179
Hydrobiologia 141 : 1 7 9 -197 (1986)
© Dr W. Junk Publishers, Dordrecht - Printed in the Netherlands
Distribution and faunal associations of benthic invertebrates at Lake Turkana, Kenya
Andrew S. Cohen
Department of Geosciences, University of Arizona, Tuscon, AZ 85721, USA
Keywords : Lake Turkana, benthic, invertebrates, Africa, ostracods
Abstract
The benthic environment and fauna of Lake Turkana were studied during 1978-1979 to determine distribution patterns and associations of benthic invertebrates . Lake Turkana is a large, closed-basin, alkaline lake,
located in northern Kenya .
Detailed environmental information is currently only available for substrate variations throughout Lake
Turkana . Water chemistry and other data are currently inadequate to evaluate their effects on the distribution
of Lake Turkana benthic invertebrates . Three weak faunal-substrate associations were discovered at Turkana .
A littoral, soft bottom association (large standing crop) is dominated by the corixid Micronecta sp. and the
ostracod Hemicypris kliei. A littoral, rocky bottom association, also with a large standing crop, is dominated
by various gastropods and insects . A profundal, muddy bottom association, with a very small standing crop,
is dominated by the ostracods Hemicypris intermedia and Sclerocypris cf. clavularis and several gastropod
and chironomid species .
Introduction
Location and water chemistry
Studies of the benthos of lakes contribute important data towards our comprehension of the lacustrine ecosystem . For a wide variety of reasons such
work has lagged behind the study of the planktonic
and nektonic elements of most lakes . Sampling
difficulties and lack of standardized methods and
presentation of results are only a few of the factors
working to limit advances in our knowledge of
lacustrine benthic organisms . Not surprisingly
therefore, the study of the lacustrine benthos in E .
Africa, where even planktonic ecology is poorly understood, can only be described as rudimentary.
In this report I present preliminary results
describing the benthos of Lake Turkana, Kenya .
This study provides an initial understanding of the
distributional ecology of the Lake's invertebrate
fauna, as well as data on abiotic factors influencing
the observed distribution patterns .
Lake Turkana, the largest lake in the Gregory
(Eastern) Rift Valley of E. Africa, lies in the
semiarid-arid northernmost part of Kenya (Fig . 1) .
Because of its remote location, it has been the least
studied of the African Great Lakes . Catchment
drainages on the east side of the lake are primarily
derived in volcanic-rift related terrains whereas the
west side of the lake drains a mixture of volcanic
and Precambrian metamorphic terrains . The lake is
a closed basin with one major perennial influent,
the Omo River, two semiperennial streams, the
Kerio and Turkwell, and numerous seasonal and
flash flood streams (Fig . 2) .
Like most other lakes in the Eastern Rift, Lake
Turkana is moderately alkaline and saline, of the
sodium chloride/sodium bicarbonate variety (Table 1) .
Alkalinity
varied between
17-20 .64 meq . I -1 total CO3 2 + HCO3 within
the lake proper during the study period, with some-
180
N
Koobi Fore
Allia Bay
0
25
Jarigole
K m.
3 ° 30'N . -
Moiti
Eliye Spgs :
-~Lolebe
Turkwell River
N. Sandy Bay
S . Sandy Bay
Porr
Fig. 1. Location map of Lake Turkana, Kenya . Rift Valley
shown by hatchured lines . From Cohen (1984) .
what higher values occurring mostly in marginal
embayments and the north basin of the lake and
lower values in the southern basin . Alkalinity of the
Omo Delta region water during flood stage was
considerably more dilute (mean 7 .63 meq .1 ).
pH for the same intervals and localities registered
8 .6-9 .5 (main lake) and 7 .7 (Omo Delta) . Additional details of dissolved gas concentrations, alkalinity and water chemistry are given in Yuretich
(1976, 1979), Hopson (1982) and Cohen (1982,
1984).
Outside of some marginal embayments Lake Turkana is unstratified with respect to dissolved oxygen and temperature. Weak daily stratification cycles develop at midday and breakdown each night .
The water column is usually supersaturated with respect to oxygen, even at depths greater than
60 meters, due to strong wind activity . Even near
the maximum depths of the lake, TDO averages
60-80% saturation . Water temperatures at maximum depths fluctuated between 24-26 .5 °C during
the study interval . Surface temperatures fluctuated
-1
Fig. 2. Bathymetric contour map for Lake Turkana . Contour interval is 20 m . Adapted from data from Hopson (1975) . Note
that the place name Loyangalani, in the SE part of the map area
appears on other maps in this paper under an older, alternative
spelling Loiengalani. From Cohen (1984) .
between 23-32'C depending on time of day and
location .
Lake Turkana water exhibits high organic and inorganic turbidity on both a seasonal and continuous basis, such that macrophyte growth is restricted
to less than one meter water depth in the extreme
north . In parts of the sediment starved Southern
Basin this depth increases to over four meters (see
Hopson, 1982 and Cohen, 1982 for details) .
Previous work
Interest in the benthic fauna of L. Turkana dates
from the Cambridge University Expedition to the
1 81
Table 1 . Water chemistry determinations for Lake Turkana, 1931 - 1979 . Turkana is a sodium carbonate-bicarbonate lake, typical
of the Eastern Rift Lakes of Africa . Values in mg/1, except alkalinity (meq/1), P0 4 (µg/1), conductivity-k 20 (µmho/cm) and pH . From
Cohen (1984) .
Author (date of
study/Ref. date)
Beadle
(1931/1932)
Beadle
(1931/1932)
Fish
(1953/1954)
Fish
(1954/unpub)
Tailing and Talling
(1961/1965)
Yuretich
(1973 - 1974/1976)
Cerling
(1975/1977)
This study
(1978/1979/-)
pH
-
Na
K
770
Ca
23 .0
5 .0
9 .5
9 .7
-
-
5 .8
Mg
Alk .
4 .0
21 .7
-
19 .4
-
21 .6
Cl
429 .0
So4
P0 4
Total P
56
-
715
320
57 .6
-
Si0 2
F
4 .2
-
5 .0
-
TDS
K20
-
2860
24
23 .0
810
9 .2
(749)
9 .2
767
8 .6-9 .3
(9 .1)
-
21
5 .7
3 .0
24 .5
475
64
-
(18 .2)
(3 .8)
(2 .3)
(19 .0)
(505)
(38)
-
-
(18 .5)
22
4 .6
2 .4
22 .2
440
-
-
22 .2
-
16 .1-21 .8
(19 .5)
East African Lakes in the early 1930s . Work at Turkana was limited by severe logistical difficulties of
the day, and was primarily taxonomic in nature .
