Long-term change in Wisconsin lake plant communities

Long-term changes in Wisconsin lake plant communities were assessed with the floristic quality index (FQI) and the aquatic macrophyte community index (AMCI). Only plant communities in natural lakes were studied, by ecoregion and water source. The median number of species (N) and the median FQI increased over time in Northern Lakes and Forests ecoregion lakes with and without inlets and North Central Hardwood Forests ecoregion lakes without inlets. The median N declined in Southeastern Wisconsin Till plains lakes with inlets and median N and median FQI declined in Southeastern Wisconsin Till Plains lakes without inlets. No significant change in median N or FQI values was found in North Central Hardwood Forests ecoregion lakes with inlets. Northern Lakes and Forests ecoregion lake plant communities increased in quality using median AMCI values. No other ecoregion-lake type showed a significant change over the time period analyzed. Regression analysis showed that four of 33 lakes showed a significant trend of quality change using FQI values, and seven of 28 lakes showed a significant trend of quality change using AMCI values. Only one of the AMCI trends was negative; all other AMCI and FQI trends were positive Although this study indicates no strong or widespread decline in aquatic plant communities, results should be interpreted with caution because declines may have occurred before sampling began, increased plant quality may indicate degradation of other water-quality parameters, and the length of record for many lakes is short.