Dynamic pollutant carriers: The quality of suspended matter in Lake Geneva

Suspended matter is an often underestimated but crucial component of aquatic environments. It binds nutrients and pollutants and transports them through the water column. A new study from Lake Geneva shows how significantly the chemical and ecotoxicological quality of these particles can change over time – and what role extreme weather events, tributaries and biological processes play in this.

Countless microscopic particles, known as suspended matter, float in the water of lakes. They act as mobile collectors of substances from the surrounding environment: dissolved substances accumulate on their surfaces and are transported further. In this way, the particles form a dynamic link between the water column and the sediment. It is precisely this dynamic that makes them particularly interesting to researchers. Whilst sediments tend to represent a long-term archive of environmental conditions, suspended matter reflects short-term changes in a lake’s material balance.

Suspended matter as a key compartment

Suspended matter serves as a food source for many organisms, yet it can also contain pollutants. Its chemical composition and potential ecotoxicity therefore provide important insights into the health of a lake. The Ecotox Centre characterised the quality of suspended matter in Lake Geneva from 23 March to 24 April. “Thanks to the LéXPLORE experimental platform, we were able to install suspended matter traps at various depths and use them for sampling,” explains project leader Rébecca Beauvais. “We then chemically analysed the particles and observed their effect on the growth and survival of ostracods.”

As expected, the concentration of suspended matter was particularly high in summer, as snowmelt leads to increased inflow via the Rhône. However, peak values were also noticeable in connection with heavy rain and high waves, which led to the resuspension of sediments in the lake and its tributaries.

Suspended matter was collected from Lake Geneva using suspended matter traps.

Effects on the mortality and growth of ostracods

A quarter of the suspended sediment samples collected had a toxic effect on ostracods; the impact on mortality tended to be highest in spring. The growth of the animals was also affected differently depending on the season: in summer, growth rates were significantly higher than in the other seasons. During periods of heavy rainfall, the animals’ growth slowed.

Metals: Some exceedances of guideline values

Metal concentrations for cadmium and lead were in line with previous sediment surveys, whilst those for chromium, nickel and zinc repeatedly exceeded their reference values. Copper concentrations fluctuated significantly. However, there was no direct correlation between the elevated metal concentrations and the observed effects on the ostracods. In some cases, mortality increased despite low metal concentrations – an indication of other toxicologically relevant factors or combined effects. The biotests confirmed that the ecotoxicological quality of the suspended matter in Lake Geneva was dynamic in both time and space.

A supplementary non-target analysis using high-resolution mass spectrometry identified 1473 organic compounds in the suspended matter, with varying patterns depending on the season. A greenish sample in spring 2023 was notable, showing a high degree of similarity to metabolites of cyanobacteria. Such compounds can potentially have a toxic effect on aquatic organisms. Indeed, the average ostracod mortality rate was highest during this period.

Implications for monitoring and water protection

The study confirms that suspended matter is a highly dynamic key compartment that reflects both inputs from the catchment area and internal biological processes. High-temporal-resolution monitoring allows critical time windows, such as algal blooms or extreme weather events, to be identified, during which there may be increased risks to aquatic organisms. In the long term, consideration should be given to whether suspended solids should be incorporated into monitoring programmes for surface waters.

Publication

Beauvais, R., Lafargue, O., Casado-Martinez, C., Ferrari, B. J. D., Dubois, N., & Pasche, N. (2026). Qualité des matières en suspension du Léman. Évaluation écotoxicologique et chimique. Aqua & Gas, 106(3), 38-46. , Institutional Repository