danish Centre for Environment and Energy No. 223. http://dce2.au.dk/pub/SR223.pdf 

 

Summary

According to EU’s Waterframe Directive, Denmark must provide a national index based benthic macroinvertebrates, as well as for other so-called biological quality elements (phytoplankton, phytobenthos & macrophytes and fish), to assess ecological quality in its lakes.

This report presents such a new macroinvertebrate index based on an already existing Lithuanian index (LLMI) that has been intercalibrated together with national indices from other countries within the Central-Baltic Intercalibration Group (CB-GIG) to which also Denmark belongs.

The Danish Littoral Macroinvertebrate Index (DLMI) is multimetric being composed of four different components being calculated as:

DLMI = (ASPT + H1 + EPTCBO + %COP)/4,

where ASPT is an index developed in the U.K. to assess ecological quality of streams, H1 is defined as exp(Shannon-Wiener Index), EPTCBO is the number of taxa of Ephemeroptera, Plecoptera, Trichoptera, Coleoptera, Bivalvia and Odonata, and %COP is the relative abundance of Coleoptera, Odonata and Plecoptera.

DLMI is calculated based on a composite kick-sample for 2 minutes on firm substrates (sand, gravel, stones) in the littoral zone using a standard net in the littoral zone.

The index was tested on a total 280 samples from 55 Danish lakes of which two lakes complied with generally accepted criteria for reference conditions. The data set included both shallow and deep lakes, however all being alkaline and basically with “clear” (not humic) water.  Thus, the index is used for both these lake types representing about half of all Danish lakes.

The index correlated well (r2 = 0.33) with a combination of both “eutrophication”, assessed using a Principal Components Analysis on a suite of physical, chemical and biological parameters and expressed by primary and best explaining axis (PCA1) scores, and the “anthropogenic pressure” in the littoral and riparian zone (assessed from a large suite of different elements and activities) where macroinvertebrate sample were taken. Further, the index correlated significantly with eutrophication alone (but not for anthropogenic pressure alone).

Moreover, DLMI correlated well (r2 = 0.48) with the macroinvertebrate common metric (index) to which all national indices within the CB-GIG were benchmarked in order to intercalibrate the national boundaries for High/Good and Good/Moderate ecological quality.

After a preliminary designation of DLMI boundary values (expressed as a Ecological Quality Ratio on a scale from 0 to 1) for High/Good, Good/Moderate, Moderate/Poor and Poor/Bad, respectively, these boundaries were intercalibrated using exactly the same procedure as already carried out for the majority of countries within the CB-GIG (see Böhmer et al. 2014). The intercalibration showed that boundaries H/G and G/M were significantly biased (0.311 and 1.091, respectively), well above 0.25, meaning that the boundaries were stricter than required. Below is shown the adjusted boundaries, based on a mean bias of 0.25 for both boundaries, and to be comparable with adjustments made by Lithuania for their index:

Boundary

H/G

G/M

M/P

P/B

DLMIEQR

0.71

0.41

0.30

0.12

 

Boundaries H/G and G/M based on a mean bias of 0 and -0.25 is also presented in the report.

Taking into account that DLMI primarily reflects the pressure of eutrophication, it is estimated that obtaining at least good ecological in shallow or deep alkaline and clear-water Danish lakes require that the total-phosphorus yearly mean  should not exceed 0.049  mg/L. This is within the range previously (yearly means: 0.045 – 0.051 mg/L) recommended by Danish EPA in regard to the other biological quality elements. 

Thus in conclusion, the report recommends that DLMI should be used as an official, national assessment method in lakes.