Colonisation of leaf litter by lotic macroinvertebrates in a headwater stream of the Phachi River (western Thailand)

Leaf litter decomposition has been widely studied during the last 60 years. However, the relative importance of various processes, such as physical abrasion, microbial activity and macroinvertebrates consumption, remains unclear in tropical streams. The aim of this study was to gain insights into the ecological processes leading to the decomposition of leaf litter in headwater streams of Thailand, with a particular focus on the role played by macroinvertebrates. The breakdown rate of two leaf species, Bambusa bambos and Lagerstroemia floribunda, was investigated in a 1st order tropical stream of Ratchaburi province (western Thailand) using litter bags submerged over an 8-week period. Litter bags were retrieved after 2, 14, 28, 42 and 56 days of immersion and characterised by two metrics, remaining AFDM and faunal composition of macroinvertebrates. Most rapidly decomposed leaves were those of B. bambos placed in riffles and most slowly decomposed were those of L.floribunda placed in pools (k = 0.030 and 0.006 d(-1), respectively). The highest amount of invertebrates per leaf bag were found after 14 days of immersion for B. bambos (174 individuals, SE = 10.0, n = 4) and after 28 days of immersion for L. floribunda (174 individuals, SE = 15.8, n = 5). Faunal composition of invertebrates differed between pools and riffles but not within leaf species. The low influence of shredders (accounting for maximum 5 % of total invertebrate abundance in leaf bags) is in accordance with previous findings in tropical zones, while there is a new thought that physical abrasion may play an important role in leaf decay. Nevertheless, non-shredder invertebrates may indirectly contribute to the decomposition of leaf material. In both pools and riffles, macroinvertebrate communities varied with time, showing a diminution of stream habitability, correlated with progressive water withdrawal.