Why rank lakes by phosphorus?

Phosphorus is usually what limits how much algae a lake can grow, so it is the earliest signal that a lake is under nutrient pressure. It is also the measurement these agencies publish most consistently, which makes it the fairest way to compare lakes across a country.

Is a low-phosphorus lake safe to swim in?

Not necessarily. A lightly loaded lake can still carry bacteria, a seasonal algal bloom, or contaminants that a nutrient reading does not cover. Check the local authority notice for the day you plan to go.

What is a typical phosphorus level in France?

The median measured lake in France reads 33 µg/L of total phosphorus. Lakes below roughly 10 µg/L are considered nutrient-poor and usually run clear; above about 30 µg/L a lake is generally nutrient-rich and more likely to green up in summer.

Cleanest lakes in France

Cleanest lakes in France: 185 named lakes have an official total-phosphorus reading. The 20 below carry the lowest nutrient load — a measurement, not a LakeQuality A–F grade.

Lake map

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Lowest phosphorus readings in France

Phosphorus is the nutrient that feeds algae growth, so the lakes at the top of this table are the least loaded ones the France monitoring programme has measured. The median measured lake in France reads 33 µg/L.

Lake	Place	Total phosphorus	Chlorophyll-a	Secchi	Year
Barrage de Lunax	Gers	10 µg/L	1.5 µg/L	2.8 m	2024
Etang de Cazaux Sanguinet	—	10 µg/L	—	—	2023
Lac Chauvet	—	10 µg/L	—	—	2023
Lac D Artouste	—	10 µg/L	—	—	2019
Lac D'Arjuzanx	Landes	10 µg/L	0.60 µg/L	8.3 m	2023
Lac D'Issarles	Ardèche	10 µg/L	6.0 µg/L	4.9 m	2025
Lac de Commanday	Landes	10 µg/L	0.60 µg/L	5.0 m	2020
Lac de Marou	Lot-et-Garonne	10 µg/L	3.0 µg/L	1.0 m	2022
Lac de Mondély	Ariège	10 µg/L	1.8 µg/L	3.0 m	2024
Lac de Tazenat	Puy-de-Dôme	10 µg/L	2.1 µg/L	7.1 m	2023
Lac des Quatre Cantons	Landes	10 µg/L	2.8 µg/L	4.3 m	2020
Petit Etang de Biscarrosse	—	10 µg/L	—	—	2023
Réservoir de Castelnau Magnoac	Hautes-Pyrénées	10 µg/L	3.0 µg/L	3.2 m	2024
Retenue de Bort Les Orgues	—	10 µg/L	—	—	2019
Retenue de la Cheze	Ille-et-Vilaine	10 µg/L	4.2 µg/L	2.8 m	2023
Retenue de Saint Pardoux	Haute-Vienne	10 µg/L	9.2 µg/L	2.7 m	2022
Retenue de Vassiviere	Creuse	10 µg/L	5.5 µg/L	2.1 m	2024
Retenue du Drennec	Finistère	10 µg/L	4.0 µg/L	3.4 m	2025
Lac de Varilhes	Ariège	12 µg/L	3.5 µg/L	2.2 m	2023
Lac de Peyralade	Tarn-et-Garonne	12 µg/L	1.9 µg/L	1.9 m	2023

Showing 20 of 185 named lakes. Open a county or place page for the rest.

Frequently asked questions

Which lakes in France are the cleanest?: 185 named lakes in France have an official total-phosphorus reading. This page ranks the 20 lowest. Phosphorus is the nutrient that feeds algae growth, so a low reading means a lightly loaded lake — it is not a swim-safety class and not a LakeQuality A–F grade.
Why rank lakes by phosphorus?: Phosphorus is usually what limits how much algae a lake can grow, so it is the earliest signal that a lake is under nutrient pressure. It is also the measurement these agencies publish most consistently, which makes it the fairest way to compare lakes across a country.
Is a low-phosphorus lake safe to swim in?: Not necessarily. A lightly loaded lake can still carry bacteria, a seasonal algal bloom, or contaminants that a nutrient reading does not cover. Check the local authority notice for the day you plan to go.
What is a typical phosphorus level in France?: The median measured lake in France reads 33 µg/L of total phosphorus. Lakes below roughly 10 µg/L are considered nutrient-poor and usually run clear; above about 30 µg/L a lake is generally nutrient-rich and more likely to green up in summer.