Weed killers change the diet of freshwater snails

In this post, Marco Konschak reports on the recently published paper “Herbicide-Induced Shifts in the Periphyton Community Composition Indirectly Affect Feeding Activity and Physiology of the Gastropod Grazer Physella acuta”.

Weed killers in freshwaters

Weed killers, also known as herbicides, are one of the main pollutants in European rivers. They accidentally enter aquatic river ecosystems where they pose a hazard to a variety of aquatic primary producers. For instance, freshwater periphyton, defined as submerged substrate-associated biofilm that mainly consists of green algae and diatoms, can be directly affected by this pesticide group. Herbicides cause adverse effects in sensitive algae species, and thus induce shifts in the periphyton community composition to more tolerant taxa. Since these freshwater biofilms are an important high-quality food source for herbivorous invertebrates (known as grazers), herbicides may also indirectly have an impact on grazers due to changes in their diet. However, little is known about these diet-related effects in grazers.

The setup

The researchers assessed diet-related herbicide effects on the feeding activity and physiology (i.e. somatic growth and energy storage) of the model snail grazer –  Physella acuta. To achieve this, snails were fed for 21 days with periphyton that grew for 15 days in the absence or presence of the model herbicide diuron. To uncover indirect herbicide effects in P. acuta via the dietary pathway, potential diuron-induced changes in the biomass, the algae community composition and the fatty acid  profile (a proxy for food quality) of periphyton were investigated.

Figure 1: An illustration of the model snail grazer Physellaacuta (graphic by M. Konschak)

Diet changes of snails

The results showed that the feeding activity of snails fed with periphyton that grew in presence of diuron was increased over the study duration compared to the test organisms that were cultured under control conditions. The enhanced feeding activity ultimately led to an increased somatic growth and energy storage. These effects on P. acuta can be explained by diuron-induced shifts in the algae community composition of periphyton, which resulted in changes in the nutritional quality of periphyton that grew in the presence of the herbicide. Indeed, fatty acids analyses revealed that periphyton exposed to diuron had an enhanced content of highly unsaturated fatty acids, which are critical for the physiological fitness of grazers. Thus, the researchers suggest that diuron exposure most likely led to a shift in the periphyton algal community composition in favor of diuron-tolerant algae that also contains higher amounts of highly unsaturated fatty acids. The increased nutritional quality of periphyton exposed to diuron, in turn, probably stimulated the feeding activity of P. acuta.

Figure 2: Physellaactua feeding on periphyton that grew in the presence of the herbicide diuron (photo by M. Konschak)

The paper was authored by Marco Konschak, Jochen Zubrod, Tomás S. Duque Acosta, Agnès Bouchez, Alexandra Kroll, Alexander Feckler, Nina Röder, Patrick Baudy, Ralf Schulz and Mirco Bundschuh, and published in Environmental Science and Technology