Water Quality: Total Phosphorus

What is Measured?

Aquatic ecosystems need nutrients, such as phosphorus, to thrive. Phosphorus exists in different forms in water. It can be dissolved, bound to particles of soil and other materials or contained within living or decaying plants and animals. Dissolved phosphorus is easily used by plants and algae and is typically found in low concentrations in unpolluted water bodies. Total phosphorus (TP) is a measure of all of these forms of phosphorus combined.

We report on the average concentration of total phosphorus in a given area and show it in micrograms per litre (μg), which is the same as parts per billion.

Why is it Important?

Phosphorus is an essential nutrient for the plants and animals that make up the aquatic food web. Phytoplankton, such as microscopic algae, and aquatic plants require phosphorus, so its quantity in a particular area is a good indicator of the productivity of an aquatic system. Phosphorus can be seen as the foundation of the food web, supporting higher levels of organisms: zooplankton, crustaceans, small fish and top predators.

Phosphorus is also an indirect indicator of the suitability of water for recreational activities, as changes in TP affect algae growth and water clarity, in turn affecting swimming, boating, fishing and aesthetic enjoyment. Without clean and safe water, many of our favourite summer activities are compromised and our sense of enjoyment of being in a natural and pristine environment is quickly lost.

Algae bloom in Sturgeon Bay.

Nutrients: High or Low?

The concentration of nutrients in a lake varies depending on how shallow it is, how warm it gets and how many nutrients it receives from the surrounding watershed (organic materials from land, erosion or runoff, for example). Phosphorus is typically much higher in the spring because of snowmelt from rivers and streams, which carries nutrients into lakes. Some of the phosphorus is consumed during spring and summer, as phytoplankton species use it to grow.

Outlet of the Key River into Georgian Bay.
Outlet of the Key River into Georgian Bay.

Nutrient-rich lakes are called “eutrophic,” and nutrient-poor lakes are called “oligotrophic.” The best examples of these are Lake Erie and Lake Superior: the former is shallow and warm, while the latter is deep and cold. Lake Erie gets high nutrient inputs from surrounding agriculture and human development; Lake Superior gets much less. There are many other factors at work, including the geology (type of rock and soils), hydrology (water flow) and the natural presence of phosphorus in sediments (internal load).

In the offshore, deep waters of Georgian Bay, total phosphorus levels have been naturally low, with a provincial target of 5 micrograms per litre, representing an oligotrophic state. The cold, open waters of Georgian Bay act as one large water mass, with two or three smaller masses along the eastern shores during spring.

Georgian Bay receives nutrients that are brought through the watershed from many rivers. These include the French, Key, Naiscoot, Magnetawan, Shawanaga, Shebeshekong, Seguin, Moon and Musquash, as well as the Trent-Severn Waterway. Organic materials and nutrients from land travel into lakes with spring runoff, raising the phosphorus levels early in the year.

In shallower, protected bays or near wetlands, phosphorus levels can be much higher, which is good for fish habitat. This type of nutrient-rich habitat is considered more productive, supporting more species of algae and phytoplankton, as well as a more diverse food web.

Natural Sources of Phosphorus

  • Soil and organic matter
  • Spring runoff
  • Wildlife wastes
  • Atmospheric deposition

Human Sources of Phosphorus

  • Wastewater-treatment plants
  • Detergents and soaps
  • Runoff from fertilized lawns
  • Runoff from agriculture
  • Failing septic systems
  • Atmospheric deposition

When nutrients are trapped or concentrated in warmer, shallow waters (such as in late summer) an algal bloom may result, with TP levels as high as 20 micrograms per litre.

Reducing Nutrients

SSEA field crew recording temperature, oxygen and conductivity profiles in Penetanguishene Harbour, Severn Sound
SSEA field crew recording temperature, oxygen and conductivity profiles in Penetanguishene Harbour, Severn Sound.

