Hydrobiologia 519: 207–210, 2003.© 2003 Kluwer Academic Publishers. Printed in the Netherlands.

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Short research note

Leaf litter processing rates in a Kenyan highland stream, the Njoro River

Michael Dobson1, Jude M. Mathooko2, Fidensio K. Ndegwa2,∗ & Charles M’Erimba2

1Department of Environmental and Geographical Sciences, Manchester Metropolitan University,Chester St., Manchester, M1 5GD, U.K.E-mail: m.dobson@mmu.ac.uk2Department of Zoology, Egerton University, P.O. Box 536, Njoro, Kenya∗Present address: Centre for Complementary Medicine and Biotechnology (CCMB), Kenyatta University,P.O. Box 43844, Nairobi, Kenya

Received 15 July 2003; in revised form 31 October 2003; accepted 11 November 2003

Key words: detritus breakdown, k-values, Syzygium, Vanguera, Rhus, Dombeya, Pittosporum

Abstract

Mass loss of leaf litter from five common riparian trees in Kenya was determined using mesh bags over a 70-day period. The study was carried out in a river which previous research has shown to be apparently devoid ofinvertebrate shredders. There was a five fold difference between the most rapid and slowest decay rates, but allwere within the fast range identified from temperate zone trees, demonstrating high rates of microbial activity. Weconclude, therefore, that the low biomass of invertebrate shredders in Kenyan highland streams cannot be explainedby poor quality of the leaf litter that enters them.

Shaded stream channels are heavily dependent upondetritus derived from the adjacent terrestrial envir-onment for their energy resources, and establishedtheory states that a key component in its processingis the activity of large invertebrate shredders. How-ever, shredding invertebrates normally occur at muchlower densities in tropical streams relative to those intemperate zones, suggesting a qualitative differencebetween streams in temperate and tropical regions.The reason for the paucity of shredders is unclear, astropical stream channels often contain plenty of poten-tial food resources for this guild. For example, Dobsonet al. (2002) demonstrated high standing stocks ofdetritus in several Kenyan highland streams, and pro-posed a series of possible explanations for the absenceor low density of shredders. One of these related toleaf litter quality; they speculated that riparian trees inKenya produce leaf litter that is much more refract-ory, and therefore of considerably lower nutritionalvalue, than that produced by riparian trees in the northtemperate zone. This proposal was based upon littlemore than a cursory examination of leaves from the

dominant riparian trees, but it was supported by Mat-hooko et al. (2000a), who determined a slow decayrate for leaves of the common riparian tree Syzygiumcordatum (Hochst ex Krauss). In contrast, it was ap-parently contradicted by a second study on this river,carried out on leaves of another common riparian tree,Dombeya goetzenii (K. Schum), which demonstrateda high decomposition rate among leaves permanentlysubmerged within the river channel (Mathooko et al.,2000b). This latter study had, however, been carriedout to compare decomposition rates in aquatic andterrestrial situations and, as the authors stated, theirinitial preparation of leaves may have altered naturaldecomposition processes.

The aim of the present study, therefore, was totest the breakdown rate of Kenyan riparian leavesby examining a wide range of leaf species in theNjoro River (0◦ 22′ S, 36◦ 56′ E) at Egerton Univer-sity, a river that is particularly interesting because itsbenthic fauna contains no species that can be allocatedto the shredder guild (Dobson et al., 2002). Break-down rate provides a good estimate of detritus quality;

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Table 1. Physico-chemical parameters measured on each sampling date. Note that temperatureswere recorded in early-mid morning, and are probably close to the daily mean.

Date Temperature Dissolved Conductivity pH

(1999–2000) ( ◦C) oxygen (µs cm−1)

(% saturation)

1 November 14.0 61 105 6.7

4 November 12.8 53 111 7.0

8 November 13.1 67 129 7.0

15 November 14.4 84 150 7.1

29 November 14.0 73 201 7.3

27 December 14.6 81 172 7.2

10 January 13.7 76 187 7.3

Table 2. Examples of coefficients of decay (K: absolute values) recorded from studies carried out inthe tropics. In all cases cited, mesh size was coarse enough to allow macroinvertebrate access. Thetable states whether the K value was based upon ash free dry mass (AFDM).

