A macrophyte bioassessment approach linking taxon-specific tolerance and abundance in north temperate lakes.

Bioassessment methods are critically needed to evaluate and monitor lake ecological condition. Aquatic macrophytes are good candidate indicators, but few lake bioassessment methods developed in North America use them. The few macrophyte bioassessment methods that do exist suffer from problems related to subjectivity and discernibility along disturbance gradients. We developed and tested a bioassessment approach for 462 north temperate lakes. The approach links macrophyte abundance to lake ecological condition via estimates of taxon-specific abundance-weighted tolerance to anthropogenic disturbance. Using variables related to eutrophication, urban development and agriculture, we calculated abundance-weighted tolerance ranges for 59 macrophyte taxa and clustered them according to their tolerance to anthropogenic disturbance. We also created a composite index of anthropogenic disturbance using 20 variables related to population density, land cover and water chemistry. We used a statistical approach to set ecological condition thresholds based on the observed abundance of sensitive, moderately tolerant and tolerant taxa in each lake. The resulting lake condition categories were usually stable across multiple survey events and largely agreed with condition rankings assigned using expert judgment. We suggest using this macrophyte bioassessment method for federal water quality reports, restoration and management on north temperate lakes.

Commonly rare and rarely common: comparing population abundance of invasive and native aquatic species.

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.