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.

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.

Tomorrow we continue to the Obersee (Upper Lake Zurich) for another round of macrozoobenthos sampling.

Originally shared on LinkedIn. Fieldwork carried out in collaboration with AWEL, Kanton Zürich (Thomas Müller) and Eawag.

Sampling approach

Below 30 m, where diving is no longer practical, mussels are collected using Ponar grabs deployed from the Eawag boat “Calypso”. A Ponar grab closes on contact with the sediment, capturing a fixed volume of the top layer. The contents — mud, shells, and anything living in between — are then washed on deck and sorted.

I will post about scientific diving soon — stay posted!

Density varies strongly with depth

The number of grabs needed per station depended heavily on depth. At 80 m, a single Ponar grab yielded enough mussels — densities were high enough that one sample was sufficient. At 200 m, densities were far lower, and three grabs per station were required to collect a comparable number of individuals.

This gradient in density is itself ecologically informative. The deep zone of Lake Constance is cold, dark, and food-limited, conditions that appear to support smaller populations — and, as this project will test, a distinctly different shell shape.

Fieldwork carried out May 2026. This project is part of a broader research initiative on Quagga mussel morphology led by Joana L. Santos (Eawag).

Why monitor these lakes?

The Quagga mussel reached Switzerland via Lake Constance and has since spread westward through major lake systems. Its arrival in the Three Lakes region represents a significant step in that dispersal. Unlike a single observation, repeated monitoring is what reveals population dynamics: how fast densities are growing, whether the mussel is expanding into new depths.

The sampling campaign

Sampling was carried out on the Eawag boat “Calypso” across all three lakes, using Ponar grabs and a benthic imaging system to collect and document benthic material at multiple depths. More information on the monitoring approach can be found at the Eawag Quagga Mussel Competence Center.

What the connected lake system means for spread

The three lakes are not isolated. The Hagneck Canal connects the Aare to Bielersee, and the Broye Canal connects Bielersee to Murtensee, which in turn drains into the Broye river leading to Neuenburgersee. This hydrological connectivity means that once the Quagga mussel is established in one basin, larval dispersal and boat traffic provide multiple pathways into the next.

Campaign led by Thomas Müller (Eawag). Sampling carried out May 2026.

Lake Zurich is a drinking water source for over 1.3 million people and a recreational lake for millions more. What most visitors don’t see is the benthic community, the invertebrates living on and in the sediment of the lake floor. Today, non-native species account for 85% of all individuals in the shallow Lower Lake. The community is no longer primarily native. It is an assembled mixture of species from across the globe, most of which arrived unintentionally.

The timeline of arrivals spans more than 100 years.

The arrivals

~1978: New Zealand mudsnail (Potamopyrgus antipodarum)

Tiny, just a few millimetres, and capable of reproducing without males. This snail spread rapidly through European waterways via boat traffic and waterbird movements. In Lake Zurich’s Lower Lake, it once reached nearly 29,000 individuals per square metre. Recent surveys found only ~676 ind. m⁻², a ~43-fold decline, likely driven by competition with later arrivals.

~1978: Zebra mussel (Dreissena polymorpha)

Native to the Ponto-Caspian region, introduced to Western Europe via ballast water. It filter-feeds intensively, encrusts hard surfaces, and quickly became the dominant bivalve of European lakes. In the Lower Lake today, densities average ~10,000 ind. m⁻², an 88-fold increase since its first detection. It currently dominates the benthic community.

~1985: Signal crayfish & Rainbow trout. Both from North America, introduced deliberately for aquaculture and sport fishing. Crayfish introductions are particularly damaging: they disturb sediment, predate invertebrates, and outcompete native crayfish, which are now largely absent.

~2000: Killer shrimp (Dikerogammarus villosus)

Also from the Ponto-Caspian region. One of the most aggressive freshwater amphipods in Europe: it attacks and kills prey it does not even eat, including native invertebrates, small fish, and other invasive species. Its arrival marked a broader wave of Ponto-Caspian invaders moving westward through European river networks.

~2010s: Asian clam, Rockpool shrimp, North American amphipod. Each adding another layer to an already heavily invaded community.

