Assessing lake trophic status using spring-emerging chironomid pupal exuviae

The objective of this study was to test the applicability of the Chironomid Pupal Exuvial Technique (CPET), and particularly for spring-emerging chironomids, in assessing trophic status of boreal lakes. We estimated chironomid taxa optima along a trophic gradient (epilimnion TP) and constructed a Chironomidae-TP inference model (using Locally-Weighted Weighted Averaging), which can be used to assess trophic status of boreal lakes. We then tested for bias in CPET assessment due to temporally varying emergence patterns of chironomids and compared classification efficiency of chironomids with species-and genus-level data sets. Time of sampling was shown to have a major effect on the CPET assessment, which was largely due to the shift in emergence from early-emerging cold-stenothermal taxa to late spring/early summer-emerging chironomids. For monitoring and assessment of boreal lakes, two spring samples are recommended, one during the early phase of spring emergence and another sample later in spring or early summer. Species-level data more accurately (81 % correct) predicted correct trophic group for the samples than genus-level data (64 % correct).