| dc.description.abstract | The broad objective of this study was to assess the limnological characteristics of man-made
reservoirs in the uplands of the Eastem Rift Valley, Kenya from a geographical perspective of
spatial comparative analysis. It seeks to assess the types of natural resources existing in the
reservoirs in order to identify suitable opportunities for multipurpose use, management and
conservation.
The morphometric, water quality, biodiversity and socio-economic characteristics in eight
small reservoirs (0.065-0.249 km2) in both the rugged escarpment landscape above the rift valley
floor and the stepped plateau above them were studied between 1998 and 2000. Fixed Area
Tri-zonal Stratified Sampling was used to select Area Sampling Units (ASUs). The reservoir and
catchment morphometry were worked out using field measurements, topographic maps and black
and white panchromatic aerial photographs. Water transparency was measured using a 20-cm
Secchi disk. Water temperature, total dissolved solids (TOS) and specific conductance with
temperature compensation at 25°C were recorded in situ from a portable Jenway probe model
4075. Similarly, water pH, dissolved oxygen (~O), percentage oxygen saturation (POS) and
oxidation reduction (redox) potential (ORP) were recorded on site from a portable WTW probe
model ProfiLine Oxi 197/197-S. Phosphorus (total-P) was determined using the molybdenum blueascorbic
acid technique while Nitrogen (total-N) was determined using the Kjeldahl method.
The sampling of phytoplankton, zooplankton, benthic invertebrate, vegetation and
waterbirds was undertaken to determine species composition and community structure. The
identification and counting of free-floating planktonic algae was conducted from water samples in
Sedgewick-Rafter chambers using a Leitz inverted microscope. The zooplankton were sampled
.with a 0.14 m-diameter, 35-l-lm mesh net hauled both horizontally and vertically. The zooplankton
were also identified and counted from Sedgewick-Rafter chambers using a Leitz inverted
microscope. Benthic samples were collected with an Ekman Grab Sampler and specimens
identified and counted using a dissecting microscope. Reservoir vegetation analysis was restricted
to the determination of the general composition of emergent, submerged, and free-floating
macrophytes. Waterbird counts were conducted monthly from an infltable rubber dinghy (Zodiac).
The analysis of reservoir configuration and catchment structure was undertaken using topographic
maps, aerial photographs, and informal on-ground interviews. Data analysis was undertaken using
EXCEL and SPSS. Prior to advanced analysis (ADA), an exhaustive exploratory data analysis
(EDA) was performed. The ADA induded regression analysis, principal component analysis (peA)
and analysis of variance (ANOVA).
The results showed that the range of mean annual depth was 2.0-4.5 m while the overall
reservoir density in the region was between 0.06-0.21 reservoirs/krn". The range of watershed
drainage density at 0.7-1.3 km/km" which was quite low when, compared to other similar areas
of the world. Water transparency was quite low (0.02-0.8 m) but similar to some large
reservoirs. The reservoirs in the rugged escarpment were more transparent than the ones in the
high elevation plateau landscape although the latter had greater hydrological stability. The mean
temperature ranged between 15° and 21°C. Most waterbodies were found to be dimictic with the
short-lived thermal stratification occurring during the transition from the dry to the rains.
The overall range of water pH was 7.0 - 8.4 while the mean range was 6.9 - 7.9. The
water pH was statistically related with rainfall and catchment drainage network. The overall
range of mean TDS was 29-82 mg/l while that of specific conductance was 37-101 /-LS/cm.
Reservoir chemo-stratification, was either very rare or non-existent because thermal stratification
was usually short-lived. The mean range of dissolved and percentage oxygen saturation was 2.0-
7.2 mg/l, 23.4.2-33.6%, respectively. The level of dissolved oxygen was quite low in most
reservoirs with dry season hypolimnial oxygen deficits in some sites and higher dissolved oxygen
, content in the more transparent reservoirs. The range of total-:;Nconcentration was 220-16 800
f.1g/l while that of total-P concentration was 30-700 /-Lg/l with the plateau reservoirs containing
more total-P than the escarpment reservoirs. The total-N was statistically related to the rainfall
and catchment drainage network while total-P was related to the state of water transparency,
altitude and human population density. The waterbodies were either hypertrophic or oligomesotrophic
according to the status of nutrient content. However most of them could still
experience both phosphorus and nitrogen limitation of phytoplankton growth.
Approximately seventy species of phytoplankton were identified with the community
being dominated by cyanobacteria, chlorophytes and chrysophytes. The diatoms were quite
rare. Most of the phytoplankton communities were related to rainfall, water transparency, water
depth, and zooplankton. Approximately forty species of zooplankton were identified in the
reservoirs but site diversity was quite low. The plateau reservoirs had high zooplankton
densities than the ones in the escarpment. The stock mainly consisted of crustaceans and
rotifers and was closely related to water temperature and depth, dissolved solute content and
nutrient loading. Total-P was not found to be a very good predictor of zooplankton biomass like
other studies have established. Some of the reservoirs had inverted pyramids of numbers and
species for both phytoplankton and zooplankton.
There were less than twenty species of macrobenthic invertebrate fauna in the
reservoirs, which was quite poor compared to other areas. The most prominent groups
consisted of Lumbriculid-oligochaeta (Annelida) and Dipteran-chironomidae (Insecta) worms.
There was strong benthic sensitivity to rainfall, water depth and the plankton communities.
A total of 49 waterbird species and an average of 60 individuals per reservoir were
recorded which translated to approximately 5-11 species and 60-80 birds per site. The plateau
reservoirs had slightly higher species than the escarpment reservoirs but the resident avifauna
in most reservoirs comprised herons, coots, ducks, geese, grebes and egrets. Large-scale
breeding by Black-headed Heron (Ardea melanocephala) and Little Egret (Egretta garzetta) was
observed on Acacia xanthophloea and Ficus trees at Gathambara but several other species
were also breeding around the reservoirs. Waterbird density and diversity were strongly related
to rainfall, reservoir area, water depth, sediment chemistry and the plankton communities. The
results did not establish any widespread movement of waterbirds between the reservoirs and
natural lakes in the Eastern Rift Valley except for the White-necked Cormorants (Pha/acrocorax
carbo) and Pink-backed Pelicans (Pelecanus rufescens).
All the reservoirs were in very bad state of domestic water quality when considered
according to the levels of water transparency, total-P and total-N. Some reservoirs contained
Bulinus, one of the two species of bilharzia causing snails. The results of cyanotoxin analysis in
selected waterbodies also indicated that the levels were well above the recommended safe
limits.The reservoirs were however found to possess a good potential for fishery development
but the ones within the escarpment areas were considered to be more promising for this kind of
exploitation. Most of the waterbodies were quite good for recreation and ecotourism
development.
Two new indices of environmental analysis and classification were developed based on
the findings of the study. The catchment morphometric index (CMI) was developed for
estimation of reservoir trophic status using catchment morphometric data. The integrated
biodiversity index was developed for the comprehensive portrayal of reservoir biodiversity using
data from all the lifeform groups, which were considered in the study.
The study proposed further research on the interactions of the plankton communities
through a more intensive sampling protocol. The actual impact of sediment chemistry on the
utilization of reservoirs by waterbirds was not established and should therefore be an important
subject for further investigation. There is also need for an impact analysis to establish the
implications of reservoir ecosystems on the local communities particularly with regard to food
security,poverty alleviation and public health. | en |