The latest arrival: Quagga mussel

In September 2024, during my own benthic field surveys, I found the Quagga mussel (Dreissena rostriformis bugensis) in Lake Zurich, the first confirmed record. Like the Zebra mussel, it originates from the Ponto-Caspian region and arrived in Western Europe via ballast water. It first appeared in Switzerland in Lake Constance in 2014.

The Quagga mussel is considered even more ecologically impactful than the Zebra mussel: it tolerates deeper water, softer sediments, and can reach extraordinary densities. In Lake Constance, established populations already reach ~8,000 ind. m⁻². In Lake Zurich today, densities are still low, between 28 and 74 ind. m⁻². The invasion is in its early stages. But history suggests what comes next.

The full picture

Looking at a century of arrivals together, a pattern emerges. The most impactful species, Zebra mussel, New Zealand mudsnail, Killer shrimp, and Quagga mussel, share the same origin (Ponto-Caspian region) and the same pathway (shipping and waterway connectivity). They are ecological generalists that thrive in disturbed, invasion-saturated conditions.

The Quagga mussel is not the end of this story. It is the latest chapter.

The invasion chronology is based on first records in the Lower Lake compiled from AWEL (Amt für Abfall, Wasser, Energie und Luft, Kanton Zürich), Burla, H. & Ribi, G. (1998). Population density of the snail Potamopyrgus antipodarum in Lake Zurich. Aquatic Sciences 60(1), 33–39, and InfoFauna – Centre Suisse de Cartographie de la Faune (infofauna.ch).

Here is that story.

A routine sampling trip

In September 2024, Jukka Jokela (Eawag) was snorkeling at Mythenquai, a public bathing area on the western shore of Lake Zurich, collecting benthic samples at 1 m depth. This was part of the benthic community survey of Lake Zurich, the same survey that forms the foundation of my first PhD project. Nothing about the day suggested it would be unusual.

Among the material collected were small mussels that didn’t look quite right for Zebra mussels (Dreissena polymorpha), which had been present in Lake Zurich since the 1970s. The shells were rounder and lacked the characteristic angular keel of the Zebra mussel.

Lars Sturm, then working at Eawag and now a PhD student at Simon Fraser University, examined the specimens and made the first identification: Quagga mussels.

Confirmation

Visual identification of Dreissena species can be uncertain: the two species overlap in shell morphology, and juveniles are particularly difficult to distinguish. The specimens were passed to lab technician Marco Giulio (Eawag), who confirmed the identification using COI barcoding, a standard DNA-based method that uses a short, diagnostic segment of the mitochondrial cytochrome oxidase I gene to identify species with certainty.

The result was unambiguous. The mussels were Dreissena rostriformis bugensis, the Quagga mussel. The first confirmed record in Lake Zurich.

The response

The cantonal authorities (Kanton Zürich) moved quickly. Within days, the findings were made public and a launch ban was imposed on several other lakes in the canton specifically Greifensee, Pfäffikersee, and Türlersee, as a precautionary measure to prevent further spread via recreational watercraft.

Boat cleaning is now central to the management response. The SMRP (Schiffsmotoren-Reinigungspflicht), mandatory boat cleaning requirements in the canton of Zurich, targets precisely the pathway by which species like the Quagga mussel move between water bodies: on and in the hulls, motors, and equipment of recreational boats. Quagga mussel larvae and juveniles are microscopic. They are invisible to the naked eye and survive for hours in residual water. Cleaning, draining, and drying watercraft between launches is the most effective measure currently available.

What comes next

The Quagga mussel was first detected in Switzerland in Lake Constance in 2016. By 2024 it had reached Lake Zurich. In Lake Constance, established populations now reach densities of around 8,000 individuals per square metre. In Lake Zurich at the time of discovery, densities were between 28 and 74 individuals per square metre.

The invasion is in its early stages. The ecological trajectory of this species in Lake Zurich, and what it means for the benthic community already dominated by invasive species, is exactly what my PhD research is designed to track. How the Quagga mussel is spreading and adapting genetically across Swiss lakes is the focus of my second PhD project on invasion genomics.

In the media

Gewässer in Gefahr: Invasion der Quaggamuschel – das können wir dagegen tun (SRF Wissen, December 2024)