pH in freshwater

In freshwater systems, pH plays an important role in determining the conditions for life.

What is pH?

pH stands for “potential for Hydrogen”. It is the measure of the acidity or alkalinity of water soluble substances.

Why is pH important in freshwater?

pH sets up the conditions for how easy it is for nutrients to be available and how easily things like heavy metals (toxicity for aquatic life) can dissolve in the water. Rivers and lakes generally range between 5 (acidic) and 9 (basic) on the pH scale. Whereas ocean water averages closer to 8.2 (slightly basic). Low pH can reduce how many fish eggs hatch and can make life difficult for fish and macroinvertebrates (the backbone of our water ecosystems).

What does a pH measurement mean?

The most important thing is to first establish a baseline for testing. Based on that number we can determine if something is influencing the water’s health. For example if the pH falls below 5 or over 10 (this can be caused by algae blooms) you will start to see fish life spans and reproduction affected.

Common values for pH in freshwater

A visual representation of the pH scale, with acids (pH less than 7) at the top and bases (pH greater than 7) at the bottom. Freshwater pH is in the middle, with values between 6 and 7.

Did you know?

  • Coloured water tends to be more acidic, so it may have a lower pH than clean water.
  • Like other parameters, you’ll need to create a baseline for what’s a normal pH level (e.g. Ottawa River’s pH is usually around 7, while the Rideau River has a normal pH level around 8).
  • pH is important because it sets up conditions for how easy it is for nutrients to be available (which can lead to algae blooms) and how easily things like heavy metals can dissolve in the water (creating toxicity for aquatic life).
  • Human influence through dumping can change pH levels. Acid rain, mining run-off, and pine forests all lower the pH of
    water systems.
  • Lakes and rivers have an optimum pH level (average 7.4)
  • Fish and organisms can’t survive above or below certain values
  • Changes in pH could indicate that an area is in trouble
  • Plants affect pH over a day through photosynthesis and respiration: pH will be highest in the afternoon and lowest  before sunrise.
  • Low pH can reduce how many fish eggs successfully hatch, and make life difficult for fish and macroinvertebrates (the small creatures that are the backbone of our water ecosystems).  Amphibians are very sensitive to low pH (their skin is sensitive).

Algae blooms

Harmful algae is a growing concern for lakes and are made worse by excess nutrients from human activity. Algae blooms have an optimum pH between 8.2 – 8.7 and usually appear in late summer/early fall. When algae grows, pH levels go up, and can reach over 10 (all fish die above 10)! Some algae is normal. However, if you you see a lot, be careful as some are toxic!

Water Rangers Protocol

We use test strips for pH. We’ve compared 15 types of test strips, and those made by Taylor have been shown to be the most accurate.

For those looking for more accuracy, inexpensive pen-style meters have proven to be very accurate, but they need to be calibrated at least once a week. We’ve tried this one and it works well (however, you need to be careful not to dunk it too far in the water).

How to test for pH in freshwater

  1. Rinse sample cup 3 times.
  2. IMPORTANT! Make sure your hands are dry, since moisture in the container will ruin strips.
  3. Shake out 1 test strip, close bottle.
  4. Dip the entire strip in the water. Remove after 2 seconds.
  5. Wait 20 seconds before reading.
  6. Compare colours with guide on side of bottle. Line up the colour strips vertically so that you can compare along the spectrum for each value.
A woman sits on the grass and reads a freshwater pH measurement by comparing her test strip to the reference bottle. pH is estimated by comparing the color change to the reference scale.

Contributing to the community!

Water Rangers is citizen-scientist led. So, if you have any questions, ideas, or notice any errors related to our tests, please tell us!