Title: Effects of litterbag mesh-size and weirs on decomposition rates of Dombeya Goetzenii (K. Scum) leaf litter in the river Njoro, Kenya

Author: Evagzard Korir Kipnusu

Supervising Institution: Egerton University

Year: 2023



Weirs are one of the most common human-related disturbances affecting the structure and functioning of riverine ecosystems. Many studies have been conducted on the effects of large dams in streams and rivers. However, there is limited information on how small dams affect leaf litter decomposition rates in Kenyan riverine ecosystems. This study aimed to assess the impact of weirs on leaf litter decomposition rates, physico-chemical variables, and macroinvertebrates abundance. The study was conducted at five study sites located at the Treetops and Njoro canning area in river Njoro, Kenya, near Egerton University. Four sites were located upstream and downstream near the weirs (i.e., about 10 m). A reference site was located 100 m from the first weir. Physico-chemical parameters were measured in-situ at each sampling site using a multi-probe water quality metre. Four replicates of water samples were collected at each site using 2 litres acid-washed bottles for nutrient analyses during each sampling occasion. The leaf litter decomposition experiment was conducted between November 2021 and January 2022. Ten kilograms (kg) of Dombeya goetzenii (K. Scum) leaves were retrieved from the riparian zone of river Njoro and air-dried at room temperature for two weeks. Five grams of the air-dried leaves were placed in 10 cm by 15 cm fine mesh size (100 μm) and coarse mesh size (500 μm) litter bags. A total of 240 fine and coarse mesh litter bags were incubated in the five study sites and retrieved after eight sampling occasions (2, 3, 7, 14, 21, 28, 35, and 42 days). The negative exponential decay model was employed to estimate decomposition rates (-k per day). The decay rates of fine mesh litter bags for the control site were higher (-k/day = 0.04) and lower (-k/day = 0.02) above the Treetop weir. The decay rates for fine and coarse mesh litterbags in the mentioned sites were significantly different (p = 0.002). The overall decay rates of Dombeya goetzenii (K. Scum) leaves were higher in coarse mesh size than fine litter bags and were statistically significant (p = 0.0006). The mean density (individual/g/leaf) of macroinvertebrates was highest (20) in control site and lowest (1) above Treetop site and Njoro canning weirs. Macroinvertebrates density between control, upstream and downstream of the first weir as well as above and below the second weir was statistically significant (p = 0.002). A positive correlation of leaf litter decomposition rates was observed between fine (r = 0.40), coarse (r = 0.67) mesh size and temperature at control site. Weirs, macroinvertebrates, and litter bag mesh size had a significant effect on the decomposition rates of Dombeya goetzenii leaf litter. The concerned government agencies should advice the surrounding community on repercussions of weirs along the streams and rivers.