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 Portugal?

The median measured lake in Portugal reads 20 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 Portugal

Cleanest lakes in Portugal: 19 named lakes have an official total-phosphorus reading. The 19 below carry the lowest nutrient load — a measurement, not a LakeQuality A–F grade.

Lake map

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Lowest phosphorus readings in Portugal

Phosphorus is the nutrient that feeds algae growth, so the lakes at the top of this table are the least loaded ones the Portugal monitoring programme has measured. The median measured lake in Portugal reads 20 µg/L.

Lake	Place	Total phosphorus	Chlorophyll-a	Secchi	Year
Albufeira Vilarinho Das Furnas	—	5.0 µg/L	—	—	2023
Albufeira Castelo Bode	—	5.9 µg/L	—	—	2023
Albufeira Paradela	—	6.0 µg/L	—	—	2023
Albufeira Salamonde	—	6.0 µg/L	—	—	2023
Albufeira Alto Lindoso	—	9.0 µg/L	—	—	2023
Albufeira Fronhas	—	10 µg/L	—	—	2023
Albufeira Caldeirao	—	11 µg/L	—	—	2023
Albufeira Pracana	—	13 µg/L	—	—	2023
Albufeira Aguieira	—	14 µg/L	—	—	2023
Albufeira Pisao	—	20 µg/L	—	—	2023
Albufeira Funcho Int C	—	28 µg/L	—	—	2018
Albufeira Santa Clara C	—	32 µg/L	—	—	2018
Albufeira Miranda	—	40 µg/L	—	—	2023
Albufeira Pocinho	—	40 µg/L	—	—	2023
Albufeira Roxo C	—	48 µg/L	—	—	2021
Albufeira Maranhao	—	62 µg/L	—	—	2023
Albufeira Vale Gaio	—	77 µg/L	—	—	2022
Albufeira Fratel	—	130 µg/L	—	—	2023
Albufeira Divor	—	150 µg/L	—	—	2023

Frequently asked questions

Which lakes in Portugal are the cleanest?: 19 named lakes in Portugal have an official total-phosphorus reading. This page ranks the 19 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 Portugal?: The median measured lake in Portugal reads 20 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.