Interactions between detritivores and microfungi during the leaf detritus decomposition in a volcanic lake (Lake Vico - central Italy)

The role of biota in the mass loss of Phragmites australis (Cav.) Trin. ex Steud was studied in the littoral belt of a central Italy volcanic lake. The research focussed on the feeding interactions between detritivores and decomposing fungi as drivers of the leaf litter decomposition. The litterbag technique was used to assess the leaf mass loss, the number of colonizing fungi and the patterns of leaf colonization by detritivores during 40 days of submersion in 16 sampling sites. Cores of bottom sediment were collected to estimate the organic content and ergosterol concentration as measure of fungal mass. The rate of leaf mass loss showed significant variability among the sampling sites and was non-linearly related to the quantity of organic depositions onto the lake bottom, peaking at about 40% of the dry matter. The rate was also positively correlated with the density of detritivore mass relative to the leaf unit mass, which increased with time. On the 20th day of litterbag immersion, when 40% of the initial leaf litter remained, we observed the best accordance between the two measures as well as the lowest difference in the detritivore mass density among sampling sites. In the absence of animals, the decomposition rate was positively related to the number of fungi on the decaying litter. The feeding activity of detritivores changed both the species richness and composition of the fungal community on the litter. The substrate reduction due to intense animal feeding appeared to limit the ability of fungi to regrow after grazing. As a result, an inverse relationship between the number of fungi and the decomposition rate was observed.