Cyanobacteria Blooms
Every week through the season, trained volunteers walk to the same places on the shore and file a report on what the water looks like. Since 2012 they have filed 21,000 of them on Lake Champlain alone. The map that displays those reports shows only today. Fourteen seasons of them, kept as one spreadsheet per year, show something the daily map cannot: the blooms are concentrated in St Albans Bay and Missisquoi Bay, season after season.
Two shallow bays at the north-east corner account for almost all of it. St. Albans Bay and Missisquoi Bay are under an alert in more than a third of all reports ever filed there, and under a high alert in a quarter. The South Lake, at the opposite end, has been under a high alert in one report in eighty. That is a difference of about twenty to one across a single lake.
High-alert rate by lake segment and year, 2012–2024
Share of that season's reports at a high alert. Blank means nobody was reporting from that segment that year — 2013's spreadsheet is the one season that recorded no segment at all, so its column is empty here although its reports are counted everywhere else. 2025 is empty for a different reason: the state never published a summary for that season, and the tracker keeps only the year that is running, so those reports are gone from both places they could have come from. 2023 stands out almost lake-wide; the ordering barely changes.
Why the blooms are in those two bays
Every measure points at the same two bays. They are shallow and warm early. Two of the basin's heaviest phosphorus sources empty into Missisquoi Bay alone — the Missisquoi, second largest load of any tributary, and the Pike, second highest yield per square kilometer. Between them they deliver about a quarter of all the phosphorus the whole watershed sends the lake, into one shallow bay. And these bays are nearly enclosed: water that enters them has no short way out, so the phosphorus that arrives stays and cycles.
The lake as a whole flushes in about two years. A bay behind a causeway does not flush on anything like that schedule.
How to read these figures
Reporting effort grew from 636 reports in 2012 to 3,493 in 2024, and volunteers go where blooms are known to happen, so the raw count of alerts is partly a count of people looking. Everything here is therefore a rate — alerts as a share of reports filed at the same places — which is far less sensitive to how many people were out.
A site can be clear all season and bloom the week you visit. The state tracker is the place to check that.
Water clarity before the tracker, 1965–1977
The tracker starts in 2012. The nearest thing to a measurement of this lake's condition before the modern programs is water clarity, and there is one: Myer and Gruendling's Limnology of Lake Champlain (1979) gathers thirteen years of secchi disk readings, 1965 to 1977, out of ten separate studies, and reports both what each basin averaged and how that average moved between the late sixties and the seventies.
South Lake water clarity, 1929 and 1965–1977
The South Lake is the only part of the lake where the report can set its own figure against a much older one.
Two readings, forty years apart, with a description between them — the line joining them is drawn dashed because nothing was measured along it. The later point is the 1965–1977 mean, plotted at the middle of its own span rather than at a year.
What the 1965–1977 readings show, and their limits
It is a decade of decline in water clarity across almost the whole lake, measured, and finished fifteen years before either the bloom tracker or the long-term monitoring program began.
No modern transparency series exists to set against these readings. And the readings themselves lean toward summer and autumn — most were taken between June and November — so they describe the season the lake is watched in rather than the whole year.