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, as the authority reports it.
Lake map
Map shows 20 lakes with coordinates
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.
- 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 other countries
Named lakes and official measurements
Directory of official lake water quality
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Where to stay near Northern Ireland
Stays near the least loaded measured lakes in Northern Ireland — live availability from Booking.com, Vrbo and Expedia.
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