Contributions of bacteria, fungi and detritivorous macroinvertebrates (shredders) to leaf litter breakdown in freshwater are studied by Hieber & Gessner (2002), in Germany. They have enclosed alder (Alnus glutinosa) and willow (Salix fragilis) leaves in coarse-mesh bags (5 g dry mass), placed them in the stream during peak leaf fall and retrieved periodically to study. Leaves have been retrieved after 1, 3, 7, 14, 28 and 55 days of submersion. In is shown that leaves decompose rapidly with the exponential breakdown coefficients k of 0.035 per day (alder) and 0.027 per day (willow). Leaves are also quickly colonized within the first 4 weeks of decomposition, when shredder biomass reach 263 and 141 mg dry mass per litter bag, respectively. Maximum bacterial numbers (5,6 and 4,8 x 10¹⁰ g^-1 detrital dry mass) are attained after 8 weeks and corresponded to biomass of 3,6 (alder) and 3,1 (willow) mg dry mass g^-1. This value is less than 5% of the maximum fungal biomass (77 and 70 mg dry mass g^-1, respectively). Aquatic hyphomycetes are equivalent to the daily conidial production of 9.4 and 2.9 mg dry mass g^-1 on alder and willow, respectively. Shredder feeding rates indicate that shredders account for the largest portion of overall leaf mass loss (64 % and 51 % on alder and willow leaves, respectively). According to Hieber & Gessner (2002), fungi contribute at least 15 % and 18 % to leaf litter background, bacterial contribution is estimated at the substantial (7 % and 9 %) levels.

As mentioned above, Hieber & Gessner (2002) have found that conditioned leaves of alder, A. glutinosa, decompose faster (92 % of initial dry mass after 8 weaks) than leaves of willow, S. fragilis (74 %). According to Gessner & Chauvet (1994), leaf decomposition rate (in stream in the French Pyrenees) decreases in the following order: ash (Fraxinus excelsior) (k = 0,059), wild cherry (Prunus avium), alder (A. glutinosa), hazel (Corylus avellana), sycamore (Platanus hybrida), beech (Fagus sylvatica) and evergreen oak (Cuercus ilex) (findings for sycamore and oak are given for comparison). Decomposition rates (in stream in the Northern Appenins, Italy) of conditioned leaves of maple, Aser pseudoplatanus (0,054), and elm, Ulmus minor (0,036), are also high (Gazzera et al., 1993).

Numerous papers have been published that directly examine decomposition in leaf mixtures as well as in the component species decaying alone (for review, see Gartner & Cardon, 2004). From these litter-mix experiments, it is clear that decomposition patterns are not always predictable from single-species dynamics. Non-additive patterns of mass loss are observed in 67 % of tested mixtures, mass loss is often but not always increased when litters of different species are mixed.

In the experiments by Hieber & Gessner (2002), macroinvertebrates have been represented chiefly by stonefly larvae Nemoura, Protonemura, Amphinemura and Leuctra, caddisfly Potamophylax as well as amphipod Gammarus fossarum. Collector-gatherers and scrapers are mayfly larvae such as Baetis and Leptophlebiidae, together accounting for an additional 44% of total numbers but only 19% of biomass. Maximum densities of macroinvertabrates have been observed to 4 weeks after leaf submersion, with an average of 708 (alder) and 422 (willow) animals per 5 g leaf pack (Hieber & Gessner, 2002).

For comparison, leaves of beech, F. sylvatica, are colonized in Irish rivers by macroinvertebrates of 76 taxa (Murphy et al., 1998).

Tiegs et al. (2008) have manipulated the quantity of leaf litter to test whether: 1) greater litter quantity promotes microbial leaf decomposition (through greater microbial inoculum potential), and 2) reduced litter quantity enhances decomposition by leaf-shredding invertebrates (because shredders aggregate on rare resource patches). Decomposition rates and macroinvertebrate colonization of alder leaves placed in coarse- and fine-mesh litter bags, an approach intended to allow or prevent access to leaves by leaf-shredding macroinvertebrates, are also determined. Tiegs et al. (2008) have found that the effects caused by manipulations of litter quantities on leaf decomposition and macroinvertebrate colonization are relatively weak, such large and highly mobile shredder as amphipod G. fossarum can play an instrumental role in causing differences in decomposition in response to litter manipulations.

Basic References

Gartner T.B., Cardon Z.G. 2004. Decomposition dynamics in mixed-species leaf litter. Oikos 104, 230-246

Gazzera S.B., Cummins K.W., Salmoiraghi G. 1993. Elm and maple processing rates: comparisons between and within streams. Annales de Limnologie 29, 189-202

Gessner M.O., Chauvet E. 1994. Importance of stream microfungi in controlling breakdown rates of leaf litter. Ecology 75, 1807-1817

Hieber M., Gessner M.O. 2002. Contribution of stream detrivores, fungi, and bacteria to leaf breakdown based on biomass estimates. Ecology 83, 1026-1038

Murphy J.F., Giller P.S., Horan M.A. 1998. Spatial scale and the aggregation of stream macroinvertebrates associated with leaf packs. Freshwater Biology 39, 325-337

Tiegs S.D., Peter F.D., Robinson C.T., Uehlinger U., Gessner M.O. 2008. Leaf decomposition and invertebrate colonization responses to manipulated litter quantity in streams. Journal of the North American Benthological Society 27, 321-331