Too much phosphorus supports rapid algae growth, which can appear as “blooms” of murky scum. Oxygen levels in water decline as the algae decompose. Reduced oxygen kills fish, invertebrates and other aquatic animals.

These algal blooms make water less attractive for boating and swimming, and if you draw lake water for drinking, it will taste and smell foul. The algal blooms can also be composed of cyanobacteria, or blue-green algae, which can have toxins dangerous to both wildlife and humans.

Since the 1970s, people in Ontario have made efforts to reduce phosphorus loads to surface water. Nutrients from sewage-treatment plants have been greatly reduced, and there are fewer source points of phosphorus. Detergents and soaps are not used directly in lake water, as they used to be. Nutrient loads from agriculture and stormwater sources remain but are much lower than they were in the past.

How is it Measured?

The State of the Bay report uses five data sets to report on phosphorus concentrations:

  1. Environment and Climate Change Canada (26 monitoring sites)
  2. Ministry of the Environment and Climate Change (135 monitoring sites)
  3. The Lake Partner Program volunteer sampling sites (39 sampling sites)
  4. Severn Sound Environmental Association (14 long-term monitoring stations)
  5. Muskoka Watershed Council, for southern Georgian Bay (16 sampling sites)

Together, these data are combined to report on total phosphorus from open-water areas in Georgian Bay to inland lakes. Collectively, these data sets cover a large enough area, over a long enough time period, to assess long- and short-term trends and show any seasonal changes (spring to fall).

We include data collected by volunteers with the LPP because we want to encourage citizen science and increase the number of sample sites in the Georgian Bay Biosphere and beyond, particularly for the nearshore, where federal and provincial boats have limited access.

Results: Where Did the Nutrients Go?

Along northern and eastern Georgian Bay, phosphorus concentrations in all areas are below the provincial water quality objective of 20 micrograms per litre, with the exception of a few locations in the French River and Sturgeon Bay.

However, surface concentrations of total phosphorus have been declining far lower than mid-1990s levels of 5 micrograms per litre. Concentrations abruptly declined around the year 2000 and have decreased to approximately 2 micrograms per litre in offshore waters.

Nuisance algal blooms

15 to 20 μg/L (reported in French River and Sturgeon Bay)

Georgian Bay nearshore

5 to 15 μg/L (nutrient-rich, productive ecosystem)

Georgian Bay offshore

2 to 5 μg/L (nutrient-poor and of concern to scientists)

How Low is too Low?

For the first time in recorded history, concentrations of total phosphorus in Georgian Bay are as low as those of Lake Superior. Georgian Bay waters are now considered ultra-oligotrophic, meaning less than 4 micrograms per litre.

These results represent an unprecedented low level of phosphorus—a critical nutrient in the open-water system that supports a healthy food web and stable fish community.

Offshore productivity of Lake Huron is thought to be negatively impacted by low phosphorous concentrations, with the most recent offshore concentrations the lowest on record and below the target set to maintain an oligotrophic state.

A. Dove and S.C. Chapra, Journal of Great Lakes Research, 2015

Why Have Nutrients Disappeared?

The reasons for the dramatic decline of TP in Georgian Bay are not completely understood, but a progressive reduction was seen over the past 40 years:

1970 to 1980s

Aggressive reduction of phosphorus discharge into Georgian Bay


Active filter feeding by mussels, thought to be related to lower TP levels


Period of relatively stable total phosphorus, but arrival of invasive zebra mussels


Unprecedented low TP showing fewer nutrients for ecosystem productivity

The invasion and rapid spread of zebra and quagga mussels has resulted in the loss of phytoplankton and zooplankton from the lake due to the mussels’ immense capacity to filter lake water. Their feeding seems to have used up most of the nutrients in Georgian Bay, and this is having a destabilizing effect on the aquatic ecosystem. Most likely, several factors are interacting, and more research will be required to understand this complex system.

