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Lake water clarity is an indicator of water quality, trophic status, and habitat condition. Changes in clarity impact lake ecosystems and may reflect land use changes or presence of invasive species. Quantifying temporal changes in water clarity can be challenging because clarity varies seasonally, annually, and spatially within and among lakes. We developed a hierarchical generalized additive model to quantify trends in water clarity (Secchi depth) from 1979 to 2018 for 909 Minnesota lakes, accounting for seasonal and spatial variability. Water clarity increased by 0.41 m across lakes from 1984 to 1988 and 2014 to 2018. Lake‐specific clarity trends varied: clarity did not change significantly in 59.0% of lakes, increased in 34.5% of lakes, and decreased in 6.5% of lakes. Water clarity dynamics caused considerable variability in littoral area between seasons and years. Our results have wide applications in aquatic ecology, including understanding changes to food webs, assessing fish habitat, and evaluating impacts of invasive species. [ABSTRACT FROM AUTHOR]

Invasive species are leading drivers of environmental change. Their impacts are often linked to their population size, but surprisingly little is known about how frequently they achieve high abundances. A nearly universal pattern in ecology is that species are rare in most locations and abundant in a few, generating right-skewed abundance distributions. Here, we use abundance data from over 24,000 populations of 17 invasive and 104 native aquatic species to test whether invasive species differ from native counterparts in statistical patterns of abundance across multiple sites. Invasive species on average reached significantly higher densities than native species and exhibited significantly higher variance. However, invasive and native species did not differ in terms of coefficient of variation, skewness, or kurtosis. Abundance distributions of all species were highly right skewed (skewness>0), meaning both invasive and native species occurred at low densities in most locations where they were present. The average abundance of invasive and native species was 6% and 2%, respectively, of the maximum abundance observed within a taxonomic group. The biological significance of the differences between invasive and native species depends on species-specific relationships between abundance and impact. Recognition of cross-site heterogeneity in population densities brings a new dimension to invasive species management, and may help to refine optimal prevention, containment, control, and eradication strategies. [ABSTRACT FROM AUTHOR]