Waterloo biologists model the big impacts of zebra mussels in Lake Simcoe

Thursday, March 19, 2015

 Martin Schwalbe)Zebra and quagga mussels, known collectively as dreissenid mussels, have changed the ecology of some of the largest lakes in the world, costing governments more than $7 billion in damage according to the Ontario Ministry of Natural Resources.

Biologists from the University of Waterloo, along with colleagues from Queen’s University, Environment Canada and the Ontario Ministry of Environment, have developed a new modeling tool that can be used to study and predict the ecological impacts of dreissenid mussels on Ontario’s Lake Simcoe.

Their results are published the current issue of Aquatic Sciences.

By combining two models – a physical mixing model with a biological one - the researchers simulated the complex negative effects mussels have on Lake Simcoe’s phytoplankton distribution (a staple food source for fish), temperature and oxygen levels in the water.

Results predict the mussels will have considerable impact on phytoplankton concentrations in nearshore areas, supporting the so-called Near-Shore Shunt hypothesis that mussels are re-engineering the lake’s ecosystem.

The model also predicts climate change will affect mussel populations, not by affecting phytoplankton growth, but by changing the physical mixing within the lake.

Dreissenids and the Near-Shore Shunt hypothesis

Dreissenid mussels essentially act like ecosystem engineers, filter out food and nutrients from the water column at an incredible rate.

One 20-millimetre quagga mussel can filter up to a litre of water per day,” says Ralph Smith, a Biology professor in the Faculty of Science and co-author on the paper.

This alarmingly high filtration rate not only strips food from the water column that could sustain other native fish, but it also clarifies the water, allowing more sunlight to reach deeper into the lake.

The additional sunlight combined with nutrient rich deposits has been proposed as the cause behind returning algae blooms in the Great Lakes, despite reductions in phosphorus entering the waterways.  

This theory, known as the “Near Shore Shunt Hypothesis”, was first proposed by Waterloo biologists Bob Heckey and Stephanie Gillford (both now at the University of Minnesota), Ralph Smith, and William Taylor in 2004.

Adding climate change to the mix

The model also shows how climate events such as storms and surface temperature warming can dramatically impact dreissenid numbers by changing the water dynamics of the lake.  A 60 per cent increase in the intensity of three summer storms is enough to increase phytoplankton supplies and expand mussel colonies, according to the model.

Lake Simcoe has undergone profound changes in the past thirty years,” says Smith. “Not only has it been hit by dreissenid mussels, climate change is now having an effect as well.”

Mussels are stationary and live along the upper edges of the Lake.  Their food supplies are limited to the upper lake levels but weather events, such as storms, act like a cocktail shaker and mix up the water layers – bringing fresh supplies of phytoplankton to the mussels and encouraging the mussels to multiply.

Warming the lake’s surface and creating a stable, stratified water layer acts in the opposite way, reducing mixing and restricting mussel numbers.

Charting Lake Simcoe’s fishing future

As climate predictions improve, managers can use this tool to predict a variety of scenarios, including how the mussels influence the viability of certain fish species, such as lake trout, small mouth bass and yellow perch.

Knowing which sports fish will thrive is an important management consideration as Lake Simcoe is one of Ontario’s top recreational fishing destinations.

Postdoctoral fellow Astrid Schwalb was first author on the paper.  Environment Canada’s Lake Simcoe Cleanup Fund funded the project.