Middle Lake Hamilton, Polk County: The One Passing Grade Came From Four Wedges of Twelve

Middle Lake Hamilton's biological record holds exactly one passing score. It is a 46, from September 3, 2020 — and two of the four parts of the lake that produced it came in at 20.2 and 31.5, well under the failing line of 43.

Polk County asked for Florida's Lake Vegetation Index, the state's rapid bioassessment of a lake's aquatic plant community, run to the Department of Environmental Protection's LVI 1000 protocol on forms FD 9000-27 and FD 9000-31. The protocol cuts a lake into twelve pie-shaped segments and the assessment team selects four. In each one the crew lists every aquatic and wetland species it can find and throws a submerged vegetation sampling tool called a Frodus along a five-meter belt transect running from the shoreline out past the submerged vegetation zone. Four metrics come out of that work: the coefficient of conservatism of the dominant plants, the share of taxa native to Florida, the share on the state's Category I invasive list, and the share rated sensitive to disturbance. Every wedge gets its own score. The lake's score is the average of the four, and below 43 the lake is impaired.

On September 3, 2020, David Eilers of the USF Water Institute assessed wedges 1, 4, 7 and 10 of Middle Lake Hamilton. They scored 20.24603, 60.90292, 70.24155 and 31.51582. The average was 46 — healthy, by three points, on a lake where two of the four sampled wedges had failed.

Ten months later, on July 9, 2021, Eilers worked wedges 3, 6, 9 and 12. Those scored 42.91831, 39.20077, 12.36772 and 13.19444, for a lake score of 27. The secchi disk read 2.75 feet in 2020 and 2.89 feet in 2021. The 2021 round logged more plants than the 2020 round, 46 species against 36, and more natives, 35 against 24. The two rounds did not cover the same water, so nothing in the record separates a change in the vegetation from a change in which quarter of the lake got walked.

Middle Lake Hamilton is the small lobe in the middle of a much larger run of water. The state's own plan puts it at 107 acres, sitting between Lake Hamilton's 2,185 and Little Lake Hamilton's 368, the three joined by constructed navigable canals a boat can cross. To the Department of Environmental Protection they are three separate waterbodies carrying three separate numbers — 15041, 15002 and 15001 — each with its own verdict, and each with a chemistry record drawn from a sampling point out in the middle. Middle Lake Hamilton's is station 21FLPOLKMDL HAMILTON1, where the Polk County Natural Resources Division has been pulling samples since 1988. That single mid-lake station is what rates the lake fair, at 62 on the trophic state index, as of 7/7/2026.

WBID 15002 is listed impaired for biology and for all three nutrient parameters — chlorophyll-a, total nitrogen and total phosphorus — with its status recorded as ongoing restoration activities. The biology listing rests on three index scores, and the Category 4e documentation submitted by Polk County Parks and Natural Resources lists them: 31 from June 20, 2013, 20 from June 24, 2015, and the 46 of September 3, 2020, marked in the file as passing the threshold of 43.

The surveys have kept coming since. The Water Atlas record for the lake runs 27 on July 9, 2021, 38 on June 1, 2022, 30 on June 27, 2023, and 21 on May 28, 2025 — all of them below 43, with the 46 still the only score in the file from 2013 forward that cleared it.

DEP accepted the Lake Hamilton Chain of Lakes Pollutant Reduction Plan in 2021, covering all three WBIDs for biology, chlorophyll a, total nitrogen and total phosphorus, filed as documentation in support of Category 4e. It commits the county to a Lake Vegetation Index survey at a minimum of every other year, to assess the vegetation and the changes restoration activities make in it, and prices one at $1,200 an event. Alongside the surveys it books the City of Lake Hamilton's septic-to-sewer conversion at roughly $838,621 — approximately 153 single-family residences and 8 commercial buildings taken off septic systems and put on central sewer treated at the Haines City facility.