Fish and plankton studies (Beadle, 1932 ; Worthington, 1932 ; Worthington & Ricardo, 1936) comprise
the bulk of the work published from this research .
Some ostracod descriptions from collections made
by this expedition were published by Lowndes
(1936) .
Arambourg's 1932-1933 expedition to Turkana
followed, from which Roger (1944) described 17
species of molluscs from the lake . It is clear however, that these were actually shell collections from on
shore, representing reworked Holocene fossils and
not living populations.
Lindroth visited Lake Turkana briefly in 1948 as
part of a study of the taxonomy and biogeography
of East Africa freshwater ostracods (1953) . He
made a number of dip-net collections in and
around Ferguson's Gulf, but took no dredge or bottom samples .
Butzer (1971), in a major study of the Omo River
Delta, described sedimentological and vegetational
regimes of the near lake and prodeltaic regions
around the river mouth . In addition, his climatic
studies have been important in deciphering the
cause and response correlations between short-term
lake level fluctuations and climatic changes .
36 .7
2600
18
-
(2488)
8 .6
2584
Severe famine in northern Kenya lead the British
Government in the 1960s to institute the Lake
Rudolf Fisheries Research Project (LRFRP), in an
effort to alleviate food shortages by introducing
fishing into the local (previously pastoral) economy. In addition to stimulating the first in-depth
study of the biology of the L . Turkana fish populations, a considerable effort was expended in studying the benthos, as part of a routine limnological
survey of the lake .
Accurate depth soundings by the LRFRP led to
the first good bathymetric map of the lake (Hopson, 1975) (Fig . 2) . Lake Turkana is divided into
two distinct bathymetric basins (North and South)
with a maximum (South Basin) depth of approximately 115 m . Valuable studies of primary and
secondary productivity in various lake environments (Ferguson, 1975), identified constraints on
any future estimates of energy flow into the detritivore food chain . Detailed results of the LRFRP are
presented in Hopson (1982) .
In connection with the LRFRP, Yuretich (1976)
conducted a sedimentological study of the lake .
Among his results were several of significance for
the present study, including a) the low organic carbon content of Turkana deep water sediments, b)
relatively high profundal sediment accumulation
rates (up to 1 mm • a - '), and c) description and
1 82
mapping of numerous textural and mineralogical
features of the lake's deep water substrates, particularly for areas not visited by the present author.
Methods
Faunal and sediment samples were collected at
331 stations
throughout
the
lake
during
July-September 1978 and July-November 1979
(Fig. 3). Samples were taken using a modified Ekman Dredge with a collecting area of 225 cm 2 and
a maximum sediment penetration of 50 cm . Details
of sampling methods are given in Cohen (1984) .
No systematic variations were observed between
samples collected at differing times of day and it
can be safely assumed that beneath the photic zone
(which in Turkana is always less than 10 m near-
SAMPLING STATION
LOCALITIES
N
3'30' N . -
shore), diurnal variations in the shelly benthos of
this lake are insignificant . However, diurnal vertical
migrations of dipteran larvae, known to occur in
other East African lakes (Burgis et al., 1973)
presented an intractable problem beyond the scope
of this study.
At some shallow water stations, shingle or heavily vegetated bottoms prevented the proper operation of the dredge and qualitative samples were collected by hand . Sampling in certain shallow water
embayments was also inhibited by the considerable
population of Crocodylus niloticus, whose cooccurrence with ecologists is often incompatible .
Immediately upon collection, a 50 gm (approx .)
surface sediment sample was removed for later
study, and then the remaining sample was sieved
using a U.S . sieve size #120 (125 micron) Nalgene
sieve. This sieve size was small enough to retain
most ostracod instars, in addition to most other
microinvertebrates . When the dredge sample was
undisturbed, epifauna was isolated from infauna
prior to sieving. Occasionally subsurface 02 measurements were also made on these undisturbed
sediments prior to sieving .
Faunal samples were immediately preserved in
50% ethanol or pH 9 buffered formalin . Only live
dipteran larvae and molluscs were counted . Both
living ostracods and whole, but empty carapaces
were examined . Rose Bengal has proven inconclusive as an indicator of soft tissues present in very
small amounts in ostracod carapaces (K . Brassil,
1979, oral commun .) and this method for recognizing recently dead specimens was not used. Empty
whole ostracod carapaces were examined to indicate species proportions of adults where live populations were extremely low . Such estimates may be
partially biased by variations in hinge complexity
between different ostracod species . The significance of such proportions and their relationship to live faunal assemblage proportions will be
discussed below.
Benthic fauna of Lake Turkana - an introduction
Fig. 3. Benthos sampling station localities ; 331 stations are
recorded from 1978 and 1979 surveys (from Cohen, 1984) .
Lake Turkana has a depauperate benthos, in
comparison with most permanent lakes of its size .
The appendix lists the taxa recovered to date based
on this and other studies . These include ; I sponge
species, I bryozoan species, 8 gastropod species, 3
1 83
bivalve species, 17 ostracod species, 23 + insect species and several hydracarines and annelids (totals
from both the lake proper and the Omo Delta) . Of
these, only a small number are found regularly
enough to be discussed further in this report .
The list clearly suggests a species dominance of
ostracods among the benthic fauna . Unlike the insects (aside from chironomid larvae) they occur frequently outside of vegetated littoral areas, and are
usually more abundant than molluscs or
chironomids in terms of both population size and
diversity in both littoral and profundal regions .
Therefore, most of this discussion will center on the
ostracod fauna, with mention of other taxa only
where appropriate.
Adequate data to assess the relationship between
local water chemistry, seasonal or temperature variations and benthic faunal distribution patterns do
not currently exist. Substrate data however, suggest
that three broad faunal-substrate associations occur in the Lake Turkana benthos :
1)
A littoral, soft bottom association.
2) A littoral, rocky bottom, and aufwuchs (encrusting) association .
3)
A profundal (sensu Hutchinson, 1967), soft
bottom association .
