Life Under the Surface

Prepared by Buzz Boles, Portland

An Ecological Overview of Big Rideau Lake

The key to any monitoring or advocacy for Big Rideau Lake is having a good understanding of how lakes ‘work’ and how human actions affect their water quality. Drawing from a wide range of scientific studies, we offer the following overview of how Big Rideau Lake works.

Virtually all biological systems on our planet are driven primarily by solar energy.  Big Rideau Lake and surrounding watershed lakes are no different.

Sunlight penetrates the water and provides the basic energy to enable chlorophyll based life forms such as phytoplankton (algae) and macrophytes (other larger water plants) to grow and thrive.  Other nutrients aiding in plant growth (i.e. fertilizers) are provided in the form of phosphorus, nitrogen and nitrates, which are deposited into the lake by inflows from the surrounding landscape such as streams and ground water seepages. These nutrients are generated through the erosion of rock and are naturally occurring in soils and through the decomposition of plant and animal matter.

In recent centuries, however, there has been an increase in nutrient inputs from human land use, including inputs from agricultural fertilizers, and septic waters from both human and livestock wastes. Too much nutrient loading into a lake can readily increase phytoplankton ‘blooms’. These phytoplankton blooms can be problematic as they consume large amounts of oxygen and nutrients from the ecosystem for their growth and decomposition, creating ‘hypoxic’ zones within the lake. As a result, the life cycle of phytoplankton blooms can drastically and negatively affect the form and function of the ecosystem. As part of the natural cycle, lake nutrients and oxygen are stirred about, primarily  in spring and fall, throughout a lake. This mechanism is commonly known as ‘turnover’ and is primarily driven by water temperature changes as well as wind and wave action.  

Plant life, particularly phytoplankton, forms the base of the food web within the Big Rideau Lake ecosystem. The energy produced from this base helps to structure and energize the animal communities further along in the lake’s web of life. Since the 1980’s, scientists have put a lot of effort into gaining a better understanding of the workings of lakes, particularly the food webs of life (i.e., interconnectivity of species) as this is key to protecting water quality and biodiversity, including humans. In the last twenty years, the knowledge and use of natural isotopes (radioactive particles) has enabled scientists to track food as it passes through the food web, which has helped tell a much fuller story of life under the surface of the water.  Also the invention of miniature tracking tags has enabled scientists to track fish and birds to gain more knowledge of their patterns of life.  

There are numerous phytoplankton species found within Big Rideau Lake. These micro plants are largely preyed upon by a wide variety of zooplankton (micro animals). The role of zooplankton in the natural system is: [1] to keep phytoplankton from reaching population levels where they can become out of balance and destructive to habitat quality and other lake life; and [2] act as a food source for higher levels of life (e.g., very small fish). When phytoplankton numbers get ahead of the zooplankton population this can readily create situations where algal populations may bloom, including the various types of blue-green algae, some of which can cause toxic situations and human health issues. Algal blooms also rapidly consume the available oxygen in the water creating hypoxic and/or anoxic conditions (i.e., very low dissolved oxygen availability). These two conditions can create uninhabitable environments for plant and animal life and, in the worst case scenarios, can cause fish kills. Massive fish die-offs have occurred in Lakes Ontario and Erie and some other smaller lakes from this phenomenon. 

NASA photo credit

NASA photo credit

Zooplankton form the second main link of the food web, as they are eaten by small fish and aquatic insects.  The small fish and insects are in turn eaten by larger fish as well as some birds, amphibians, reptiles and mammals. And thusly, starting with the solar energy and nutrients supporting phytoplankton life forms, and onwards up through the food chain, the apex predators are sustained in their life cycles and as a presence on the lake.  It is the apex predators who are the main predators of small fish and insects, which eat the zooplankton which in turn eat the phytoplankton. The more small fish and insects eaten by apex predators leaves more zooplankton alive to eat the algae which can cause a deterioration of water quality. On Big Rideau Lake these apex predators include great blue and other herons, ospreys, eagles, loons, cormorants, mergansers, terns, kingfishers, seagulls, and mammals such as mink and otter and a variety of larger fishes, and some snakes and turtles, and also a variety of other waterfowl which summer here or stop in on the lake while migrating.  Human beings are also part of the food web and act as apex predators through their fishing activities.

