Coarse Particulate Organic Matter Drift in the Njoro River, Kenya, as a Function of Drift Net Mesh size, Exposure time and Position of Drift sampler

Abstract

Streams transport organic materials to downstream areas and provide food resources and habitat to downstream consumers. Organic matter drift has been reported in streams and is affected by factors such as human-induced disturbances (e.g., damming), stream geomorphology and discharge. However, there is still a paucity of information on the influence of the mesh size of drift nets, drift sampler position and sampling duration (i.e., exposure time) on the drift dynamics of allochthonous particulate organic matter (POM) in Kenyan streams. In the current study, the downstream movement of POM was assessed in different habitats (i.e., pools and riffles) of the Njoro River, between 3rd January and 28th March, 2017. Drift samplers with varied net mesh-sizes (100 µm, 250 µm and 500 µm) were used and POM sampling time varied between 5-120 minutes. The average amount of POM that drifted (pooled data) differed significantly between the riffle and pool habitats (p < 0.001). The average amount of POM that drifted through the three drift nets did not vary significantly at the riffle (p > 0.05) and pool (p > 0.05) habitats. Based on the mesh size of drift samplers, the 100 µm drift sampler had the highest mean amount POM compared to the other (i.e. 250 and 500 µm) drift samplers at the pool habitat. The pool habitat had relatively low average POM content in drift samples. Organic matter drift was significantly affected by the duration of sampling at the riffle (p < 0.001) and pool habitats (p < 0.001). In general, POM drift increased with increase in the duration of sampling. Drift sampler position on the stream bed significantly influenced the mean amount of POM that drifted in both stream habitats (p < 0.001). This research indicates that the mesh size of drift samplers’ nets, duration of sampling and drift sampler position on the stream bed should be considered when characterizing POM drift in running water ecosystems. In future, research on drift should assess the effects of other factors, such as stream size and seasonality on POM drift.