There is considerable overlap between taxa of
these three associations . In optimal areas for each
habitat type, the associations are variable in terms
of population dominance, with typical species
sometimes absent from what might seem ideal localities . Population densities and zoobenthic biomass data for all associations are given elsewhere
(Cohen, 1984) . Each association will be briefly
summarized below. The term association is used in
preference to community, because the results
presented in this paper are principally distributional in nature ; preliminary data on feeding biology,
competition, predation, etc., are at best circumstantial . Thus, to use the term `community', would be
misleading, given the current status of knowledge
of the Lake Turkana benthos .
The littoral, soft bottom association
The littoral zone of Lake Turkana is extremely
heterogeneous in terms of substrate texture and
composition, sediment accumulation rate, water
chemistry (due to varying degrees of evapotranspiration, photosynthetic activity, surficial and
groundwater discharge) and vegetation . The areal
extent of this zone is limited by high turbidity to
embayments and nearshore regions along open
coastlines . Rare marshlands occur in the Omo Delta, the Kerio Turkwell Delta and in S . Central Alia
Bay, all areas of surface or groundwater discharge .
Silty mud substrates occur in most protected embayments on the West shore, as well as sporadically
on the East shore, north of Alia Bay . Sands and
sandy silt substrates (predominately composed of
quartz and volcanic rock fragments) occur in
lengthy segments along the Northwest shore, and
sporadically elsewhere. Details of nearshore environments are given in Cohen (1982) and Cohen et
al . (1986) .
Insects, particularly corixid and naucorid water
bugs, and the ubiquitous ostracod Hemicypris kliei
are -the most common faunal elements of this association . In extremely shallow lagoonal areas, the
corixid Micronecta sp . and Hemicypris kliei are
usually the only macrofauna present, apparently
grazing on algal mats, often in great numbers . In
slightly greater depths (greater than 0 .5 m), the ostracods Ilyocypris gibba, Potamocypris worthingtoni (juveniles), Cyprideis torosa and an unidentified naucorid beetle may be found . Several species
of swimming beetles (listed in the appendix) are
also associated with vegetated soft bottoms .
Faunal densities and diversities in littoral soft
bottom habitats are highest in areas of discontinuous vegetation . They drop off slightly in areas of
continuous Cyperus and 7ypha, and almost completely on coarser, sandy bottoms . Pulmonates,
which might be expected in marshy or vegetated
bottom habitats are conspicuously absent, except in
the Omo Delta, where the large snail Pila wernei
occurs .
The littoral, rocky bottom association
Rocky littoral environments occur primarily in
the southern part of the lake, where volcanotectonic activity has been most intense during the
Holocene. Mixed rocky gravel and shingle substrates are common along the southeast shoreline
and on the volcanic islands in the center of the lake .
Extensive continuous cliffs occur in the southernmost regions of Lake Turkana . They are also found
more sporadically around Kokoi, between Jarigole
and Moiti and between Porr and El Molo Bay on
184
a
M
a
O
U
U
a
a
N
N
a
a
N
w
w
'
W
W
C
E
x
0
0
W
m
Shingle-Hard Bottom
Gravel-Gravelly Sand
Medium-Coarse Sand
Fine-Silty Sand
19
Silt-Silty Clay
7
28
44
3
Thixotropic
Inorganic Silty Clay
Gyttja
W
v
W
E
.y
W
m
0
N
c
a
U
a
L
a
0
U
a
Shingle-Hard Bottom
11
4
3
Fine-Silty Sand
Silt-Silty Clay
1
18
1
38
11
7
Gyttja
a
m
W
c
W
4,
0
E
m
E
0
c9
O
.5
y W
Z
a
0
t0
E
(0
W
0
E
W
2
W
0
n
W
W
E`
E
W
W
a
a
a
24
23
31
18
60
27
185
W
Pal
a
T
0
Shingle-Hard Bottom
Gravel-Gravelly Sand
4
Medium-Coarse Sand
Fine-Silty Sand
26
Silt-Silty Clay
Thinotropic
6
2
2
3
Inorganic Silty Clay
Gyttja
11
4
Fig. 4. Faunal substrate relationships . Common benthic invertebrate taxa are figured in relation to 7 important substrate textures . Bar
widths and percentages to the left of the bar indicate the frequency (presence vs . absence only) at which the given species occurred in
the samples from that textural class . Most ostracod species occur frequently on a wide variety of substrates, while infaunal and aufwuchs
insects (see text) are more selective.
the east side of the lake, and near Todenyang on the
west side.
The littoral, rocky bottom faunal association is
dominated by insect larvae and molluscs which
graze or forage on the epilithic algae . The insect
larvae (several unidentified species of baetid stoneflies and taeniopterygid mayflies) are found
primarily in cryptic environments such as crevices
and the undersides of rocks, whereas the gastropods (Gabbiella roses, Ceratophallus natalensis
and ?Tomichia n . sp .) occur exposed on surface
aufwuchs . One, as yet unidentified spongillid
sponge and one leech (Placobdella fimbriata) have
also been observed on the undersides of boulders
near Loiengalani . Ostracods are rare on both
vegetated and barren littoral rocky bottoms, except
where they border on mud bottoms .
The profundal, muddy bottom association
The profundal zone in Lake Turkana occurs at
depths greater than 5 meters throughout the lake,
and shallower in the turbid North Basin . With few
exceptions sublittoral substrates are silty muds
(8-12 phi mean grain size), consisting primarily of
clay minerals, quartz, feldspar and calcite and poor
in organic carbon (usually less than 1%) . Yuretich
(1976, 1979) described systematic variations in
profundal substrate mineralogy throughout the
lake.
Chironomids (4 spp), gastropods (Melanoides
tuberculata, Cleopatra bulimoides, Gabbiella roses
and ?Gyraulus sp.) and a variety of ostracods in-
habit the profundal zone on soft substrates . All of
these taxa are apparently detritivorous in Lake Tur-
1 86
kana, although for some (i .e. Melanoides tuberculata, Darwinula stevensom) this is almost certainly facultative. All of the gastropods and most
of the ostracods are stunted and thin shelled relative to their conspecifics in other African lakes,
perhaps due Jo Ca 2-1- undersaturation (uncommon
littoral gastropods in Turkana are also thin shelled) .
Population densities tend to be low (total profundal invertebrate dry weight standing crops range
from 10 to 150 mg • m -2), reflecting the general
absence of detritus below 20 m (Cohen, 1984) . The
proportions of individual taxa in this association
are more uniform between localities than for the
other two.
