Jeanette Schlief

Long-term leaf litter decomposition and associated microbial processes in extremely acidic (pH <3) mining waters

Leaf packs were exposed in extremely acidic mining waters, removed at intervals throughout a two-year period and analysed for mass loss, microbial respiration and fungal biomass (ergosterol content). The mass loss followed an exponential trend with asymptotic values of 45-53 % of initial mass reached within the first 5 months. After this initial stage, leaves were completely encrusted by iron oxyhydroxide precipitates. The highest respiration occurred during the initial period and subsequently decreased to low levels comparable to those on an inert substrate (plastic strips) that was covered by iron precipitates. These precipitates create a barrier against microbial attack and mechanical abrasion of leaf litter and trap considerable amounts of organic matter. Ergosterol contents were highest after 3 months and varied throughout the subsequent exposure period with higher contents in autumn/winter. Unidentified HPLC-peaks may indicate that dead fungal hyphae are enclosed by iron plaques instead of being degraded. The formation and accumulation of refractory leaf litter/iron plaque layers has implications for the future development of the mining waters.

Palatability of Leaves Conditioned in Streams Affected by Mine Drainage: A Feeding Experiment with Gammarus Pulex (L.)

Both the absence of leaf shredding macroinvertebrates and low microbial activity are of major importance in determining slow and incomplete leaf decay in extremely acidic (pH<3.5) mining streams. These streams are affected by a heavy ochre deposition causing the formation of massive iron plaques on leaf surfaces that hinder microbial exploitation. An investigation was carried out to determine whether iron plaques and leaf conditioning status (acid conditioned with and without iron plaques, neutral conditioned, unconditioned) affect the feeding preference of the shredder Gammarus pulex (L.). Leaf respiration rates and fungal biomass (ergosterol contents) were measured to determine microbial colonization. Neutral conditioned leaves had significantly higher microbial colonization than acid conditioned leaves with iron plaques. Notwithstanding, leaves of both conditioning types were consumed at high rates by G. pulex. The microbial colonization had no influence on feeding preference in the experiment. It is presumed that iron adsorbed organic material caused the high palatability of leaves with iron plaques. The results indicate that the large deposits of leaves coated with iron plaques will be available to the stream food web when water quality will be restored to neutral as planed in scenarios for the future development of mining streams.

Contrasting habitats but comparable microbial decomposition in the benthic and hyporheic zone

Input of allochthonous leaf litter is the main carbon source for heterotrophic metabolism in low-order forested streams. A major part of this leaf litter is accumulated at benthic retention structures or buried in the hyporheic zone. As a result of hyporheic sediment characteristics, hyporheic physicochemistry differs from that of the benthic zone selecting the microbial community. The present study aimed at understanding the influence of the hydrological and physiochemical differences between the benthic and hyporheic zone on microbial leaf litter decomposition and on the structure and function of the associated microbial community. Leached leaves of Alnus glutinosa were exposed for 62days in 250-μm mesh bags in the benthic zone and buried in the hyporheic zone at a depth of 2-3cm. Decomposition rates were comparable for both zones. In contrast, respiration, bacterial abundance, ergosterol content, fungal spore production and richness of fungal morphotypes were lower associated with hyporheic than with benthic leaves. Microbial community structure displayed zone-dependent temporal dynamics. Thus, the microbial community carried out leaf litter decomposition independently of its structure. These results suggest that carbon processing is not necessarily impaired by environmental constraints because the community structure may compensate those constraints (i.e. functional redundancy).