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 better 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 (1 st -4 th ) 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 chemical and physically protected leaves with low nutrient content. According to several experiments, fungal colonization facilitates the access of invertebrates to the energy trapped in deciduous leaves in streams. Some invertebrates have strategies to cope with low quality food (leaves with low microbial biomass or high chemical defenses). Those include high mobility, small size, compartmentalized digestion in the gut, presence of endosymbionts, and the capability to decrease respiration rates. The relative importance of fungi and invertebrates in the incorporation of plant litter material into secondary production varies across rivers and biomes. Shredder invertebrates seem to play a key role in litter breakdown in headwaters, but their importance appears to decrease downstream. In the same way, some systems where leaves are hard or protected, shredder invertebrates may be less abundant and the energy may be mainly recovered from litter by fungi. Eucalyptus plantations are systems with low diversity of invertebrates and aquatic hyphomycetes. Streams running through eucalyptus plantations seem therefore ideal to experimentally investigate relationships between structural parameters (biodiversity) and function. Finally, our research has been extended to other climatic areas including the Mediterranean and tropical streams. We reported a wide variety of situation in those systems. A general rule applying to all...

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With the increase in global mean surface temperature predicted for the near future, stream water temperature will also increase. Simultaneously, water quality is likely to decrease (e.g., due to increases in nutrient and pollutant concentrations). The objective of this study was to evaluate the individual and combined effects of increases in water temperature and nutrients concentration on periphyton respiration, as a surrogate for stream metabolism. Stones naturally colonized with periphyton in an unpolluted mountain stream in Central Portugal were sampled seasonally over a year, and incubated in the laboratory under two water temperatures (ambient and 4°C elevated) and two nutrients concentration levels (ambient and ∼6× higher inorganic dissolved nitrogen, ∼2× higher soluble reactive phosphorous concentrations). Overall, increases in water temperature stimulated periphyton respiration to a larger extent than did increases in nutrients concentration. In spring, the simultaneous increase in water temperature and nutrients concentration stimulated periphyton respiration beyond expected from the individual effect of each factor. These results indicate that synergistic interactions between factors might occur under certain environmental conditions, suggesting that care should be taken when predicting the combined effect of changes in multiple factors from their individual effects. The observed stimulation of periphyton respiration promoted by increased temperature and nutrients concentration can lead to changes in streams carbon budgets, with a positive feedback for global warming, as more CO2 might be released to the atmosphere.