The keystone role of fungi in nutrient-mediated organic matter transformations

Fungi and bacteria are important decomposers of organic matter in detritus-based streams and contribute to the transfer of energy and nutrients to higher order consumers.  Aquatic fungi play a key role in modifying coarse particulate organic matter (CPOM) by affecting its breakdown rate, quality, and conversion to fine particulate organic matter (FPOM).  We hypothesized that these transformations would be accelerated under elevated nutrient concentrations and tested whether fungal presence was critical to rates of change in organic matter resources.  We manipulated the presence and absence of fungi, nutrients, and invertebrate consumers in a full factorial laboratory experiment and determined their effects on CPOM quality and mass loss, and FPOM quality and quantity.  Quality of both CPOM and FPOM, breakdown rates of CPOM and quantity of FPOM increased most in nutrient addition/fungi present treatments.  We found significant interactions between nutrients and fungi on most response variables, indicating that nutrient effects on organic matter transformations were dependent on the presence of fungi.  Our results suggest that fungi play a keystone role in facilitating energy and nutrient flow through both CPOM and FPOM food web pathways and are critical to changes in organic matter transformations due to nutrient enrichment.