How would you assess the quality of aquatic plant life in your lake? If you’re like me and most other people, you would have a pretty simplistic answer. Like, “not very weedy,” or “overgrown with coontail,” or “lots of good cabbage weeds for fish cover.”
It turns out there are scientific ways to evaluate lake vegetation, and in particular to express how closely it mirrors an undisturbed state, such as before humans took up residence on the shoreline. Lake scientists commonly use three measurements: Simpson’s Diversity Index, Quotient of Conservatism, and Floristic Quality Index. Calculated during plant surveys, these tools are used to asses overall lake quality.
Plant surveys follow specific methods. In transect sampling, scientists first divide a map of the lake areas where vegetation is present into sections. Then in each section they use a rake to collect a series of vegetation samples along a line (transect) within that section perpendicular to the shore. The samples are taken in different depth zones along that transect.
The plant species on each rake sample are then identified and each is assigned a ranking based on its density (abundance). The results are tabulated to note the points of collection and in each case all the species present and their density.
Another method is the point intercept survey, taken based on a grid system, using specific points spaced a specified distance apart (for example, 30 meters apart for a small lake, greater distances for larger lakes). Again, plant species are identified and density rankings given.
Whichever sampling method is used, the data collected forms the basis for indices that measure lake quality. Simpson’s Diversity Index looks at the frequency of occurrence and the density of the plant species found. The higher the index, the greater the diversity. The median Simpson’s Diversity Index for lakes in Wisconsin ranges from 0.80 to 0.90 (the maximum is 1.0).
The Coefficient of Conservatism assigns each plant species a value from zero to 10, reflecting the likelihood that the plant would live in undisturbed habitat. For example, invasive plants receive a score of zero; plants with ratings of 8 to 10 would likely be found only in high-quality waters and natural areas. The average overall coefficient for Wisconsin lakes averages 5.5 to 6.9.
The individual plants’ Coefficients of Conservatism then can be used to determine the lake’s Floristic Quality Index, which measures how close the plant community is to an undisturbed condition. This index can then be used as a basis to identify areas worthy of conservation, to monitor lake quality over time, and to see how human activity is affecting a lake.
So, what is the practical value of the arcane numbers these measurements present? For one they provide quantitative information that can be used to help create lake management plans. For example, they can help identify shoreline areas affected by erosion or other conditions that could damage water quality. They also make it possible to compare the quality of one lake to another.
Studies like these are conducted by lake science professionals. They’re not easy or inexpensive, but they can provide information of substantial value.
Ted Rulseh resides on Birch Lake in Harshaw and is an advocate for lake protection and improvement. His Lakeland Times and Northwoods River News columns are the basis for a book, “A Lakeside Companion,” published by The University of Wisconsin Press. Ted may be reached at [email protected].