EFFECTS OF EXPERIMENTAL AND CULTURAL LAKE ACIDIFICATION ON LITTORAL BENTHIC MACROINVERTEBRATE ASSEMBLAGES

A simple method for the assessment of littoral benthic macroinvertebra te (BMI) assemblages in lakes was developed at the Experimental Lakes Area (ELA) in northwestern Ontario and applied to 64 lakes in central Ontario. The presence (1) or absence (0) of BMI taxa was established a t five sites on each lake, using a kick-and-sweep net technique. Summi ng presence/absence scores across sites ranked the importance of each taxon in each lake on a scale of 0 to 5. Relationships between the BMI assemblages were assessed using nonmetric multidimensional scaling (N MDS), based upon the Kendall's tau correlation matrix. Correlations be tween NMDS dimension 1 and 2 scores and environmental variables identi fied factors that may structure BMI assemblages. The BMI assemblages o f three experimentally acidified and seven reference lakes at the ELA were correlated strongly with lake pH. The BMI assemblage structure of central Ontario lakes was predicted by lake area and sensitivity to a cidification (NMDS dimension 1) and by lake elevation (NMDS dimension 2). The BMI assemblages of small or Ca-poor and acidic lakes include f ewer oligochaetes, mayflies, and Cryptochironomus and Stictochironomus (chironomids) and more Odonata, Trichoptera, Chironomus, Conchapelopi a, Microtendipes, and Procladius (chironomids), and Crangonyx (Amphipo da) than other central Ontario lakes.