Leaf litter processing in low order streams

Abstract

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...

Respiration regimes in rivers: Partitioning source‐specific respiration from metabolism time series

“…4a) were needed to maintain the estimated leaf litter contribution to ER. This range is comparable to results from field studies in temperate streams reporting leaf litter standing stocks of as low as 4 g of ash free dry mass (AFDM) m À2 (Sycamore Creek, Sonoran Desert, Wallace et al 1999) and up to 450 g AFDM m À2 (deciduous forest streams in Portugal, Graça and Canhoto 2006). Direct observation in one of the study sites, Walker Branch, allowed a more refined assessment of the model results and confirmed that model estimates of leaf litter-related respiration for this site are reasonable.…”

Invasion of Native Riparian Forests by Acacia Species Affects In-Stream Litter Decomposition and Associated Microbial Decomposers

“…In forest streams, the plant matter supplied by the riparian vegetation (e.g., leaves, wood) represents a major source of organic carbon for aquatic food webs [5]. In these ecosystems, plant litter decomposition is a fundamental ecosystem process mediated by the activities of microbes, namely fungi and bacteria, and invertebrate detritivores [6]. Fungi, particularly aquatic hyphomycetes, and bacteria produce a large variety of extracellular enzymes that promote leaf maceration and increase leaf palatability to invertebrate detritivores [6,7].…”

“…In these ecosystems, plant litter decomposition is a fundamental ecosystem process mediated by the activities of microbes, namely fungi and bacteria, and invertebrate detritivores [6]. Fungi, particularly aquatic hyphomycetes, and bacteria produce a large variety of extracellular enzymes that promote leaf maceration and increase leaf palatability to invertebrate detritivores [6,7]. Aquatic hyphomycetes are key decomposers of litter particularly in the initial stages of the process, while bacteria only increase their contribution when litter has been partially…”

Dendritic network location mediates detritivore community structure and associated processing of leaf litter in a riverine ecosystem

“…It appears headwaters created more favorable environments for consumer-resource interactions to develop. The decrease in stream width and discharge may increase retention time for leaves (Graça and Canhoto 2006) and possibly promote consumer mobility (Gjerløv et. al.…”