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 Ireland?

The median measured lake in Ireland reads 17.5 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 Ireland

Cleanest lakes in Ireland: 207 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 Ireland

Phosphorus is the nutrient that feeds algae growth, so the lakes at the top of this table are the least loaded ones the Ireland monitoring programme has measured. The median measured lake in Ireland reads 17.5 µg/L.

Lake	Place	Total phosphorus	Chlorophyll-a	Secchi	Year
Upper Glendalough	Wicklow	5.0 µg/L	3.1 µg/L	—	2022
Lough Cummernamuck	Kerry	10 µg/L	2.6 µg/L	—	2022
Lough Currane	Kerry	10 µg/L	4.7 µg/L	—	2022
Lough Muckross	Kerry	10 µg/L	2.0 µg/L	—	2022
Lough Namona	Kerry	10 µg/L	3.9 µg/L	—	2022
Lough Fadda	Galway	11 µg/L	2.2 µg/L	—	2022
Annaghmore Lough	Roscommon	11 µg/L	3.4 µg/L	—	2023
Lough Acoose	Kerry	11 µg/L	4.4 µg/L	—	2023
Lough Coosan	Westmeath	11 µg/L	2.9 µg/L	—	2022
Lough Fad Meendoran	Donegal	11 µg/L	2.3 µg/L	—	2022
Lough Glenbeg	Cork	11 µg/L	5.1 µg/L	—	2023
Lough Lene	Westmeath	11 µg/L	2.2 µg/L	—	2023
Lough Cullaun	Clare	12 µg/L	2.8 µg/L	—	2023
Lough Owel	Westmeath	12 µg/L	1.9 µg/L	—	2023
Lough An Tseisigh	Donegal	12 µg/L	2.3 µg/L	—	2023
Lough Ardderry	Galway	12 µg/L	1.9 µg/L	—	2023
Lough Bane Noggin Hill	Meath	12 µg/L	2.5 µg/L	—	2023
Lough Brin	Kerry	12 µg/L	4.9 µg/L	—	2023
Lough Caragh	Kerry	12 µg/L	2.8 µg/L	—	2023
Lough Carra	Mayo	12 µg/L	2.2 µg/L	—	2023

Showing 20 of 207 named lakes. Open a county or place page for the rest.

Frequently asked questions

Which lakes in Ireland are the cleanest?: 207 named lakes in Ireland 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 Ireland?: The median measured lake in Ireland reads 17.5 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.