Faunal-substrate associations
Figure 4 shows the frequency of association for
each abundant species with the most common substrate types for the lake. Most ostracod taxa show
only weak and irregular associations with a particular substrate, being found instead on a wide variety of bottom types. Aquarium studies of several
species of ostracods from Lake Turkana shed some
light on this subject .
Four species studied in detail to date
(Plesiocypridopsis newtoni, Hemicypris intermedia, Darwinula stevensoni, and Sclerocypris cf.
clavularis) in my aquarium show one of two consistent locomotion patterns. Crawling is restricted to
firm, usually vegetated surfaces, particularly on
macrophyte leaves and stems . Where ostracods occur over soft, unvegetated substrates, they almost
perpetually hover over the sediment-water interface
(excluding nonswimmers like Darwinula or infrequent swimmers like Ilyocypris), usually between
1-10 cm above the bottom . They will alight on the
substrate only occasionally (presumably to grasp a
particle of food) and remain on the bottom for
only a few seconds . None of the ostracod species
examined so far in my aquarium are infaunal, and
no live dredge haul specimens have been observed
in the substrate, despite numerous searches . I conclude therefore, that most of the Lake Turkana ostracods are epifaunal . (However, related species of
Ilyocypris, Darwinula, and Cyprideis are infaunal
elsewhere ; R . Forrester, written commun . 1985 ; P.
DeDeckker, written commun . 1985.) Thus, their tie
to any specific substrate is considerably reduced .
Sandy, high energy substrates have almost no os-
tracods associated with them for the simple reasons
that; 1) the ostracods cannot remain in position on
the bottom for long enough to grasp their food,
and 2) most food particles of a size range and quality appropriate for ostracods are winnowed out of
areas with strong wave or current activity . On the
other hand, where macrophytes have been able to
stabilize such areas (usually quite restricted `toeholds') or where logs have been deposited, crawlers
like Hemicypris kliei may occur in abundance (even
in areas that are otherwise barren of ostracods),
having a firm surface to cling to during the near
continuous water motion . These abrupt faunal discontinuities do not correlate with significant water
chemistry changes, but do suggest that substrate is
an important factor for ostracod distribution in
this instance.
Chironomids which make shallow burrows have,
not surprisingly, a closer relationship with substrate
texture than is the case with the epifaunal ostracodes. Chironomid sp . A and B tolerate a wide variety of predominately silty and often organic rich
substrates at medium depths . The less common,
deeper water species C and D, were found exclusively on very fine grained bottoms (either inorganic or
organic in the case of C, but only inorganic for D),
where they are often found in small (less than 1 cm
long), fragile, vertical tubes . The tubes are agglutinated from clay flocs, pellets, and a muccilaginous
binder. Like chironomid tubes elsewhere (Pennak,
1978) they are probably used to assist the organism
in water filtration . Species A and B occur in silt,
and apparently do not construct tubes .
The mayfly larva Povilla sp. was found burrowing in the mud at 10 meters depth by the LRFRP,
but was not recorded in this study . Corixids are associated with algal mats, which themselves develop
on a variety of underlying sedimentary textures .
The gastropods Melanoides tuberculata and
Cleopatra bulimoides were primarily restricted to
soft, mud bottoms of various types, with only an
occasional specimen found on coarse substrates .
Melanoides tuberculata is a shallow burrower,
while Cleopatra bulimoides may be found both inand epifaunally. Gabbiella rosea is found in rocky
areas as well as on soft bottoms, but always epifaunally. ?Gyraulus sp. may prefer sandier bottoms, but its rarity makes any generalization dubious at this time . Ceratophallus natalensis was also
found only rarely in this study but sampling of the
1 87
rocky south end of the lake and South Island was
minimal and A. Hopson (pers . commun ., 1979) informs me that they are very abundant on rocky
shorelines of that area .
Geographic distribution of benthic invertebrates
The separation of the lake into distinct physiographic basins and water masses inevitably leads to
the question of whether faunal `regions' exist, isolated by geography in addition to substrate type.
For example, variations in benthic faunas from
different parts of lake basins have been observed
for Lake Tanganyika (ostracods and gastropodsCohen and Johnston, unpub.), Naivasha (decapods, chironomids-Litterick et al., 1979) and Chad
(molluscs, chironomids-Dejoux et al., 1971) . Major
environmental variations between parts of each
lake (in particular, water chemistry, major substrate
changes and vegetation) can usually be called upon
to explain these faunal boundaries .
In an effort to test this proposition, the distributions of seven common ostracod taxa were plotted
on maps of the lake, with contouring expressed as
a percentage of the total ostracod fauna counted
for each station . The results are shown in
Figures 5a-5g .
In these maps there is little to suggest any major
geographic zonation within the lake as a whole .
Two taxa (Sclerocypris cf. clavularis and
Hemicypris intermedia) show clearly defined concentric distribution patterns which approximately
follow depth contours, simply becoming more
abundant (as a percentage) in deeper water (and
generally on finer substrates) . Cyprideis torosa,
which reaches its maximum percentage abundance
at 5-10 meters depth, clearly shows this on the
map, but again, both basins of the lake are inhabited by this species . Many of the areas with large
numbers of C. torosa are adjacent to regions of significant Na+ and K+ enriched groundwater discharge (for example the regions immediately north
and south of Ferguson's Gulf and the area near the
Turkwell Delta) . Na -1- concentration and groundwater discharge areas have been found to be important in regulating the distribution of this species
elsewhere (Cohen et a l., 1983 ; P. DeDeckker, written common . 1985 ; R . Forrester, oral commun .
1985) .
Hemicypris kliei and Ilyocypris gibba are more
restricted in their distributions . Since shallow,
vegetated areas are most common in the north
(from Loelia north on the west side, and from Alia
Bay north, on the east side), their distributions reflect this habitat variance. However, it can be seen
that in the few localities in the South Basin (i .e., at
Loyangalani Bay) where vegetated habitats do oc-
A
Fig. 5. Geographic ranges for selected common ostracod taxa .
Percentages refer to 076 of total live ostracod fauna for each sample station (100 individuals counted at each station) .
5a) Hemicypris kliei.
5b) Ilyocypris gibba.
5c) Gomphocythere angulata.
5d) Darwinula stevensoni.
5e) Cyprideis torosa.
5f) Sclerocypris cf . clavularis.
5g) Hemicypris intermedia.
1 88
36E.
