2006: Leaf litter processing in low order streams

Forests produce a large amount of detritus, that inevitably end up in streams, subsidizing aquatic systems with organic matter and nutrients. Here we review some of the research carried out at the University of Coimbra with the objective of getting a bet-ter understanding of the breakdown process of these materials and its incorporation to secondary production. Litter-fall in deciduous forests in Central Portugal can reach up to 750 g AFDM of leaves m-2 yr-1, with 73 % of the litter produced between October and December. In several retention experiments, we measured a 90 % leaf retention in low order (1st- 4th) streams within 15 – 70 m, and a standing stock of up to 450 g AFDM m-2. The amount of nutrients in the water and the plant physical and chemical defenses can be an indicator of the rate at which plant material is incorporated into secondary production or exported as dissolved and fine particles of organic matter. Respiration rates of decomposing leaves incubated with fungicides were severely reduced, supporting the idea that fungi are very important agents in litter breakdown. The fungi group known as aquatic hyphomycetes are capable of producing enzymes able to cause leaf maceration, and by 2 to 3 weeks, up to 15 % of the decomposing leaf biomass corresponds to fungi. Shredder invertebrates are also biological agents involved in litter breakdown. Given their densities and feeding rates, we measured consumption rates of 12 – 54 g of leaves m-2 yr-1 in a stream in Central Portugal, corresponding to 2 to 9 times the litter standing stock. Feeding rates were high in nutrient rich leaves and low in che-mical and physically protected leaves with low nutrient content. According to several experiments, fungal colonization facilita