A stable interconnected food-web is essential to maintain healthy functioning ecosystems. If instability or imbalance (e.g. excessive nutrient loading creating algal blooms) occurs in the linkages then detrimental impacts to the natural form and function of the food-web will likely result.  The natural lake system has some built in flexibility to such impacts through the numbers and diversity of species present which to some degree can absorb or cushion some of these negative impacts. However, the natural buffering capacity of an ecosystem is finite, and can readily and eventually fail in some way if the imbalance or disturbance increases and/or persists. Beyond that point water quality begins to degrade and other life forms, including humans, are negatively impacted. This has happened elsewhere in larger lakes such as Lakes Ontario and Erie and many other smaller water bodies as well.

Over the past several decades we are also seeing the spread of invasive plant and animal life to Ontario’s lakes and rivers; and this brings some added level of impact and concern. Zebra mussels and cormorants are prime examples of this. With regards to potential acid rain impacts, Big Rideau Lake is naturally protected by the calcium carbonate make-up of much of the underlying and surrounding geology, which buffers such acidic inputs. Calcium eroded from the surrounding rocks can also bond with nutrients such as phosphorous and then settle to the lake bottom without contributing to algae growth and thereby contributing to the clarity of the water and helping to reduce potential nutrient overloading.

Studies of the Rideau Lakes Basin in the 1990’s have specifically identified the need to protect the presence of apex predators in order to protect lake water quality. The Township bylaws for lake edge and near water property development and habitation are based on these studies so as to help protect the water quality and other attributes of the lake.  In 2015 these studies were reviewed for accuracy and suitability and confirmed by the Township authorities and consultant scientists and Rideau Valley Conservation Authority as being appropriate as a basis to work from.   

Multiyear studies by the Rideau Valley Conservation Authority  reported on in 2014 show Big Rideau Lake as having a ‘fair’  [65-79/100] rating, Lower Rideau Lake having a ‘very poor’ [0-44/100] rating and Upper Rideau Lake having a ‘poor’ [45-64/100] rating based on a standard national scale. Human activities tend to introduce added nutrients into the lake which can reduce water quality and create noxious situations such as algal blooms, which are becoming all too familiar to lake users. The natural mechanisms outlined above plus well vegetated shorelines and well founded and respected bylaws work towards maintaining higher water quality and to reduce nutrification of the lakes.  One specific line of defence open to us all against such noxious blooms is to ensure that septic fields are in proper working order and that the escape of nutrients from septic fields and agricultural activities is minimized.

With the advent of recent global climate changes and the general warming of the planet we can expect that some changes are in store for the Big Rideau watershed. Changes in length and severity of the seasons will likely be the most evident ones along with some possible modification to water levels, water chemistry and the life forms dependent on the lake water.  Which specific impacts will occur and the degree of these impact is yet to be seen. 

The Ontario government along with The Rideau Valley Conservation Authority and other conservation authorities continually monitor the water quality and the well-being of wildlife in Big Rideau Lake and many other lakes and rivers.  The Big Rideau Lake Association will continue to review information provided by these authorities or others and bring them to the attention of our membership to inform and encourage their continued participation in protecting our lake environment. 

References and Suggested further readings: 

  • Brönmark, C., & Hansson, L. A. (2005). The Biology of Lakes and Ponds (Second Edition). Great Britain: Oxford University Press.  
  • Rideau Lakes Subwatershed Report 2014. A report on the environmental health of the Rideau Lakes. Prepared by the Rideau Valley Conservation Authority and Conservation Ontario. Available at or   tel: 613-692-3571.
  • Rideau Lakes Basin Carrying Capacities and Proposed Shoreline Development Policies. Prepared by Michael Michalski Associates and Anthony Usher Planning Consultant for Rideau Valley Conservation Authority, Feb. 1992.