Species Location K AFDM? Reference

Trema micrantha Costa Rica 0.559 No Irons et al. (1994)

Croton gossypifolius Colombia 0.065 Yes Mathuriau & Chauvet (2002)

Vanguera madagascariensis Kenya 0.047 No This studyPittosporum viridiflorum Kenya 0.043 No This studyRhus natalensis Kenya 0.026 No This studyClidemia sp. Colombia 0.024 Yes Mathuriau & Chauvet (2002)

Hibiscus tiliaceusa Hawaii 0.023 No Larned (2000)

Syzygium cordatum Kenya 0.022 No This studyDombeya goetzeni Kenya 0.021 Yes Mathooko et al. (2000b)

Pithecellobium longifolium Costa Rica 0.020 No Irons et al. (1994)

Dombeya goetzeni Kenya 0.010 No This studyPsidium guajavaa Hawaii 0.008 No Larned (2000)

Syzygium cordatum Kenya 0.001 Yes Mathooko et al. (2000a)

aNot native, but naturalised in the study area.

most breakdown processes are biologically mediated(microbial decomposition, invertebrate consumption),and breakdown rate is an indirect measure of biolo-gical activity and therefore of potential litter quality asa food resource.

The study site was heavily shaded by trees, butwith nearby cultivation on each side. The river chan-nel was ca. 4 m wide and averaged 30–40 cm depthduring the period of the experiment. Five leaf spe-cies were used, from Dombeya goetzeni, Syzygiumcordatum, Pittosporum viridiflorum (Sims), Rhusnatalensis (Berhn.) and Vanguera madagascariensis(Gmel.). All are common evergreen riparian species atthe study site and between them they account for mostof the tree species encountered in the area (Magana,2000) Leaves were collected from the trees (an appro-

priate collection method, as a major input of leaf litterto streams in this region is direct removal from treesduring storms), air dried for at least two weeks, andthen placed into coffee tray wire mesh cages (150 ×100 × 100 mm), mesh size 5 × 5 mm. Each meshbag contained approximately 8 g of one species ofleaf. Forty two bags of each species were made inthis way. The bags were then arranged into sets offive, each set containing one bag from each species.The five were arranged into a random order, and thentied onto a string, far enough apart to avoid any over-lap among bags. Each string was then secured in thestream channel by tying to tree roots or to steel polesthat had been hammered into the stream bed. Poleswere placed randomly but far enough apart to avoidbags from different strings coming into contact with

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Figure 1. Mass loss recorded from each of the study species. The percentage remaining in each individual mesh bag is plotted over time, alongwith the fitted exponential decay curve.

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each other. Leaves were not pre-leached. Bags wereplaced into the stream on 1 November 1999 and sixrandomly chosen sets were removed after 1, 3, 7, 14,28, 56 and 70 days. The strings were numbered insuch a manner that one could read the cage numberswithout disturbing the others during retrieval. Acci-dental losses in the field were low. Three of the stringscollected on day 70 had suffered severe sedimentation,and so were abandoned, leaving only three replicates.Apart from this, however, only a single set was lost(on day 14).

Upon removal, leaves were carefully cleaned ofextraneous material, then dried at 60 ◦C for 48 h andreweighed. Mass loss was fitted to an exponentialmodel, allowing calculation of the breakdown rateconstant, k.

Various physico-chemical parameters were meas-ured on each sampling date (Table 1). The valuesrecorded are typical of the Njoro, being close to thoserecorded by the long-term study of Magana (2000)between January 1998 and June 1999.

