Procesado de hojarasca en arroyos del Parque Nacional de la Sierra de Guadarrama (Madrid)

Le af litter breakdown in streams of Sierra de Guadarrama National Park (Madrid) The main goal of this study was to identify the main drivers of natural variability of leaf litter processing in six well preserved Mediterranean mountain streams in the Sierra de Guadarrama, through the analysis of the breakdown rates of alder litter, content and dynamics of leaf nutrients, and associated fungal and invertebrates communities. In general, breakdown rates of alder leaves, both exponential (k = 0.013± 0.005 dd –1 , k ′ = 0.041± 0.011 d –1 ) and lineal (b = 0.470± 0.121% PSLC dd –1 , b ′ = 1.522± 0.153% PSLC d –1 ) were very high. Variability in breakdown rates among streams was not correlated to environmental characteristics (discharge, slope or water temperature) neither to stream water characteristics (nutrient concentration, pH, or conductivity). This may be due to the low environmental heterogeneity observed among these streams and also because the physical factors that could regulate this process were outweighed by biological processes that mask them. The highest breakdown rates were recorded in the coldest waters where the activity of decomposers aquatic fungi was very low (measured by sporulation rates, 98.36±148.84 conidia mg –1 PSLC d –1 ), suggesting that consumer invertebrates played a key role in the faster leaf litter processing rates in these sites. Family richness (13.67±3.44 fam. bag –1 ) and macroinvertebrate density (25.33±10.42 ind bag –1 ), as well as shredder density (16.17±7.44 ind bag –1 ) did not show any relationship with the breakdown rates (p> 0.100, neither linear nor exponential rates). A positive relationship was only found between percentage of leaf litter lost and abundance of Limnephilidae (p< 0.010), that pointed out a higher relevance of shredder traits than their total abundance. Limnephilidae are caddisfly shredders adapted to low thermal regimes, they have large body and are able to feed on unconditioned litter debris and poor colonized by aquatic hyphomycetes, which seem to exert a decisive control over leaf litter processing rate in this type of aquatic ecosystems. Therefore, we can suppose that in colder headwater mountain streams consumer macroinvertebrates (shredders) might have a major relevance in the processing of litter debris whereas decomposers (aquatic hyphomycetes) would play an increasing role in warmer streams.