/Ilyocypris gibba
5-25%
X25% low
cur, both of these species may be fo
The distributions of Gomphocyt
and
stevensoni are more puz
are anomalously rare in some parts
while common elsewhere on very similar s
and no physiographic features correspond to either
of these distributions, Many lacustrine ostracod
distribution patterns are regulated by groundwater
seepage patterns (R . Forrester, written common .
1985), but insufficient data exists at present on local water chemi
sibility.
JbhunoylvI wo
d a number of
to were on the w
e lake. Specimens recovered from the east
were all juveniles .
was,
llusc, Ceratophallus natalenlacobdella JObrAW
he only
these taxa north of Central Island, despite many
searches on appropriate rocky habitats . These species may be limited by the increasing alkalinity of
the North Basin (Hart & Fuller, 1974). Certainly
the vagility of the leech Placobdella (which is a
temporary parasite of fish) would be adequate to
spread it throughout the lake, were its distribution
not being regulated by some environmental factor.
Tomichia? n, sp. also appears to be restricted to
calm water, western inlets, north of Ferguson's
Gulf, where it occurs on small cobbles and plants,
189
although its water chemistry and temperature tolerances are unknown. The remaining molluscan species of the lake proper are all widespread . The Omo
Delta-Sanderson's Gulf fauna, indicated by asterisks on the faunal list, is not found elsewhere, but
none of these species are truly lacustrine.
The reasons for the apparently widespread nature of the Lake Turkana benthos are not difficult
to understand . Despite some geographic barriers at
shallow depths, the profundal zone provides an
easy corridor for passive dispersal of the few,
vagile, cosmopolitan, deeper water species which
are present . Kornicker and Sohn (1971) have shown
that ostracod eggs can be transported alive in the
digestive systems of fish .
Shallow water populations are more isolated by
habitat barriers. However, La
portant migratory water
known to be significant dispersal agents for ostracods (Klie, 1939 ; Sandberg, 1964; McKenzie,
1970) . Thus, there is probably a regular transport
of shallow water species between all coastal areas
of the lake, populations being at least potentially
established wherever the habitat is appropriate .
There is very little endemism displayed by the
Turkana benthic invertebrate fauna. Except for a
small number of endemics (eg . Hemicypris kliei)
most species of benthic invertebrates in Lake Turkana have widespread geographic ranges beyond
the lake, and some of them are truly cosmopolitan
on a global scale (i .e., Darwinula stevensoni,
Melanoides tuberculata) .
1 90
Ostracod depth ranges
Figure 6 illustrates the depth ranges of the 9
most common ostracod species found in Lake Turkana, as well as their mean abundance for each
depth range (expressed as a percentage of the total
fauna) . Below 50 meters, extremely small population sizes (rarely more than 1 live individual per
dredge haul) occurred . Thus it was necessary to
supplement the live ostracod ratios (for the greater
than 50 meter depth range) with adult dead valve
ratios . The great similarity between ; 1) live ostracod
ratios from the 20-50 meter range ; 2) somewhat
deeper (max. sampled depth 84 meters) dead valve
ratios, and 3) rare, live specimen ratios from deep
water suggest that this is a valid approach, and that
the data are not significantly skewed by valve re-
working . Furthermore, the deepest water ratios do
not change any conclusions which could not otherwise be gained from only examining the evidence to
a depth of 50 meters . The faunal composition of
the shallowest part of the lake (less than 5 meters)
is quite distinct from greater depths, being dominated by Hemicypris kliei to the near exclusion of
other species . Ilyocypris gibba and Cyprideis torosa are found in most samples, but in relatively small
numbers . Potamocypris worthingtoni was found
primarily in juvenile (instars II -IV) forms in shallow water in 1979, but during the more restricted
sampling season of 1978 (E . Turkana, from Alia
Bay to the Omo Delta only) it was quite rare .
Plesiocypridopsis newtoni (not shown in Fig . 6)
may be locally abundant in shallow water, but was
not found nearly as prolifically as apparently oc-
191
m
xl
o~
.U.
T
Z
E
n
n~
a
.E
0
m
C
mT
9
m
c
m
U
ql
E
p
m
m
o
6'
m
U
c
E
0
>
T
0
0
E
m
E
0
0
T
x
U
3
c
m
5
oI
m
o
O
m
b
m
E
R
DEPTH
RANGE
(m.)
E
0
S
a
m
1
83
10
4
2
1-2
77
10
2
6
11
1I
1I
i
2-5
79
1
5-10
15
7
37
4
4
1
16
18
1
10-20
12
20-50
2
50
1
1
2
1
2
2
2
2
minimum
recorded depth
0
0
0
maximum
recorded depth
3.5
64
34
85
1
8
19
26
11
i
33
30
60
34
63
0
.5
1 .5
1 .5
1 .5
44
60
72
85
85
Fig. 6. Ostracode depth ranges . Nine common taxa are shown . Values to the left of the bars indicate mean percentage of each sample
made up by the taxa in question (n = 100) . Minimum and maximum depths for which each species has been recorded live are shown
below each column. Hemicypris kliei is exclusively shallow water, while H. intermedia and Sclerocypris cf . clavularis are most frequent
in profundal environments .
curred when Lindroth (1953) sampled in the Ferguson's Gulf area.
At moderate depths (5-20 meters), Hemicypris
kliei disappears and Cyprideis torosa becomes
much more abundant . The species that dominate
the deeper water assemblages (Hemicypris intermedia and Sclerocypris cf. clavularis) appear abundantly for the first time . Gomphocythere angulata
is most abundant at this depth range .
There is little evidence of a `dominant' ostracod
species in the 5-20 meter depth range . While
is
most
common
from
Cyprideis
torosa
Sclerocypris
cf.
clavularis
for
the
5-10 meters and
10-20 meter interval, the variance on these statistics are large, and in any given sample in the
5-20 meter range, any one of several species (including Gomphocythere angulata, Potamocypris
worthingtoni and Hemicypris intermedia in addition to the above named species) may be most
abundant . P worthingtoni at this depth range is
represented largely by adults, unlike its shallow water occurrences, but the implications of this peculiar distribution pattern are unclear . It may be significant in this regard that Lindroth collected at
Ferguson's Gulf between 15-23 March (1948) during the height of the rainy season, whereas my collections were made during the dry seasons .
