Quagga Mussel Monitoring in Lake Zurich — Tracking an Invasion in Real Time

We just completed a Quagga mussel monitoring survey in Lake Zurich together with the Amt für Abfall, Wasser, Energie und Luft (AWEL), Kanton Zürich, led by Thomas Müller. The results are a clear signal of how quickly this invasion is progressing.

The population is growing fast

In 2024, we had our first record of the Quagga mussel (Dreissena rostriformis bugensis) at Mythenquai. Now, at 10 m depth off Männedorf, Erlenbach, and Oberrieden, we are already finding a substantial population. We still need to sort and process the samples from the other depths, but the pattern at 10 m alone is a strong indication of how established the species already is.

Left: Looking across the lake toward the sampling area near Männedorf. Right: Sorting a sample by hand — shells, plant material, and debris need to be separated before mussels can be counted. Photos: © Julie Conrads.

Last week, using a different sampling method, we also confirmed the species at 55 m depth. That the population is already present at that depth is worth taking seriously.

Why this matters beyond the count

This is directly connected to the benthos monitoring work I’ve been doing on Lake Zurich. We first came across Quagga mussels during a macrozoobenthos sampling campaign, and I’m continuing to sample benthos specifically so we can track what the invasion does to the wider community over time.

What makes this particularly valuable is the timing: we caught the invasion at a very early stage, one that is normally missed entirely. Most monitoring only picks up an invasive species once its population has already grown large enough to be obvious. Catching it this early gives us a rare opportunity to follow the impact on the community from close to the beginning, rather than only after the fact.

Lake Zurich is already dealing with the effects of multiple invasive species, and I am currently writing up a paper on how the lake’s benthic community has been affected so far. The Quagga mussel invasion will very likely add another detrimental layer on top of that. Continuing the benthos monitoring means we can follow this as it happens, not reconstruct it after the fact.

More on this ongoing work can be found on the Benthic Ecology: Lake Zurich project page.

A single handful from the sample contains three invasive species at once: Asian clam (_Corbicula fluminea_), Zebra mussel (_Dreissena polymorpha_), and Quagga mussel (_Dreissena rostriformis bugensis_) — several of the Quagga mussels are already fairly large. Photos: © Julie Conrads.