Rates of mass loss are presented in Fig. 1. Thespecies had differing decay rates, with the most rapid,Vanguera, decaying almost five times more rapidlythan the slowest, Dombeya. By the end of the study,Vanguera and Pittosporum leaves had almost entirelydisappeared. Syzygium leaves decayed at a much fasterrate in this study than in the previous one (Mathookoet al., 2000a), because they did not show the increasein biomass observed in the first experiment; this in-crease remains unexplained and is worthy of furtherinvestigation, to determine whether it was indeed algalgrowth, as suggested by Mathooko et al. (2000a), ormineralisation.

This is one of the first studies of decay rates thathas examined the range of leaf types that will fall intoa given tropical stream, rather than just selected spe-cies. Calculation of exponential decay coefficients hasbeen rightly criticised as an often inappropriate wayof presenting decomposition data (Boulton & Boon,1991). Indeed, in some cases it is clearly a poor fitto the breakdown patterns observed (e.g., Dudgeon,1982). However, it is a reasonably good fit to our data,and it is so widely used that we have adopted it here toallow comparison of our decay rates with those repor-ted in other studies. The decay rates recorded are rapidby the standards of temperate leaf species, the slow-est rate of 0.01 d−1 being at the boundary proposed

by Petersen & Cummins (1974) between medium andfast decomposition rates. However, they fall withinthe ranges recorded elsewhere in the tropics (Table 2),despite the apparent absence of shredders in the NjoroRiver (Dobson et al., 2002), ensuring that mass lossis mediated by microbial and physical factors. Thisis in accordance with Dudgeon & Wu (1999), whodrew a similar conclusion from work carried out inHong Kong. Flows in the Njoro River remained lowthroughout the study, with little evidence of severelosses through abrasion, so we conclude that microbialactivity was the main processing agent. Rapid decayrates are normally associated with detritus that is ahigh quality food resource for shredders. Confirma-tion of this awaits detailed analysis of our chosen leafspecies, but the implication from these results is thatthe paucity of shredders in the Njoro River cannot beexplained in terms of poor detritus quality.

References

Boulton, A. J. & P. I. Boon, 1991. A review of the methodologyused to measure leaf litter decomposition in lotic environments:time to turn over an old leaf? Australian Journal of Marine andFreshwater Research 42: 1–43.

Dobson, M., A. Magana, J. M. Mathooko & F. K. Ndegwa, 2002.Detritivores in Kenyan highland streams: more evidence for thepaucity of shredders in the tropics? Freshwater Biology 47: 909–919.

Dudgeon, D., 1982. An investigation of physical and biologicalprocessing of two species of leaf litter in Tai Po Kau ForestStream, New Territories, Hong Kong. Archiv für Hydrobiologie96: 1–32.

Dudgeon, D. & K. K. Y. Wu, 1999. Leaf litter in a tropicalstream: food or substrate for macroinvertebrates? Archiv fürHydrobiologie 146: 65–82.

Irons J. G. III, M. W. Oswood, R. J. Stout & C. M. Pringle, 1994.Latitudinal patterns in leaf breakdown: is temperature reallyimportant? Freshwater Biology 32: 401–411.

Larned, S. T., 2000. Dynamics of coarse riparian detritus in aHawaiian stream ecosystem: a comparison of drought and post-drought conditions. Journal of the North American BenthologicalSociety 19: 215–234.

Mathooko, J. M., A. M. Magana & I. M. Nyang’au, 2000a. De-composition of Syzygium cordatum leaves in a Rift Valley streamecosystem. African Journal of Ecology 38: 265–368.

Mathooko, J. M., C. M. M’Erimba & M. Leichtfried, 2000b. De-composition of leaf litter of Dombeya goetzenii in the NjoroRiver, Kenya. Hydrobiologia 418: 147–152.

Mathuriau, C. & E. Chauvet, 2002. Breakdown of leaf litter in aNeotropical stream. Journal of the North American Bentholo-gical Society 21: 384–396.

Petersen, R. C. & K. W. Cummins, 1974. Leaf pack processing in awoodland stream. Freshwater Biology 4: 343–368.