Characterization of coase particulate organic matter (CPOM) standing crop and its retentive structures along a low order tropical stream: Sagana river, Kenya
Odhiambo, Charles Oduor
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Retention capacity of coarse particulate organic matter (CPOM) was studied in a low order forested tropical stream, Sagana River, Kenya from February to October 2003. Sampling was carried out bi-weekly along a 100 metres stream stretch. Retentive features were examined for their abudance, type, size and distribution along the stream reach. The characteristics (length, breadth, area and volume) of the retentive features were measured and related to the amount of CPOM retained. A Hess sampler (area = 0.0299m2) was used for collecting benthic organic matter (BOM), which was then sorted into leaves barks, twigs, fruits, roots, wood debris and others. Prior to sorting, all associated macroinvertebrates were picked, identified, enumerated and classified to functional feeding group categories. The distribution and abudance was determined and related to the amount of CPOM retained by the retentive features. Materials from retentive features were ashed and weighed. The horizontal projection area (HPA) ranged from 0.64m2 to 3.76m2 and the volume of debris dam from 0.38m3 to 3.7m3. At the exposed riffle gravel bar (ERGB), the HPA varied greatly from 0 to 32.62m2. Large woody debris (LWD) retained the highest amount of BOM totaling 68.89g AFDWm-2 followed by ERGB with 65.34g AFDWm-2 and debris dam with a total of 58.83g AFDWm-2. Leaf litter dominated the BOG inputs accounting for over 47% at the debris dam Ficus thorningii leaves dominated the leaf inputs accounting for upto 44% of the total BOM. Correlation analysis showed that the most important factor influencing the accumulation of leaf litter was the volume of the debris dam (r = 0.83; p < 0.01). The highest microinvertebrate abudance was recorded at the exposed riffle gravel bar totaling to 675 individuals followed by LWD with 671 individuals and debris dam with a total of 537 individual. However, diversity and evenness was highest at the debris dam (H1 = 1.96; E = 0.67). Among the retention structures the main functional feeding groups were the collector-gatherers. The abundance increased greatly due to increased abudance of midges (Chironomids) and collector gathering mayflies (Baetis sp. Caenis sp. Afronurus sp. and Choroterpes sp.) Contrary to the predictions of the River Continuum Concept (RCC), macroinvertebrates belonging to the shredding functional group that were dominated by Scitidae (Coleoptera) and Tipulidae (Oligochaeta) constituted a small proportion of the total faunal abudance among the retentive structures. Organic debris dams and LWD, therefore, are extremely important components of the stream ecosystem. The present study showed that debris dam supported high diversity of rare macroinvertebrate taxa which is due to the highly heterogeneous architecture of dam, that offers a multitude of microhabitats for many functional feeding groups. They retain and regulate the size and amount of organic matter input within the system, thereby allowing it to be processed into finer size fractions rather than transported downstream in a coarse particulate form, which in turn affect the community structure of the stream ecosystem. It is therefore concluded that most of the relationships between macroinvertebrate assemblages and the BOM accumulation among the retentive features were due to the existence of good substrate grounds for refuge or attachment and as habitats for prey insects which various taxa feed upon and not BOM as food source.
- MST-Zoological Sciences