Lake Mattie, Auburndale: The Lake Florida Measures Other Lakes Against

In November 2019, Florida's set of reference lakes — the minimally disturbed waters the Department of Environmental Protection measures other lakes against, chosen because so little has been done to the land around their shorelines — held 71 sites spread across 25 lake regions. One of them was Lake Mattie, 1,058 acres of water in unincorporated Polk County, in the Withlacoochee River watershed, averaging eight feet deep. The Polk County boat access directory lists it as canal access only: you come in from Lake Juliana, when the water is high enough. Juliana's outlet canal, on its eastern side, discharges into Mattie.

The department began its reference lake sampling program in 2016, to record water quality and plant communities in minimally disturbed lakes across as many of the state's lake regions as it could reach. Qualifying takes a number. The Landscape Development Intensity index scores the ring of land within 100 meters of a lake by what people have made of it: natural, undeveloped cover carries a value of one, and the Florida scale runs to a maximum near 42. At or below two, a lake counts as minimally disturbed.

On November 5, 2019, DEP presented a Reference Lakes Status Update at the biocriteria meeting in Palatka. One slide was headed Low Scoring Lakes. Of the reference sites sampled between 2016 and 2019, twelve had come back with Lake Vegetation Index scores under 50, and listed among them under the Southwestern flatlands region was Lake Mattie, at 41. The slide closes on a question the department put to itself: do these lakes qualify as minimally disturbed?

The Lake Vegetation Index is how Florida scores a lake's plants. A qualified crew divides the water into twelve pie-shaped segments, works four of them, lists every aquatic species found, and grades the result on how tightly those plants track undisturbed Florida. Below 43, the lake is impaired.

Polk County had the index run on Lake Mattie on May 18, 2023. David Eilers and Karina Gonthier of the USF Water Institute scored it 27. The previous assessment, on June 10, 2020, had scored 38. Averaged, 32.5, and the report classifies the lake as impaired.

The same report put the Landscape Development Intensity of Mattie's 100-meter buffer at 1.64, inside the threshold of two — 83 percent natural land, another 14 percent agricultural, shorelines recorded as typically undeveloped, with most stormwater reaching the lake as sheet flow across natural cover. The crew counted 39 species of wetland vegetation across the four segments, and 32 of them were native.

What pulled the score down was which plants had taken over. Peruvian primrose-willow, Ludwigia peruviana, was dominant in segments 3 and 6 and co-dominant in 9. Water hyacinth, Eichhornia crassipes, was co-dominant in 9 and 12. Both carry a coefficient of conservatism of 0.00, the floor of a scale that runs to 10. No submerged aquatic vegetation was observed anywhere in the survey. A Secchi disk vanished at 0.7 meters, about 2.3 feet, which the report notes is near the ten-year mean for the lake.

The last slide of the 2019 update lists, first among next steps, reviewing and updating the reference lake sites, then prioritizing lakes in regions with no data: six lake regions had no sample set at all.

Lake Mattie's 2023 species list still runs to saw-grass, Cladium jamaicense, at a coefficient of conservatism of 8.00, recorded in two of the four segments. The highest-scoring plant on the whole list is swamp lily, Crinum americanum, at 9.00, and the crew found it in one.