Leaf breakdown and retention of four different sized leaf species in Rutanga Stream, Gombe Stream National Park, Tanzania

Introduction The relationship between terrestrial and aquatic ecosystems is particularly important in small, forested streams (Benfield 1996, Wallace et al. 1999). All stream organisms obtain their energy from either autochthonous sources such as photosynthetic algae or from allocthonous sources such as the surrounding riparian vegetation. Small, forested streams rely heavily on allocthonous inputs of leaves and woody debris as the primary source of organic matter instead of algae due to light-limitation (Benfield 1996). In order for this organic matter to be utilized, it must first be retained in the stream and then broken down. Leaf litter breakdown is an integral part of carbon cycling, nutrient uptake, and more specifically, the food web of stream ecosystems. Leaves enter the stream following abscission and accumulate on rocks, twigs, and stream margins to form debris dams (Mathooko 2001). These debris dams provide food and habitat for macroinvertebrate shredders such as insects and crabs as well as microbes that colonize the leaves. Microbes use enzymes to obtain nutrients from the leaves while softening them in preparation for macroinvertebrate shredding. Together, microbes and macroinvertebrates break the leaves down from complete leaves to fine particulate organic matter (FPOM) that becomes suspended and carried downstream and is eventually broken down to inorganic nutrients. The length of time of leaf breakdown, from abscission to complete decomposition, varies widely, providing streams with a constant supply of nutrients during the months without leaf litter inputs (Benfield 1996). Many studies conducted in the temperate zone have shown a stream-specific continuum of leaf processing ranging from slow (more than a year) to fast (a month) (e.g. Peterson and Cummins 1974). In tropical streams, however, very little is known about leaf breakdown rates. Previous studies have identified many factors affecting the rate at which leaves are processed. It remains unclear which of these factors is the most influential. The physical and hydrological characteristics of the stream are important for initial leaf retention, while characteristics such as temperature, velocity, ambient nutrients, and pH affect breakdown rate by altering the abundance and communities of organisms present (e.g. Ardòn et al 2006, Irons et al. 1994). In addition to the importance of stream characteristics, leaf characteristics are also crucial in determining breakdown rate. Leaf quality varies based on the amount and form of carbon molecules and the concentration of nutrients such as nitrogen and phosphorus. High quality leaves contain higher concentrations of small labile carbon molecules such as simple sugars that are quickly and easily broken down by microbes. Low quality leaves, however, contain higher concentrations of larger carbon molecules, such as lignin, cellulose, and hemicellulose that are slower and more difficult to break down since microbes must use specialized enzymes to process them. Without taking other factors into consideration, there is evidence that leaf quality and nutrient content (N and P) are positively correlated to breakdown rate (Ardòn et al 2006). Essentially, microbes obtain carbon, nitrogen and phosphorous from the leaves, although it is undetermined to what extent they also rely on ambient nutrients for N and P uptake (e.g. Royer and Minshall 2001, Ardòn et al 2006). Leaf quality differs based on leaf species but may also be affected by precipitation, soil quality, age of leaf, access to light (either via the leaf’s position on the tree or the tree’s position in the canopy), and possibly leaf size. This study examines the retention and breakdown rates of four different leaf species in a tropical stream. Tree species that bear large leaves may devote more energy to the production of structural components such as lignin, resulting in lower quality leaves. Thus, large-leaved species may break down at a slower rate than small-leaved species as a result of differences in quality. On the other hand, it is possible that large leaves are more likely to be retained in the stream and thus breakdown more rapidly due to a pre-existing assemblage of organisms.