Stronger effects of fungicides in the hyporheic than in the benthic zone

In this blog post, Eric Bollinger provides a brief overview of their recently published paper regarding fungicide effects on microbial leaf decomposition in sediments.

Organic matter processing in streams

A large fraction of energy is provided via the input of terrestrial plant material (e.g., leaves), especially in headwaters. A diverse community of microbes plays a principal role in the decomposition of this material. Particularly aquatic hyphomycetes (i.e., a class of endophytic fungi) are regarded as key organisms since they degrade the most recalcitrant substrates and increase the food quality and availability for macroinvertebrate shredders and filter-feeders. 


Given that fungal diseases are a major threat to agronomical productivity, results in fungicides making up almost half the sales of pesticides in the EU (mass-based). Eventually, after their application, fungicides get transported into freshwater systems via spray drift or surface runoff. In these systems, fungicides are reported to have adverse effects on microbial leaf decomposition and the fungal community.

Objective of the study

Although most of the organic material in rivers is buried in the sediment (i.e., the hyporheic zone), the effects of fungicides on leaf decomposition have been exclusively studied above the sediment (i.e., the benthic zone). This poses a major knowledge gap since both physico-chemical parameters (e.g., oxygen content) and microbial community composition are vastly different between these zones. Therefore, we assessed the effects of a mixture of commonly applied synthetic fungicides (Azoxystrobin, Carbendazim, Cyprodinil, Quinoxyfen, Tebuconazole) at three levels on microbial leaf decomposition in a test system that features a benthic-like and hyporheic-like compartment.

Caption 1: Schematic representation of the experimental setup. After distributing microbial inoculum homogeneously in and above the sediment, three leaf bags were positioned both in the benthic and the hyporheic zone. Each of the six possible variable combinations (i.e., two sediment types and three fungicide sum concentrations) was replicated seven times, containing one benthic and one hyporheic sample (each with three pseudoreplicates). The medium was renewed weekly to ensure continuous fungicide and nutrient exposure. The experiment was run for 21 d (Bollinger et al. 2021, ©2021 The Authors. Limnology and Oceanography published by Wiley Periodicals LLC on behalf of Association for the Sciences of Limnology and Oceanography, license:, full article:

Study Highlights

In line with other studies, microbial community composition was different between the benthic and hyporheic zone, and leaf decomposition was vastly impaired in the latter. Although fungicides affected leaf decomposition in both zones, effect sizes were almost two-fold in the hyporheic zone either via combined stress (i.e., oxygen deficiency + fungicides) or a community shift towards less efficient decomposers. Although this study also questions the suitability of classical methods to address fungal structure (i.e., ergosterol as biomass proxy and sporulation for species composition), these results show that the impact of fungicides on freshwater ecosystem functioning can be underestimated when ignoring the hyporheic zone.

Caption 2: Mean relative leaf mass loss with 95% confidence intervals. A: Filled circles and open diamonds refer to benthic and hyporheic data (pooled for both grain sizes), respectively. B: Open circles and squares represent fine and coarse sediment data, respectively. Solid lines with asterisks connect samples between which a significant fungicide effect was detected, while asterisks between symbols indicate a significant difference between the benthic and hyporheic zone or fine and coarse sediment, respectively, at a specific fungicide concentration (Bollinger et al. 2021, © 2021 The Authors Limnology and Oceanography published by Wiley Periodicals LLC on behalf of Association for the Sciences of Limnology and Oceanography, license: , full article: ).

The paper was authored by Eric Bollinger, Jochen Zubrod, Marco Konschak, Lenz Sulzer, Jacob Schnurr, Verena Schreiner, Ralf Schulz and Mirco Bundschuh. This study is part of the project- Project “AQUA-REG”, which is funded by the German research foundation (DFG )and has been published under open-access license in Limnology and Oceanography Volume 67.