Performance of profundal macroinvertebrate assessment in boreal lakes depends on lake depth

Profundal macroinvertebrates (PMI) are commonly used in assessment and monitoring of lakes. Traditional PMI community typologies and indices have been developed and tested for large and deep lakes with well-defined pelagic and profundal zones, whereas small lakes have received less attention; despite their large number. We evaluated the performance of PMI assemblage assessments in boreal lakes with data from 255 Finnish lake basins, which were divided into four lake mean depth categories (shallow, intermediate, deep and very deep). We first described natural community variation with DCA ordination and then, within each depth category using ANO-SIM, compared community composition between minimally disturbed reference (REF) basins and basins impacted mainly by nutrient enrichment (IMP). We assessed the status of PMI assemblages using three metrics (Benthic Quality Index, Percent Model Affinity and Shannon diversity) that aimed to meet the normative criteria set by the EU Water Framework Directive. Judging by conventional criteria, there was a consistent improvement in bioassessment performance of PMI with lake depth. Assemblages in shallow REF and IMP sites were similar and measures of ecological status showed neither difference between REF and IMP sites nor response to nutrient pollution. In deep and very deep IMP lakes the communities differed significantly from those in REF lakes and community and metric variation was more strongly related to phosphorus concentration. Our results suggest that PMI assemblages of deeper boreal lakes respond predictably to anthropogenic nutrient enrichment, whereas assemblages of shallow lakes are either 'naturally eutrophic' and thus resistant to change or unpredictably variable making it difficult to detect any impacts of anthropogenic nutrient enrichment on communities. Future studies are thus needed to evaluate the performance of littoral macroinvertebrate assemblages for shallow lake bioassessment.