Striving for consistency in a national lake assessment: defining reference status and littoral macroinvertebrate condition in lakes across the conterminous United States.

Benthic macroinvertebrates are widely used for assessing lotic ecosystems, however, their use in assessing lake condition has been more limited-especially at large, continental extents. We used data collected during the U.S. EPA's National Lake Assessment between 2007-2022 to develop and validate a national macroinvertebrate multi-metric index (MMI) of lake condition across the conterminous U.S. As part of that process, we identified least-disturbed ecoregional reference lakes by filtering all sampled lakes by using specific physical, chemical, and disturbance variables to remove disturbed lakes. To account for natural variability, different criteria values were used for each of the nine national ecoregions. This allowed for a regionally explicit and reproducible definitions of lake reference condition for current and future analyses. Because of insufficient reference lake numbers in some of the nine ecoregions, macroinvertebrate MMI development was done independently for each of five aggregate national ecoregions. All 126 candidate macroinvertebrate metrics were screened for reproducibility, responsiveness, and redundancy to identify the best metric in each of six group types: composition, diversity, feeding group, habit, richness, and pollution tolerance for each ecoregion. The six chosen metrics were summed to calculate the MMI. Condition benchmarks (good/fair/poor) for assessing biological condition were defined for each ecoregion based on reference lake MMI percentiles. Using these five MMIs with the 2022 survey data, an estimated 44% of the lakes were in good condition, whereas 27% were in poor condition. Our MMIs offer managers valuable tools for assessing lakes at large ecoregional and continental extents.