Limnocythere africana occurs commonly, but at
low frequencies, in the 1-20 meter depth range . In
many East African alkaline lakes L. africana is
quite abundant in littoral and sublittoral waters .
Cohen et al. (1983) and Nielsen (1984) however, suggest that this species may persist in Lake Turkana
near the lower limit of its alkalinity range .
Below 20 meters the two species Hemicypris intermedia and S. cf . clavularis occur in far greater
proportions than any others. Below 65 meters,
valve assemblages usually contain only these two
species, with the occasional Darwinula stevensoni
and Gomphocythere angulata . The latter of these
1 92
has not yet been recovered either alive or as empty
valves from depths greater than 72 meters . Unfortunately it has not yet been possible to sample below 85 meters . Such depths (85-115 meters) however represent only about 1% of the total lake
bottom area .
Note the apparent differentiation in ranges between the two species of Hemicypris in Lake Turkana, H. kliei being found strictly on or near
vegetated substrates while H. intermedia is almost
always profundal. The two samples containing live
H. intermedia from shallow water were both from
nonvegetated bottoms in turbid water . Lindroth
(1953) described H. intermedia from swampy
habitats in the Ngong Hills of southern Kenya (although he gives no specific environment) .
Analysis of faunal associations
In order to assess the degree of co-occurrence
among members of the soft bottom benthic faunas
of the lake, an association matrix (Fig . 7) was developed for the 15 most common taxa . Jaccard's
coefficient was used in the determinations of
species-species co-occurrences for this matrix . The
coefficient is expressed as :
C
N I +N2 -C
where C iss the number of samples in which the two
species . being compared co-occur, N l is the total
number of occurrences of sp . #1 and N 2 the num-
a
O
U
0
V
Q1
•
•
E
7
U
N
m
¢ m
3
C
a
•
•
Q)
0
Gabbiella rosea
Sclerocypris cf . clavularis .
HemicvDria intermedia
gvorideig toros a
0
V)
Hemicvorig
Klil:i
corixid sp . A
x
0
U
®®®M®E®®
,M®EM®®®® .07 .03 .00 .00
09 .04
.01
M
.01- .15
04 .05 .02 .00
.16- .30
, .9 0
II
.08 .03 .04 .00
®~®~®
M,E®®®®
08 .10 .04 .03
.05 .02 02 .31- .45
05 .04 .00 .00 .00
Melanoides tuberculata . . . .
chironomid sp . B
V
0o
Cleopatra bulimoides
chirono mid sp . A
•
N
Q)
a
.02 .07 .00 .00 .05 .00
parwinula ~tevensoni . . .
icbba
O
•
.10 .10®®® .00
Potamocypris worthinptoni ,
)IVOCVDri§
•
2
Gomphocvthere anqulata . . .
Limnocvthere africana . .
0
•
0
•
•
m
•
¢
•
•
Ce
No
E∎
®, 08 .04 .03 .04 .00
M® , . 30 ®M® .00
® ∎U,® .20 04
®, E .30 .03
.46- .60 ∎
> .60
∎
20 .05
..
NEON NONE . :. '"EM11
M
i∎∎
∎∎. ∎
Fig. 7. Benthos association matrix . 15 invertebrate taxa are shown here in a Jacquard Coefficient of Association data matrix . Note the
large cluster composed of deeper water taxa in the upper left and the small shallow water cluster at the lower right . Calculation of association values is discussed in the text.
193
ber of occurrences of sp . #2. For this analysis, 264
soft bottom samples from both the 1978 and 1979
field seasons were used . The Jaccard coefficient is
used here in preference to other indices of association because of its conservatism and symmetry
properties . Valentine (1973) has suggested that Jaccard's coefficient be used where sampling is assumed to be relatively complete and few elements
of a local fauna are missing from any given sample,
conditions largely met by this study. The data were
compiled into a best fit matrix, with association
"zones" clustering around nuclei of maximum association .
Two association zones are apparent from this
analysis, one of which is both larger in number of
associations and stronger in depth of associations
than the other. The larger and stronger zone centers
around
the mutual
associations
between
Sclerocypris cf . clavularis, Hemicypris intermedia,
Gomphocythere angulata and Cyprideis torosa .
Hemicypris intermedia (with 86 occurrences) and
Sclerocypris cf . clavularis (with 93 occurrences)
were found together 85 times and these constitute the strongest element of this association .
Potamocypris worthingtoni and Darwinula stevensoni are also grouped into this zone, but at a somewhat lower level of association. This first association arises from the numerous co-occurrences of all
of these taxa at depths ranging from about
7 -10 meters (see Fig . 6) .
The smaller and looser association occurs
around the taxa Ilyocypris gibba, Hemicypris kliei,
chironomid sp . A and -Limnocythere africana. This
is the core of the shallow water (less than 5 meters
water depth), soft bottom assemblage .
Cyprideis torosa crosses over with strong associations to both zones. Limnocythere africana and Ilyocypris gibba are frequently associated with Gomphocythere
angulata
in
the
transition
(5-10 meters) between the two assemblages .
The relatively infrequent occurrence of the remaining insect (chironomid sp . B, corixid sp. A
(= Micronecta sp.) and the three molluscan taxa
listed) keep them from forming strong associations
with any of the other taxa . It is clear from their
relative frequencies of association however, that the
molluscs all belong with the deep water association
and the insects with the shallow water association .
Conclusions
A two year study of Lake Turkana, Kenya was
conducted to provide data on the distributional
ecology its benthic invertebrates . Lake Turkana is a
large alkaline lake with internal drainage. Ekman
dredge hauls at 331 sampling localities, shoreline
surveying and 02 , alkalinity, water temperature,
pH and Secchi measurements form the primary
data base for this study.
Substrate variability is very high in shallow
waters, typical of large, tectonic lake basins . Much
of the lake's shoreline is sand or rock-shingle bottoms, particularly on the south and west sides .
Muddy and vegetated shallows are more restricted.
Deep water substrates are almost entirely fine
grained silty muds .
Oxygen and temperature data show that the lake
is holomictic except in a few shallow silled embayments . 02 content is almost always well above
saturation .
Three benthic faunal associations have been
identified for Lake Turkana :
1) A littoral, soft bottom association, dominated
by the ostracod Hemicypris kliei and the corixid
Micronecta sp. This association is found throughout the basin in water depths less than 2 m . Most
lakeside sloughs and lagoons contain these two species exclusively.
