Blue-green algae — their ancestors were in Muskoka long before any of the rest of us
By John Mason | October 3rd 2026
In a previous article, I discussed the fascinating history of the Muskoka River watershed. But what about the organisms that lived here? While various creatures came and went over the immense period since the first formation of the Grenville Province 1.8 billion years ago, one group stands out — the blue-green algae, or cyanobacteria.
Life on the planet commenced with single-celled microbial organisms — the Archaea and Bacteria, including cyanobacteria which, in the absence of plants, were major photosynthesizers. They were widely distributed in the abundant shallow seas located on most of the continents in Proterozoic times (3.5 billion to 540 million years ago). The continent’s shapes and locations were very different in ancient times; continental drift was in play and Muskoka was in the tropics at the start of our story.
In Western Australia, carbonate rocks 3.44 billion years old contain fossils of the oldest microbial colonies, known as stromatolites; they are widely distributed through the ancient world. A stromatolite is a layered microbial mat — a thin layer of microbial organisms together with trapped sediments, and precipitated calcium carbonate, which is then replaced by a new layer on top and so on, building up an impressive overall thickness of carbonate layers. The microbial organisms were predominantly photosynthetic bacteria — ancestors of modern cyanobacteria.
Within Ontario, there are excellent examples of less ancient stromatolites. In northwestern Ontario, at the Steeprock Lake location, in Atikokan, are Ontario’s oldest (2.8 billion years) and best-preserved shallow-water carbonate platforms, created by cyanobacterial stromatolites. The host rock is the Mosher Carbonate formation, which reaches up to 500 metres thick. The stromatolites occur as giant domes, columns, and pseudocolumnar shapes built by the ancient microbial mats.
Northeast of Thunder Bay, near Nipigon Ontario and located now on the shores of Lake Superior, are stromatolites of the Rossport Formation. These rocks are 1.45 billion years old. Just as a footnote, we think of Lake Superior as a huge freshwater body of water but it would have been dwarfed by the seas of ancient times!
There are no fossil stromatolites known in Muskoka, but we learn from these other locations. These early life forms likely existed here as well, as ancient seas covered much of Ontario.
The fascinating fact about stromatolites is that there are present-day species living in Shark Bay, Australia and in Chile, Argentina and the Bahamas, in hypersaline marine environments. During their three-billion-plus year history, photosynthesis by stromatolites released free oxygen (O2) into the environment, creating an atmosphere suitable for the evolution of animals.
Blue-green algae, today’s cyanobacteria and close cousins of stromatolites, are now a concern in some Muskoka Lakes because they occasionally become very abundant (bloom) and may produce toxins. Cyanobacteria have always been present here and are always present in all our lakes. Blooms may form when some, or all of the following occur: high levels of nutrients (phosphoros and nitrogen), calm weather and waters, strong sunlight, or high air and surface water temperatures. Our changing climate may be a key reason for the increase in reports of algal blooms. If we could learn why they are more frequent, it might be possible to reduce the tendency to bloom.
In Ontario, there is no governmental or regulatory agency completing regular, ongoing monitoring for algal blooms, algal abundance, or toxicity in the Muskoka River watershed. To help address this monitoring gap, MWC partnered with lake associations on a program to monitor algal abundance in Muskoka. The program has been running for several years and will be accepting additional lake associations for 2027. Contact us if you are interested in this program.
This article was first published by MuskokaRegion.com.
John Mason
This is article No. 39 in the current series, Nurturing Our Watersheds, from Muskoka Watershed Council. Its author is John Mason, D.Sc., professional geologist, recent Muskoka resident and a director of Muskoka Watershed Council. The series is edited by Peter Sale.