INFLUENCE OF LARGE WOODY DEBRIS ACCUMULATIONS ON MACROINVERTEBRATE DISTRIBUTION IN A LOW ORDER FORESTED TROPICAL STREAM, SAGANA RIVER, KENYA

Abstract

Woody debris accumulations are important ecological components in low order forested streams. They provide forage and refuge for a macroinvertebrates, which are key trophic components for riverine fish. The occurrence of woody debris in tropical streams is however threatened by large scale destruction of catchment forests thereby threatening the riverine biodiversity. Despite their importance, the current  status  of  woody  debris  accumulations  in  tropical  forested  streams  is unknown. In this study we determined the densities of debris dams in a forested stream section, their effectiveness in coarse particulate organic matter (CPOM) retention and the associated  macroinvertebrates along a 100m stream stretch within the mid‐reaches of Sagana River. Volume of woody debris was estimated from length, width and height. All components retained were collected, sorted and dried  to  a  constant  weight  at  85oC.  Prior  to  drying  of  the  organic  matter, macroinvertebrates  in  the  CPOM  were  picked,  sorted  into  different  taxa  and enumerated. The average density over the whole study period was 2 woody debris dam accumulations per 100m stream stretch. The macroinvertebrates occurring at the  debris  dams  were  dominated  by  Coleoptera  (47.3%),  followed  by  Diptera (22.7%) and Ephemeroptera (19.2%). The Coleoptera were dominated by Gyrinidae, Elmidae and Scirtidae. Woody debris play an important role as long‐term retention structures  for  organic  matter  and  food  resources.  They  forms  an  important component  of  riverine  ecosystems  with  regard  to  CPOM  retention, macroinvertebrates distribution and abundance which are indicators of habitat condition.  There  is  therefore  need  for  re‐introductions  of  such  structures  for effective  channel  restoration.  We  recommend  maintenance  of  buffer  strips  of riparian vegetation along river banks to protect riverine communities from adverse effects of land‐use changes within Mt. Kenya catchment region.