2) A littoral, rocky bottom association, composed of stonefly and mayfly larvae, gastropods, a
leech and a sponge . This association is mostly
found in the southern part of the lake, where hard
bottoms are common.
3) A profundal, muddy bottom association,
composed of stunted gastropods, chironomids and
ostracods . This association occurs throughout the
basin at depths below 2-5 meters .
Sandy bottoms are generally devoid of benthos
at all water depths . Infaunal invertebrates, particularly bivalves, which frequent high energy sandy
bottoms in other African lakes, are absent from
Lake Turkana. Epifaunal ostracods are prevented
from feeding on shifting sandy substrates, and
macrophytes also have difficulty in colonizing
them .
Geographic distribution of benthic invertebrates
within the lake mostly follows habitat variations
with depth . With the exception of some of the
rocky bottom species from the South Basin, all
1 94
common taxa occur throughout the lake wherever
local substrate, water chemical and feeding conditions are appropriate . Most of the invertebrate species present in the lake benthos have adaptations
for long range, passive dispersal .
Depth range and faunal association studies of
the common invertebrate taxa show two associations which can be related to water depth and
which parallel the two soft bottom associations
mentioned above. Most probably, these associations are only secondarily correlated with water
depth, being principally regulated by food resource
availability.
Acknowledgements
I would like to thank Leo Laporte and Kay
Behrensmeyer and the University of CaliforniaSanta Cruz for financial support of this project .
Funding was provided by grants from NSF
(#EAR77-2349), the University of California-Davis
Chancellor's Patent Fund and an ARCO Student
Research Grant . Analytical field gear was provided
by Jere Lipps and Charles Goldman, University of
California-Davis . I am particularly indebted to my
field assistants, Karen Higgins and Nancy Dickinson for all their help. The staff of the Kenya
Department of Fisheries and Wildlife, particularly
Messrs . P. C . Kongere and B. Ogilio provided me
with tremendous logistical support, without which
this research would have been impossible. Thanks
also go to Mr. E . K. Ruchiami of the Office of the
President, Government of Kenya, for his assistance .
Many of the ideas presented here arose from conversations with Leo Laporte, Hilde Schwartz and
Kay Behrensmeyer. Leo Laporte, Richard Cowen,
Peter Ward, Rick Forester, Patrick DeDeckker and
Mary Burgis read early versions of the manuscript,
and Koen Martens and Dirk Van Damme provided
invaluable assistance with the crustacean and molluscan taxonomy, though of course, all errors are
my own.
Appendix - Checklist of benthic macroinvertebrates recorded from Lake Turkana
Reference*
Phylum Porifera
F . Spongillidae
sp . inident .
Phylum Bryozoa
F . and sp, inident . (statoblasts only)
This report
Harbott (pers .
commun ., 1980)
Phylum Mollusca
CI . Gastropoda
Sub . Cl . Prosobranchia
Ord . Mesogastropoda
F. Thiaridae
Melanoides tuberculata
Cleopatra bulimoides
F. Potamiopsidae
Tomichia? n . sp .
F . Bithyniidae
This report
Gabbiella rosea
F . Ampullariidae
Pila wernei**
Sub . Cl . Pulmonata
Ord . Basommatophora
F. Planorbidae
Gyraulus? sp .
Ceratophallus natelensis
Segmentorbis angustus
Cl . Bivalvia
Ord . Eulamellibranchia
F . Mutelidae
Verdcourt, 1960
195
Appendix (continued) .
Spathopsis wahlbergi hartmanni**
Caelatura aegyptiaca**
F. Etheriidae
Etheria elliptica**
Phylum Annelida
Cl . Oligochaeta
F . and sp . inident .
Cl. Hirudinea
F . Glossiphonidae
Placobdella fimbriata
Phylum Arthropoda
Cl . Crustacea
Sub . Cl. Ostracoda
Ord . Podocopida
F. Cyprididae
Hemicypris fossulatus
Hemicypris kliei
Hemicypris intermedia
Oncocypris worthingtoni
Oncocypris sp .
Potamocypris mastigophora
Potamocypris worthingtoni
Plesiocypridopsis newtoni
Sclerocypris cf. clavularis
Sclerocypris bicornis
Strandesia minuta
F . Ilyocyprididae
Ilyocypris gibba
F. Darwinulidae
Darwinula stevensoni
F . Cytheridae
Cyprideis torosa
Gomphocythere angulata
Limnocythere africana minor
Limnocythere africana africana
Cl . Arachnida
Ord . Hydracarina
F . and sp . inident .
Cl . Hexapoda
Sub . Cl. Insecta
Ord . Plecoptera
F. Taeniopterygidae
several sp, inident.
Ord . Ephemeroptera
F . Baetidae
sp . inident.
F. Polymitarchidae
Povilla sp .
Ord . Odonata
F . and sp . inident .
Ord . Hemiptera
F . Corixidae
Micronecta ras
Micronecta sp .
sp . A
sp . B
LRFRP-Ferguson, 1975
Klie, 1939
Lowndes, 1936
This report
This report
This report
This report
This report
This report
LRFRP-Prog .
Rept ., 1974
This report
This report
This report
This report
196
Appendix (continued) .
F . Naucoridae
sp . A
sp . B
F . Notonectidae
Anisops worthingtoni
Anisops balcis
Ord . Diptera
F . Chironomidae
sp . A
sp . B
sp . C
sp . D
Ord . Coleoptera
F . Dyticidae
Eretes sticticus
Eretes sp .
Canthydrus biguttatus
Laccophilus umbrinus
Cybister tripunctatus
F . Hydrophilidae
Coleostoma sp .
This report
This report
Worthington, 1930
This
This
This
This
report
report
report
report
Worthington,
Worthington,
Worthington,
Worthington,
Worthington,
1930
1930
1930
1930
1930
Worthington, 1930
* References are listed as This report, if collected in Lake Turkana for the first time during this survey . Unreferenced species were
collected in this survey and by earlier workers . Referenced species were not collected during this survey, but were recorded by the
referenced author .
** Collected in the Omo River Delta only .
References
Beadle, L . C ., 1932 . Scientific results of the Cambridge expedition to east African lakes 1930-1931 . The waters of some
East African lakes in relation to their fauna and flora . Zool.
J. linn . Soc. 38 : 157-211 .
Burgis, M . J., P. E. Darlington, I . G. Dunn, G. G. Ganf, J . J .
