THE INFLUENCE OF NUTRIENTS ON FUNGAL GROWTH, PRODUCTIVITY, AND SPORULATION DURING LEAF BREAKDOWN IN STREAMS

In streams, aquatic hyphomycetes degrade leaf litter and transform it into a more suitable resource for invertebrate detritivores. Previous studies have demonstrated that fungi comprise a major portion of the microbial biomass associated with decomposing leaves and that inputs of leaves in the autumn reduce the concentration of soluble phosphorus in the water. These observations indicate that fungi obtain inorganic nutrients from the water passing over leaf surfaces. However, very little is known concerning the effects of nutrients on the activity of the fungi growing within leaves. To examine this question, I determined fungal biomass (from ergosterol concentrations) and sporulation rates during the breakdown of yellow poplar (Liriodendron tulipifera) leaves in three streams that differed in water chemistry. I also estimated instantaneous growth rates by measuring rates of [C-14]acetate incorporation into ergosterol and calculated fungal productivity from growth rates and biomass. Growth rates and productivity determined by this method showed good agreement with Values calculated from changes in fungal biomass during early stages of leaf breakdown for two of the streams examined. Highest concentrations of fungal biomass and greatest fungal activity occurred in the stream with the highest concentrations of nutrients. However, differences in total fungal production and biomass were not as great as were differences among sporulation rates in the three streams suggesting that fungal allocation of resources to growth and reproduction can vary depending on nutrient availability.