The Wisconsin DNR maintains an archive of citizen-collected water quality data from Diamond Lake, in recent years headed by DL resident Ed Walen. Included are Secchi Disk values (water clarity), temperature-depth data, and Trophic State Index (TSI), (link here). Results over time may be viewed quickly in plots here and here.
The figure just above is the DNR plot of Trophic State Index for Diamond Lake from our citizen data for the last 46 years showing index values for the Secci visibility and two chemicals, phosphorous and chlorophyll, related to nutrient runoff from our properties. Points plotting in the darker blue zone toward the bottom of the figure are characteristic of oligotrophic lakes having clear water, low plant nutrients and high oxygen values even at depth. Values plotting in the lighter blue are characteristic of mesotrophic lakes, and green are eutrophic lakes which have low available oxygen making survival difficult for aquatic organisms.
Based primarily on the Secci values (black circles), Diamond Lake data plot near the boundary between oligotrophic and mesotrophic, and its classification by the DNR over time has changed accordingly, recently more often mesotrophic (see individual TSI reports in the link above). The data paint a nuanced story with our water quality varying just above and below the oligotrophic threshold for many years, with a slight trend toward mesotrophc recently. The take away from this that is most important is keeping the water quality in Diamond Lake high and working toward small improvement over time. This will result from efforts to keep chemical runoff (including lawn fertilizers) and sediment from entering the lake. Preventing shoreland erosion from high boat wakes is also a key. Thanks to everyone for help with this and please keep up the good work.