Gwahaba & L. M. McGowan, 1973 . The biomass and distribution of organisms in Lake George, Uganda. Proc. R . Soc .
Lond. B 184 : 271-298 .
Butzer, K. W., 1971 . Recent History of an Ethiopian Delta . Univ.
Chicago, Dep. of Geogr . Res . Pap. 136, 184 pp .
Cohen, A . S., 1982 . Ecological and Paleoecological Aspects of
the Rift Valley Lakes of East Africa . Ph.D. Diss . Univ.
California-Davis, 314 pp .
Cohen, A. S ., 1984. Effect of Zoobenthic standing crop on laminae preservation in tropical lake sediment, Lake Turkana, E .
Africa . J. Paleontol . 58 : 499-510 .
Cohen, A. S ., R . Dussinger & J. Richardson, 1983 . Lacustrine
paleochemical interpretations based on Eastern and Southern
African ostracodes . Paleogeogr. Paleoclimatol . Paleoecol . 43 :
129-151 .
Cohen, A . S ., D. Ferguson, P. Gram, S. Hubler & K . Sims, 1986
(in press) . The distribution of coarse grained sediments in
modern Lake Turkana : - Implications for clastic sedimentation models of Rift Lakes . Geol . Soc. London, Symp.
Sedimentation Afr. Rift System . Blackwell Publishing Co .,
Lond .
Dejoux, C ., L . Lauzanne & C . Leveque, 1971 . Nature des fonds
et repartition des organismes benthique dans la region do Bol .
Cah . O .R.S .T.O.M . Ser. Hydrobiol . 5 : 213 -223 .
Ferguson, A. J. D ., 1975. Invertebrate production in Lake Turkana . Symp. on the hydrobiology and fisheries of Lake
Turkana-Molo. Lake Rudolph Fish . Res . Proj . 13 pp.
Hart, C . W. & S. L. H . Fuller (eds), 1974 . Pollution Ecology of
Freshwater Invertebrates . Academic Press, N.Y., 389 pp.
Hopson, A . J. (ed.), 1975 . Lake Rudolf Fisheries Research Project Progress Report . 14 pp . (unpub.) .
Hopson, A . J . (ed .), 1982 . Lake Turkana: A report on the findings of the Lake Turkana Project . Univ. Stirling, 1605 pp.
Hutchinson, G. E., 1967 . A Treatise on Limnology 2 . Introduction to Lake Biology and Limnoplankton . Wiley & Sons Inc .,
N.Y., 1115 pp .
Jonasson, P. M ., 1969. Bottom fauna and eutrophication . In
Eutrophication : Causes, Consequences, Correctives . Natn .
Acad . Sci., Wash . D.C . : 274-305 .'
Klie, W., 1939 . Ostracoden aus dem Kenia-Gebeit, vorhehmilich
von dessen Hochgebirgen . Int . Revue ges. Hydrobiol . 39 :
99-161 .
Kornicker, L . S. & I . G. Sohn, 1971 . Viability of ostracod eggs
egested by fish and effects of digestive fluids on ostracode
shells : ecologic and paleoecologic implications. Paleoecologie
Ostracodes . Pau 1970 . Bull . Cent . Res . Pau-SNPA 5 :
125-135 .
Lindroth, S ., 1953 . Taxonomic and zoogeographic studies of the
ostracode fauna in the inland waters of East Africa . Zool.
Bidr. Uppsala Univ . 30 : 43-156 .
Litterick, M ., J. Gaudet, J . Kalff & J. Melack, 1979 . The limnology of an African Lake, Lake Naivasha, Kenya . Soc. Int . Limnol . Wkshop Afr. Lakes, 73 pp.
197
Lowndes, A . G., 1936. Scientific results of the Cambridge Expedition to the East African lakes 1930-1931, 16 . The smaller
crustacea . Zool . J. linn . Soc . 40, 31 pp .
McKenzie, K . G ., 1971 . Paleozoogeography of freshwater ostracoda . Bull . Cent . Res . PAU-SNPA 5 : 207-237 .
Nielsen, C ., 1984 . Ostracods as paleochemical indicators at Lake
Elmenteita, Kenya. Am . Quart. Ass . Bienn. meeting . Boulder
Co., USA . Prog . with abstract 8 : 94 .
Pennak, R . W., 1978 . Freshwater Invertebrates of the United
States, 2nd Edn. John Wiley & Sods, NY., 803 pp .
Roger, J ., 1944 . Mollusques fossiles et subfossiles du Bassin du
Lac Rudolphe. In C . Arambourg (ed .), Mission Scientifique
De LOmo (1932-1933) . Mus . Natn . Hist. Nat., Paris 2 :
119-155 .
Rome, D. R., 1962 . Ostracodes . In Exploration Hydrobiologiques Du Lac Tanganika (1946-47) . Inst . r. Sci . nat . Belg . 3,
305 pp .
Sandberg, P. A., 1964 . The ostracode genus Cyprideis in the
Americas . Stockholm Contr. Geol. 12: 1-178 .
Valentine, J. W., 1973 . Evolutionary Paleoecology of the Marine
Biosphere. Prentice Hall Inc . . Englewood Cliffs, N .J., 511 pp .
Verdcourt, B., 1960 . Some further records of molusca from N .
Kenya, Ethiopia, Somaliland and Arabia, mostly from arid
areas . Rev. Zool . Bot . Aft. 61 : 221-265 .
Worthington, E . B,, 1932. A report on the fisheries of Uganda
investigated by the Cambridge Expedition to the East African
lakes 1932-33. Crown Ag. Colon ., 88 pp.
Worthington, E . B . & C . K. Ricardo, 1936 . Scientific results of
the Cambridge Expedition to the East African lakes,
1930-31, 15 . The fish of Lake Rudolf and Lake Baringo .
Zool . J . linn . Soc. 39 : 353-389 .
Yuretich, R . F., 1976 . Sedimentology, geochemistry and geological significance of modern sediments in Lake Rudolf (Lake
lurkana), Eastern Rift Valley, Kenya . Ph.D. Diss ., Princeton
Univ., Princeton, 305 pp .
Yuretich, R . F., 1979 . Modern sediments and sedimentary
processes in Lake Rudolf (Lake Turkana), Eastern Rift Valley,
Kenya. Sedimentology 26 : 313-331 .
Received 10 July 1985 ; in revised form 31 January 1986 ; accepted 26 March 1986.