THE FAUNA IN THE UPPER STONY LITTORAL OF DANISH LAKES - MACROINVERTEBRATES AS TROPHIC INDICATORS

1. The macroinvertebrate fauna living on stones in the exposed stony l ittorals of thirty-nine Danish lakes were examined by multivariate num erical methods. The data were derived from 125 semi-quantitative sampl es and a species list of 126 taxa. The mean number of individuals per sample was 960, and among the most common taxa were Asellus aquaticus, Gammarus, Oulimnius, Tinodes, Cricotopus and Dicrotendipes. 2. The to tal number of species and fourteen individual taxa were positively cor related to mean depth of the lakes and eleven taxa were correlated to the total phosphorus concentration. The Shannon diversity was negative ly correlated to the chlorophyll a concentration ([Chl a]) 3. Communit y patterns were examined by detrended correspondence analysis (DCA), a nd the relationship between species data and selected environmental va riables was analysed by canonical correspondence analysis (CCA). Mean lake depth was found to be the strongest environmental variable in exp laining the species data. The [Chl a] and Secchi depth also explained significant variation in the distribution of the stony littoral invert ebrates. Wind fetch and relative exposure did not explain any variatio n in the faunal composition among sites. 4. The abilities of the macro invertebrates to predict the lake trophic state, expressed as log ([Ch l a]), were explored by means of weighted averaging (WA) regression an d calibration. Two tolerance-weighted WA models using inverse and clas sical regression for deshrinking are presented. The models were assess ed by the root mean square error (RMSE) of prediction, using bootstrap ping as cross validation, and by the correlation between observed and inferred log ([Chl a]) The model using inverse deshrinking had a RMSEb oot = 0.41 and r(2) = 0.63. By using classical regression, the predict ability in the ends of the gradient was improved but the RMSE increase d: RMSEboot = 0.46. 5. Although the factors determining faunal distrib ution patterns in the Danish lowland lakes were highly multivariate an d difficult to disentangle, it seems reasonable to use the WA estimate d species optima and tolerances to [Chl a] ina bio-assessment model.