Hydrobiologia 141 : 1 7 9-197 (1986)

179© Dr W. Junk Publishers, Dordrecht - Printed in the Netherlands

Distribution and faunal associations of benthic invertebrates at Lake Turkana, Kenya

Andrew S. CohenDepartment 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 distri-bution 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 LakeTurkana. Water chemistry and other data are currently inadequate to evaluate their effects on the distributionof 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 theostracod Hemicypris kliei. A littoral, rocky bottom association, also with a large standing crop, is dominatedby 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 gastropodand chironomid species .

Introduction

Studies of the benthos of lakes contribute impor-tant data towards our comprehension of the lacus-trine ecosystem. For a wide variety of reasons suchwork has lagged behind the study of the planktonicand nektonic elements of most lakes . Samplingdifficulties and lack of standardized methods andpresentation of results are only a few of the factorsworking to limit advances in our knowledge oflacustrine benthic organisms. Not surprisinglytherefore, the study of the lacustrine benthos in E .Africa, where even planktonic ecology is poorly un-derstood, can only be described as rudimentary.

In this report I present preliminary resultsdescribing the benthos of Lake Turkana, Kenya .This study provides an initial understanding of thedistributional ecology of the Lake's invertebratefauna, as well as data on abiotic factors influencingthe observed distribution patterns .

Location and water chemistry

Lake Turkana, the largest lake in the Gregory(Eastern) Rift Valley of E. Africa, lies in thesemiarid-arid northernmost part of Kenya (Fig . 1) .Because of its remote location, it has been the leaststudied of the African Great Lakes . Catchmentdrainages on the east side of the lake are primarilyderived in volcanic-rift related terrains whereas thewest side of the lake drains a mixture of volcanicand Precambrian metamorphic terrains. The lake isa closed basin with one major perennial influent,the Omo River, two semiperennial streams, theKerio and Turkwell, and numerous seasonal andflash flood streams (Fig . 2) .

Like most other lakes in the Eastern Rift, LakeTurkana is moderately alkaline and saline, of thesodium chloride/sodium bicarbonate variety (Ta-ble 1) . Alkalinity varied between17-20.64 meq . I -1 total CO3 2 + HCO3 withinthe lake proper during the study period, with some-

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Fig. 1. Location map of Lake Turkana, Kenya . Rift Valleyshown by hatchured lines . From Cohen (1984) .

what higher values occurring mostly in marginalembayments and the north basin of the lake andlower values in the southern basin . Alkalinity of theOmo Delta region water during flood stage wasconsiderably more dilute (mean 7 .63 meq .1-1 ).pH for the same intervals and localities registered8.6-9.5 (main lake) and 7 .7 (Omo Delta) . Addi-tional details of dissolved gas concentrations, alka-linity and water chemistry are given in Yuretich(1976, 1979), Hopson (1982) and Cohen (1982,1984).

Outside of some marginal embayments Lake Tur-kana is unstratified with respect to dissolved oxy-gen and temperature. Weak daily stratification cy-cles develop at midday and breakdown each night .The water column is usually supersaturated with re-spect to oxygen, even at depths greater than60 meters, due to strong wind activity . Even nearthe maximum depths of the lake, TDO averages60-80% saturation. Water temperatures at maxi-mum depths fluctuated between 24-26.5 °C duringthe study interval . Surface temperatures fluctuated

NKoobi Fore

Allia Bay

Jarigole

MoitiEliye Spgs :

-~Lolebe

N. Sandy Bay-

Turkwell River

S . Sandy Bay

Porr

0

25

K m . 3 ° 30'N . -

Fig. 2. Bathymetric contour map for Lake Turkana . Contour in-terval is 20 m. Adapted from data from Hopson (1975) . Notethat the place name Loyangalani, in the SE part of the map areaappears on other maps in this paper under an older, alternativespelling Loiengalani. From Cohen (1984) .

between 23-32'C depending on time of day andlocation .

Lake Turkana water exhibits high organic and in-organic turbidity on both a seasonal and continu-ous basis, such that macrophyte growth is restrictedto less than one meter water depth in the extremenorth. In parts of the sediment starved SouthernBasin this depth increases to over four meters (seeHopson, 1982 and Cohen, 1982 for details) .

