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Nr. 373: Revised Danish macroinvertebrate index for lakes

Wiberg-Larsen, P. & Rasmussen, J.J. 2020. Revised Danish macroinvertebrate index for lakes - a method to assess ecological quality. Aarhus University, DCE – Danish Centre for Environment and Energy, 42 pp. Scientific Report No. 373. http://dce2.au.dk/pub/SR373.pdf

Summary

This report is a revision of a previous report (Wiberg-Larsen & Rasmussen 2017) that was based on a software programme that has subsequently been found to erroneously subtract parts of the data base layer used to calculate the "metrics" included in DLMI. These shortcomings have now been rectified, new so-called anchor points used to calculate EQR values for each of the metrics provided, as are the suggestion of new boundaries between the five status classes. However, much of the content in the present report are similar to that in Wiberg-Larsen & Rasmussen (2017).

According to EU’s Waterframe Directive, Denmark must provide a national index based on 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 a revised 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 (Hill’s 1) 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 national lake types (9 & 10) representing about half of all Danish lakes.

The index correlated well (r2 = 0.45) 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 each of the two pressures, although strongest with eutrophication. 

Moreover, DLMI correlated strongly (r2 = 0.85) 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 (0.80), Good/Moderate (0.60), Moderate/Poor (0.40) and Poor/Bad (0.20), 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, the latter exceeding 0.25, meaning that the boundary would be less strict than required. We therefore recommend the following adjusted boundaries, based on a mean bias < ±0.25 as requested by the EU (and in case close to 0) for both boundaries:

Boundary

H/G

G/M

M/P

P/B

DLMIEQR

0.77

0.55

0.36

0.18

Taking into account that DLMI primarily reflects the pressure of eutrophication, it is estimated that obtaining at least good ecological status in shallow or deep alkaline and clear-water Danish lakes require that the total-phosphorus yearly mean should not exceed 0.045 mgL-1 (yearly mean). According to internal guidelines from Danish EPA, good ecological status for chlorophyll-a, fytoplankton and macrophytes may be achieved if summer mean total-phosphors does not exceed 0.053 and 0.031 mgL-1 for shallow and deep lakes, respectively. A conversion of the yearly mean of 0.045 mgL-1 for DLMI to summer mean, result in 0.051 and 0.048 mgL-1 for shallow and deep lakes, respectively. For deep lakes, this is above the Danish EPA boundary to obtain good ecological status.    

The report recommends that DLMI is suitable as an official, national assessment method in alkaline lakes (national lake types 9 and 10). However, for several reasons (most of all the weak relationship between DLMI and total-phosphorus), it is also recommended that is more advisable to focus efforts to achieve at least good ecological status based on the boundaries established for chlorophyll-a, phytoplankton and macrophytes, rather than on a boundary estimated for macroinvertebrates.