Impact of deforestation on benthic macroinvertebrate communities in tributaries of
Lake Tanganyika, East Africa
Student: Robert F. Swarthout
Mentor: Catherine O’Reilly
Abstract
Benthic macroinvertebrate communities of Lake Tanganyika tributaries in pristine, forested, and impacted,
deforested, watersheds were examined. Samples were collected using a Surber-sampler, and identified
under
a
dissecting
microscope.
Family
biotic
indices
(FBI),
percent
of
Ephemeroptera/Plecoptera/Trichoptera (%EPT), relative Chironomid abundance (RCA), and taxa richness
of five impacted streams in western Tanzania were compared, using paired t-test and single factor
ANOVA, to five pristine streams located within the boundaries of Gombe Stream National Park during the
dry season (July 2003). FBI, %EPT, and RCA were found to be significantly different, while taxa richness
was not. Watershed deforestation has influenced the composition of benthic macroinvertebrate
communities at the impacted sites.
Introduction
Lake Tanganyika has long been acclaimed as an evolutionary marvel. Endemic species flocks of cichlid
and non-cichlid fish are a protein staple of the rapidly expanding populations of its international shoreline.
In spite of the many reasons for the preservation of water quality in Lake Tanganyika and its tributaries,
increases in deforestation of surrounding watersheds to meet the energy and spatial needs of this growing
population have resulted in accelerated anthropogenic pollution.
The ubiquitous nature of benthic lotic macroinvertebrates makes them ideal indicators of changes in water
chemistry, sedimentation and other environmental factors (Rosenburg and Resh, 1993). The long life span
of these organisms allows for a view of stream conditions integrated over a longer time period than the
snapshot provided by traditional methods involving water chemistry analysis.
Benthic invertebrate communities have long been used as tools to assess the effects of anthropogenic
stressors on water quality in the temperate regions of the northern hemisphere. Environmental tolerances
of many invertebrate taxa have been evaluated in Europe and North America (Hilsenhoff, 1988). Certain
taxa, such as chironomid larvae, have demonstrated an increased capacity to withstand siltation (Buss,
2002). Species of Coleoptera relying on a bubble or plastron for breathing, on the other hand, have been
shown to be sensitive to slight increases in sediment pollution (Hauer and Resh, 1996). Changes in soilderived nutrients may also affect lotic invertebrate assemblages (Buss, 2002). In addition, homogenization
of benthic habitats has been shown to reduce species diversity of lentic benthic invertebrates, and will
likely have a similar effect on lotic organisms (Cohen et al., 1993).
However, data on African tropical lotic system invertebrate assemblages is scarce (See Dobson et. al.,
2002, and Eggermont, 2003). Applicability of temperate region tolerance values for similar tropical taxa
remains questionable. The apparant dearth of traditional shredding detritivore species is a testament to lack
of information about the roles existing species may play in tropical streams (Dobson et al., 2002). Any
study conducted on tropical streams will provide valuable additions to the sparse current knowledge of
these systems.
Methods
Site Description
Locations and descriptions of the ten sampled streams were those described by Caruso (2002). Sampling
occurred between the dates of July 18 and July 27, 2003.
Field Collection
Physical habitat assessments of each stream were conducted using the key provided in Hauer and Lamberti
(1996). Three one-ft2 sites were sampled from each of the ten streams using a 500 µm mesh Surber
sampler to collect benthic macroinvertebrates from a 10 m stream reach. All sites were riffles with
substrata composed of small cobbles and coarse to fine sediments. Cobbles within the sample area were
gently scrubbed with a toothbrush, and sediments were disturbed by hand, allowing invertebrates to drift
into the sampler. The net was then inverted and immersed in a bucket containing stream water and shaken
clean. Bucket contents were poured through a 4 mm sieve on top of a 500 µm sieve to remove coarse
debris. Large invertebrates were removed from the 4 mm sieve and placed in a 250 ml wide-mouthed
Nalgene bottle. Contents of the 500 µm sieve were rinsed with several buckets of stream water, and
sediments and invertebrates were washed into the sample bottle using a squirt bottle containing 70%
ethanol as a preservative.
Laboratory Sorting
Sites were sub-sampled by weight due to time constraints, and were sorted under an Olympus binocular
dissecting microscope. Macroinvertebrates were enumerated and identified to the lowest possible
taxonomic level, family for insects and order for other invertebrates, using keys provided in Thorp and
Covich (1991) and Hauer and Lamberti (1996). Sorted sub-samples were placed in 20 ml glass vials
containing fresh 70% ethanol for long-term storage and future species identification.
