Diversity and activity of aquatic fungi under low oxygen conditions

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 x 10(4) versus 1.3 x 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.