Classifying lake plant communities: Challenges and approaches

Background/Question/Methods

The structure and composition of plant communities vary geographically and within lakes in response to environmental conditions and human disturbance. Because aquatic plant surveys are traditionally conducted at the lake level, rather than community or stand level scale, aquatic communities are often neglected in regional vegetation classifications. Identifying and describing these communities may help resource managers select high quality sites for protection and manage others.

We have analyzed Minnesota’s extensive lakewide plant datasets and described patterns in taxonomic richness and composition in relationship to water clarity, trophic status and alkalinity. However, site-based data are needed to describe within lake community diversity. For Minnesota lakes, data are lacking because of the difficulties associated with collecting quantitative plant data in littoral areas. Basic questions such as appropriate plot size and measurable attributes have not been addressed.

We initiated plot-based vegetation sampling in Minnesota lakes to assess the feasibility of collecting quantitative estimates of plant community composition. Plots, measuring 250 square meters, were subjectively placed within vegetated areas of lakes. Thirty quadrats (one meter squared) were systematically placed in each plot. Surveyors recorded all plant taxa observed, water depth and substrate type. Estimates of plant density and cover were also attempted. 

Results/Conclusions

Plot-based sampling can provide a standardized method where multiple surveyors can quantitatively describe a lake plant community and assess potential changes in composition.  Species-area curves indicated that a 250 square meter plot was adequate to estimate taxonomic richness within diverse communities. Visual estimates of density and cover were not consistent among surveyors. However, surveyors could consistently collect information on species occurrence (presence within quadrat) with visual observation or with rakes in turbid water.

Plot data collection has focused on plant communities of relatively undisturbed sites of clear, hard-water lakes of north central Minnesota. Additional data from soft-water and alkaline lakes are needed for a statewide lake plant community classification. Data from a gradient of sites, including impaired sites, are needed to identify indicator species and to describe the range of conditions within a plant community class.