Statistics
FBI, percent EPT, taxa richness, and relative Chironomid abundance from impacted and non-impacted sites
were compared by paired t-test and single factor ANOVA.
Results
Significant differences were found between the FBI (ANOVA: p=0.00038, df=29; t-test: p=0.0023, df=14),
percent EPT (ANOVA: p=0.00005, df=9; t-test: p=0.0004, df=4), and relative Chironomid abundance
(ANOVA: p=0.007, df=9; t-test: p=0.014, df=4) between impacted and non-impacted sites. Taxa richness
was not significantly different between streams in deforested and forested watersheds (ANOVA: p=0.117,
df=9; t-test: p=0.074, df=4). Figure 1 illustrates the inverse correlation between FBI score and relative
Chironomid abundance, and physical habitat assessment score. Positive correlation between percent EPT
and physical habitat assessment score is shown in Figure 2.
Discussion
Benthic macroinvertebrate assemblages have undergone a significant change in the abundance of numerous
taxa in accordance with the rapid deforestation of the surrounding watershed, as is adequately illustrated by
the significant difference between the FBI scores, relative Chironomid abundances, and EPT percentages of
the two watershed types.
EPT percentages may prove good indicators of increased anthropogenic waste run-off as many families of
Ephemeroptera, such as Heptageniidae, and Plecoptera, such as Perlidae, are known to be sensitive to low
dissolved oxygen concentrations (Thorp and Covich, 1991). However, it may not be an effective indicator
of sediment pollution because some net spinning Trichopterans, such as Hydropsychidae, and
Ephemeropterans, such as Caenidae, thrive in heavily sedimented streams (Thorp and Covich, 1991). Also,
as these stressors often occur simultaneously, EPT percentage may not be the best measure of water
quality.
The inverse relationship between FBI and physical habitat quality shown in Figure 1 supports the stated
hypothesis. Temperate region tolerance values reported in Hauer and Lamberti (1996) appear to be
adequately applicable to this region of the tropics for determining family biotic indices as the scores are
inversely related to physical habitat quality. While the FBI scores determined above show that different
macroinvertebrate assemblages exist within impacted and non-impacted watersheds, further studies should
are needed to assess the tolerances of tropical taxa to specific stressors.
Midge larvae were encountered at all sites, and in significantly different abundances. The universal nature
of chironomids suggests that relative abundances of the larvae may be a useful measure of water quality in
this region. Chironomidae larvae may be more effective indicators of increased siltation, as they have been
shown to become dominantly abundant under these circumstances (Eggemont and Vershuren, 2003).
Blood-red chironomids, found in abundance at impacted sites, are able to withstand low levels of dissolved
oxygen due to their use high affinity hemoglobin, and thus are indicative of high levels of organic pollution
(Thorp and Covich, 1991). Accordingly, a plethora of fecal matter was observed in the riparian zone of the
deforested watersheds.
The complete absence of perlid stoneflies and potomonautid crabs from deforested watersheds nominates
them as indicators of good water and habitat quality. Absence of crabs is most likely due to a lack of
habitat complexity and thus increased predation. Perlid absence is probably due to high organic input, and
thus low dissolved oxygen concentrations due to bacterial respiration (Wetzel, 2001). Hirudinidae, on the
other hand are obvious indicators of poor water quality, as they were found only in the impacted streams.
This is possibly only a result of an ample supply of human hosts, but is still an indicator of anthropogenic
impact.
Lower abundances of individual macroinvertebrates observed at all non-impacted streams are in
accordance with the concept that as stream canopy cover increases primary production decreases, and can
thus support fewer invertebrates than a stream that is not light limited (Hauer and Lamberti 1996). Taxa
accumulation was not as is typical of pristine streams at non-impacted sites in this study, and was most
likely an artifact of small sample size in conjunction with only sampling riffles. As sample areas increase,
it would be expected that the homogeneity of habitat type in the impacted streams, due to sediment
pollution, would lead to slower taxa accumulation, while taxa accumulation would increase in nonimpacted sites with a far more diverse range of habitats. Future studies should sample more area and
different habitat types.
High FBI scores from Bwavi indicate that there may be some anthropogenic disturbance caused by a
nearby park ranger-housing complex. The FBI scores are bolstered by high phosphate concentrations and
diatom communities reported by concurrent studies (Lombardozzi, 2003 and Bellinger, 2003). Water
quality at this site should be monitored closely in upcoming years.
