Diversity and activity of aquatic fungi under low oxygen conditions

Summary 1. The objective was to test whether a decrease in oxygen concentration in streams affects the diversity and activity of aquatic hyphomycetes and consequently leaf litter decomposition. 2. Senescent leaves of Alnus glutinosa were immersed for 7 days in a reference stream, for fungal colonization, and then incubated for 18 days in microcosms at five oxygen concentrations (4%, 26%, 54%, 76% and 94% saturation). Leaf decomposition (as loss of leaf toughness), fungal diversity, reproduction (as spore production) and biomass (ergosterol content) were determined. 3. Leaf toughness decreased by 70% in leaves exposed to the highest O 2 concentration, whereas the decrease was substantially less (from 25% to 45%) in treatments with lower O 2 . Fungal biomass decreased from 99 to 12 mg fungi g −1 ash‐free dry mass on exposure to 94% and 4% O 2 respectively. Sporulation was strongly inhibited by reduction of dissolved O 2 in water (3.1 × 10 4 versus 1.3 × 10 3 spores per microcosms) for 94% and 4% saturation respectively. 4. A total of 20 species of aquatic hyphomycetes were identified on leaves exposed to 94% O 2 , whereas only 12 species were found in the treatment with 4% O 2 saturation. Multidimensional scaling revealed that fungal assemblages exposed to 4% O 2 were separated from all the others. Articulospora tetracladia , Cylindrocarpon sp. and Flagellospora curta were the dominant species in microcosms with 4% O 2 , while Flagellospora curvula and Anguillospora filiformis were dominant at higher O 2 concentrations. 5. Overall results suggest that the functional role of aquatic hyphomycetes as decomposers of leaf litter is limited when the concentration of dissolved oxygen in streams is low.