by André Martel and Jacqueline Madill

During some of the hottest days this summer, my colleague Jacqueline Madill and I are donning our snorkeling gear and looking at native freshwater mussels (Unionidae family) in riffle and high-current sites in the Rideau River, Ontario.

A man floats at the surface with his face in the water.
Here, I am looking for native freshwater mussels (Unionidae family) in a square quadrat (71 cm × 71 cm, or 0.5 square metres) while snorkeling in the Rideau River, Ontario. Image: Jacqueline Madill © Canadian Museum of Nature

From 1999 to 2001, a multi-disciplinary study called the Rideau River Biodiversity Project led by the Canadian Museum of Nature was undertaken in collaboration with universities, governments and other agencies.

Our mussel team selected and described eight ideal habitats for native freshwater mussels from Smiths Falls to Ottawa: Old Slys, Kilmarnock, Andrewsville, Burritts Rapids, two locations around Manotick Island, Billings Bridge and Sandy Hill. This summer we are revisiting these sites.

A view of the river.
This historical hotspot for freshwater mussels in the Rideau River is at Billings Bridge in Ottawa. Image: André Martel © Canadian Museum of Nature

Our return prompts the question, “If we did a complete survey in 1999–2000, why are we doing it all over again”? Some of these sites were amazing “hotspots” of diversity and abundance for the native freshwater pearly mussels, especially the upstream sites, which had not yet been subjected to the impact of the invasive zebra mussel (Dreissenidae family).

Our native freshwater pearly mussels are known to be good indicator species for the health of the environment. In 2016, we will evaluate the health of the Rideau River by studying the native mussel populations and reporting on how they have been affected by this invasion—and possibly by other stressors—during the past 16 years.

A hand holds a mussel.
A live eastern elliptio (Elliptio complanata, Unionidae family) from the Rideau River at Sandy Hill, which is a neighbourhood in Ottawa. Image: André Martel © Canadian Museum of Nature

North America has the richest native freshwater mussel fauna in the world (300 species and subspecies). During the past century, unfortunately, mussel populations have declined worldwide. Water pollution, river-shoreline and wetland degradation, impoundments (dams and weirs), agricultural, industrial and urban runoffs, and siltation have all contributed to the freshwater mussels’ decline. In North America, the most recent stressor has been the introduction of the invasive zebra mussel, originally from the Ponto-Caspian region of Eastern Europe and Asia.

A man holds mussels in both hands under water.
Here, I’m gently lifting freshwater mussels from the river bottom. Notice that the freshwater mussels (Unionidae family) are covered with many zebra mussels (Dreissenidae family). Image: Jacqueline Madill © Canadian Museum of Nature

The zebra mussel has made a swift impact since our last surveys of the Rideau River. In 2001 we found that all species of native freshwater mussels had been extirpated from the Mooney’s Bay area of the Rideau River in Ottawa during an eight-year span (e.g., Martel et al, 2001).

While our native mussels live partly buried in the sediment on the bottom of the river, the invasive zebra mussel attaches itself to just about any hard object, from rocks to native-mussel shells to water pipes. When the zebra mussels attach to their shells, our native freshwater pearly mussels eventually suffocate, unable to move freely or eat plankton.

A hand holds a mussel that is half covered in smaller mussels.
Zebra mussels (Dreissenidae family) encrust this weathered freshwater mussel shell (Unionidae family) from the Rideau River at Manotick, Ontario. Image: André Martel © Canadian Museum of Nature

In the past, the Rideau River basin has had an abundant and diverse native freshwater pearly mussel fauna (more than 12 recognized species). We know this from records of deposited shells in the national mollusc collection at the Canadian Museum of Nature, as well as from field surveys and field collecting conducted during the 1980s and 1990s. The Rideau River probably had one of the richest populations of native freshwater pearly mussels in Eastern Ontario.

A hand holds a mussel.
This live freshwater mussel is called the flutedshell (Lasmigona costata, Unionidae family) and was found near Manotick. The arrow points to a clump of black byssal threads that had been used by zebra mussels to attach to the shell. Image: André Martel © Canadian Museum of Nature

Sixteen years after our latest surveys, we are wondering how the Rideau River is doing and what has happened to its rich native mussel fauna. What are we expecting to find? We are hopeful that some populations of the native mussel species will have survived the invasion of zebra mussels. How might some survive this invasion?

Studies have shown that some populations can remain in certain shallow water habitats, or deltas, where soft sediment and adjacent aquatic plants are not favourable to the invasive zebra mussels. Such shallow-water, soft-sediment habitats can allow the native mussels to burrow completely, for extended periods of time, and in this way get rid of zebra mussels that have attached to their shells. By doing so, native mussels literally suffocate the attached zebra mussels; after some time, the native pearly mussels return to the surface in their normal position.

A view of the river.
Near Manotick at David Bartlett Park is another portion of the river where freshwater mussels had previously been found. Image: André Martel © Canadian Museum of Nature

The zebra mussel is there to stay, but it is our hope that in the Rideau River there are a few habitats, or refugia, where some populations of native freshwater pearly mussels will thrive.

Stay tuned for our next article and we will let you know what we find.