Niches filled by the species of potamonautid crabs found in the non-impacted sites should be further
investigated. Reports from other tropical stream studies have suggested that these crabs may contribute to
the shredding of large amounts of detrital input, a role currently relegated primarily to bacterial activity
(Dobson et.al., 2002). Crayfish, also decapods, have been shown to derive up to two thirds of their carbon
from allochthonous sources (Wetzel, 2001). Carbon-13 and Nitrogen-15 stable isotope studies will be
conducted over the upcoming year, as these crabs possibly occupy a similar niche to crayfish in temperate
regions.
Rapid population growth on the shores of Lake Tanganyika has had a significant impact on the quality of
the water that the responsible communities are dependent upon for their survival and livelihood. In order to
make any improvements in water quality vast efforts must be made toward community education,
alternative energy use and waste disposal methods. Until such changes can be made, benthic
macroinvertebrate assemblages may prove a useful tool for water quality monitoring.
References
Buss, D.F., D.F. Baptista, M.P. Silveira, J.L. Nessimian and L.F.M. Dorville (2002). Influence of water
chemistry and environmental degradation on macroinvertebrate assemblages in a river basin in south-east
Brazil. Hydrobiologia, 481, 125-136.
Caruso, B. (2002). A survey comparing streams from forested and deforested watersheds to assess impact
of land use change on the northeastern shore of Lake Tanganyika. Nyanza Project Report, 1-4.
Dobson, M., A. Magana, J.M. Mathooko and T.N. Ndegwa (2002). Detritivores in Kenyan highland
streams: more evidence for the paucity of shredders in the tropics? Freshwater Biology, 47, 909-919.
Eggermont, H., and D. Verschuren (2003). Impact of soil erosion in disturbed tributary drainages on the
benthic macroinvertebrate fauna of Lake Tanganyika, East Africa. Biological Conservation 113, 99-109.
Hauer, F.R., and V.H. Resh (1996). Benthic Macroinvertebrates. In: Hauer and Lamberti (Eds.), Methods
in Stream Ecology. Academic Press, 339-365.
Resh, V.H., M.J. Meyers and M.J. Hannaford (1996). Macroinvertebrates as indicators of Environmental
Quality. In: Hauer and Lamberti (Eds.), Methods in Stream Ecology. Academic Press, 647-663.
Thorp, J.H. and A.P. Covich. Eds. Ecology and classification of North American freshwater
invertebrates. San Diego, CA: Academic Press, 1991.
Wetzel, R.G. Limnology: lake and river ecosystems. San Diego, CA: Academic Press, 2001.
Table 1: Stream characteristics and biotic indicators of stream water quality.
FBI
Taxa
Relative Chironomid
Habitat Score
Site Name score %EPT richness Abundance (individuals/m2) (out of 120) Water Quality
Mtanga
5.56
30.9
14
1689
40
Fair
Mtanga A
6.17
3.6
12
7641
35
Fairly Poor
73.4
14
100
Fair
Bwavi
5.7
340
Mkenke
4.77
61.7
12
261
105
Good
Kasakera
4.91
62.3
8
100
110
Good
Rutanga
4.49
76.6
16
1302
110
Good
71.9
Mitumba
4.97
12
803
110
Good
Mwamgongo 6.01
30.2
17
5050
40
Fairly Poor
Bugamba
5.65
24.9
18
3242
85
Fair
Kibiza
6.39
11.2
16
3623
50
Fairly Poor
Table 1: Water quality descriptions are based on corresponding FBI score ranges from Hauer and Lamberti
(1996). Relative Chironomid abundances are scaled up from subsamples.
10000
6.5
8000
6.0
6000
5.5
4000
5.0
2000
4.5
0
4.0
20
40
60
80
100
120
Physical Habitat Assesment Score
RCA
RCA Regr
FBI
FBI Regr
F ig u re 2 : P e rc e n t E P T a s a fu n c tio n o f
p h y s ic a l h a b ita t a s s e s m e n t s c o re
100
80
%EPT
60
40
20
0
20
40
60
80
P h y s ic a l H a b ita t A s s e s m e n t S c o re
% EPT
% EPT R egr
100
120
FBI score
2
RCA (individuals/m )
Figure 1: Relative Chironomid abundance and family biotic
index as a function of physical habitat assesment score