Which lakes in Northern Ireland are the cleanest?

20 named lakes in Northern 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 Northern Ireland?

The median measured lake in Northern Ireland reads 55 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 Northern Ireland

Cleanest lakes in Northern Ireland: 20 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

Loading map…

Lowest phosphorus readings in Northern Ireland

Phosphorus is the nutrient that feeds algae growth, so the lakes at the top of this table are the least loaded ones the Northern Ireland monitoring programme has measured. The median measured lake in Northern Ireland reads 55 µg/L.

Lake	Place	Total phosphorus	Chlorophyll-a	Secchi	Year
Lough Fea	Northern Ireland	20 µg/L	—	—	2024
Lough Island Reavy Reservoir	Northern Ireland	20 µg/L	—	—	2024
Spelga Dam	Northern Ireland	20 µg/L	—	—	2024
Castlehume Lough	Northern Ireland	30 µg/L	—	—	2024
Lough Scolban	Northern Ireland	30 µg/L	—	—	2024
Silent Valley Reservoir	Northern Ireland	30 µg/L	—	—	2024
Upper Lough Macnean	Northern Ireland	30 µg/L	—	—	2024
Lower Lough Macnean	Northern Ireland	35 µg/L	—	—	2024
Lough Melvin	Northern Ireland	40 µg/L	—	—	2024
Cam Lough	Northern Ireland	50 µg/L	—	—	2024
Lough Mourne	Northern Ireland	60 µg/L	—	—	2024
Lower Lough Erne	Northern Ireland	60 µg/L	—	—	2024
Lough Ross	Northern Ireland	70 µg/L	—	—	2024
Upper Lough Erne	Northern Ireland	70 µg/L	—	—	2024
Stonyford Reservoir	Northern Ireland	80 µg/L	—	—	2024
Clea Lakes	Northern Ireland	110 µg/L	—	—	2024
Lough Beg	Northern Ireland	130 µg/L	—	—	2024
Lough Neagh	Northern Ireland	140 µg/L	—	—	2022
Lough Gullion	Northern Ireland	200 µg/L	—	—	2024
Portmore Lough	Northern Ireland	260 µg/L	—	—	2024

Frequently asked questions

Which lakes in Northern Ireland are the cleanest?: 20 named lakes in Northern 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 Northern Ireland?: The median measured lake in Northern Ireland reads 55 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.