Lake Huron and Georgian Bay Have Seen:

  • Higher fish biomass in nearshore areas, where nutrients come from watersheds.
  • Decreased primary production (phytoplankton, algae) in open water.
  • The disappearance of typical spring blooms of phytoplankton.
  • Decreased chlorophyll levels in all seasons.
  • Ultra-low TP in water from open areas has decreased TP levels in the nearshore.

As you read the rest of this report, think about the trends in decreasing phytoplankton, zooplankton, small fish and top predators. How much are these trends related to the loss of nutrients in Georgian Bay?

What Can You Do?

We still need people to reduce their phosphorus pollution, because an increase in nutrients along the shore will not benefit the offshore deep water. TP can still accumulate in nearshore areas, creating nuisance algal blooms—sometimes toxic ones.

  • Keep a buffer of vegetation along shorelines, which reduces nearshore nutrients.
  • Lower your household phosphorus pollution by avoiding detergents and soaps with phosphates.
  • Maintain your septic system properly to avoid leaks and nutrient spills into water.
  • Volunteer to monitor water quality near you!

Water Quality Monitoring

Sign up with the LPP so we can use your data in our next State of the Bay report. Sign up at desc.ca.

This provincial program is free and will analyze your water samples. Join the 600 volunteers sampling at over 800 sites. There are at least 30 sites in Georgian Bay that need volunteers to collect samples to help scientists better understand nutrient trends.

For volunteers, nutrient monitoring is easy. Once registered online at no cost, sampling equipment is sent to you (a water clarity disk, glass tubes, recording charts, prepaid postage for water samples, etc.), and training videos are available online. Total phosphorus and calcium samples are collected once per year, in the spring, and water clarity measurements are taken every two weeks throughout the summer.

I’ve been a water volunteer for over 20 years. It only takes an hour to learn the sampling procedures and involves roughly 20 hours per year,” says Anne Stewart from Bayfield Nares. “This includes the time required to travel to the sampling locations, log weather and water clarity information, collect and mail the samples. This program has really helped our ratepayer association to understand water quality conditions and trends in our area. I also appreciate that program support is always available from the LPP and GBBR.

Anne Stewart

If your group is already involved in nutrient monitoring and you have experienced high TP levels and/or algal blooms, you may wish to contact GBBR staff to help assess options for advanced monitoring.

For more information, download the Enclosed Bays and Inland Lakes Phosphorus Monitoring Guideline.

If your group would like further information, please contact David Bywater at [email protected].

Partner Profile: Severn Sound Environmental Association

Phosphorous has been monitored in Severn Sound since 1969. The sound was formerly listed as a Great Lakes Area of Concern, as the 1970s and 1980s marked a period of high nutrient loading from sources including wastewater-treatment plants, private septic systems and agricultural and stormwater runoff. Severn Sound was considered eutrophic (nutrient-rich) at that time and experienced excessive algae growth, particularly in Penetanguishene Harbour.

Annual Median Total Phosphorus Concentrations (μg/L) for Long-Term Stations in Severn Sound From 1969 to 2017.

A combination of remedial actions, such as controlling runoff from farms, as well as stewardship activities, septic upgrades and water-treatment upgrades, combined with ecological changes (such as the introduction of zebra & quagga mussels) have led to significant reductions in total phosphorus and algae growth. The Remedial Action Plan (RAP) targets for total phosphorus of 20 μg/L for Penetanguishene Harbour and 15 μg/L for the rest of Severn Sound continue to be met.

There have been no significant trends in total phosphorus since the mid-1990s, except for a decrease in the inner Penetanguishene Harbour. Although Severn Sound is now considered lower in nutrients, it is important to continue with remedial actions and monitoring, as climate change and invasive species continue to affect water quality in often unpredictable ways.

Severn Sound Environmental Association open-water monitoring stations.
Severn Sound Environmental Association field crew sampling bottom water chemistry in North Bay, Honey Harbour.

Next Chapter: Lower Food Web