Previous work

Interest in the benthic fauna of L. Turkana datesfrom the Cambridge University Expedition to the

East African Lakes in the early 1930s . Work at Tur-kana was limited by severe logistical difficulties ofthe day, and was primarily taxonomic in nature .Fish and plankton studies (Beadle, 1932 ; Worthing-ton, 1932; Worthington & Ricardo, 1936) comprisethe bulk of the work published from this research .Some ostracod descriptions from collections madeby this expedition were published by Lowndes(1936) .Arambourg's 1932-1933 expedition to Turkana

followed, from which Roger (1944) described 17species of molluscs from the lake . It is clear howev-er, that these were actually shell collections from onshore, representing reworked Holocene fossils andnot living populations .

Lindroth visited Lake Turkana briefly in 1948 aspart of a study of the taxonomy and biogeographyof East Africa freshwater ostracods (1953) . Hemade a number of dip-net collections in andaround Ferguson's Gulf, but took no dredge or bot-tom samples.

Butzer (1971), in a major study of the Omo RiverDelta, described sedimentological and vegetationalregimes of the near lake and prodeltaic regionsaround the river mouth. In addition, his climaticstudies have been important in deciphering thecause and response correlations between short-termlake level fluctuations and climatic changes .

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Table 1 . Water chemistry determinations for Lake Turkana, 1931 - 1979 . Turkana is a sodium carbonate-bicarbonate lake, typicalof 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 . FromCohen (1984) .

Severe famine in northern Kenya lead the BritishGovernment in the 1960s to institute the LakeRudolf Fisheries Research Project (LRFRP), in aneffort to alleviate food shortages by introducingfishing into the local (previously pastoral) econo-my. In addition to stimulating the first in-depthstudy of the biology of the L. Turkana fish popula-tions, a considerable effort was expended in study-ing the benthos, as part of a routine limnologicalsurvey of the lake .

Accurate depth soundings by the LRFRP led tothe first good bathymetric map of the lake (Hop-son, 1975) (Fig . 2). Lake Turkana is divided intotwo distinct bathymetric basins (North and South)with a maximum (South Basin) depth of approxi-mately 115 m . Valuable studies of primary andsecondary productivity in various lake environ-ments (Ferguson, 1975), identified constraints onany future estimates of energy flow into the detriti-vore food chain . Detailed results of the LRFRP arepresented in Hopson (1982) .

In connection with the LRFRP, Yuretich (1976)conducted a sedimentological study of the lake .Among his results were several of significance forthe present study, including a) the low organic car-bon content of Turkana deep water sediments, b)relatively high profundal sediment accumulationrates (up to 1 mm • a - '), and c) description and

Author (date ofstudy/Ref. date)

pH Na K Ca Mg Alk . Cl So4 P04 Total P Si0 2 F TDS K20

Beadle(1931/1932) - 770 23 .0 5 .0 4 .0 21 .7 429 .0 56 4 .2 - - 2860

Beadle(1931/1932) 9 .5 - 19 .4 - 715 5 .0 -

Fish(1953/1954) 9 .7 - - 5 .8 - 21 .6 320 57 .6 - 24

Fish(1954/unpub) 23 .0

Tailing and Talling(1961/1965) 810 21 5 .7 3 .0 24 .5 475 64 - 2600 18

Yuretich(1973 - 1974/1976) 9 .2 (749) (18 .2) (3 .8) (2 .3) (19 .0) (505) (38) -

- (18.5) - (2488)Cerling(1975/1977) 9 .2 767 22 4 .6 2 .4 22 .2 440 36 .7 -

- 22.2 8 .6 2584This study

(1978/1979/-) 8.6-9 .3 - - 16.1-21 .8(9 .1) (19 .5)

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mapping of numerous textural and mineralogicalfeatures of the lake's deep water substrates, particu-larly for areas not visited by the present author.

Methods

Faunal and sediment samples were collected at331 stations throughout the lake duringJuly-September 1978 and July-November 1979(Fig. 3). Samples were taken using a modified Ek-man Dredge with a collecting area of 225 cm 2 anda maximum sediment penetration of 50 cm . Detailsof sampling methods are given in Cohen (1984) .

No systematic variations were observed betweensamples collected at differing times of day and itcan be safely assumed that beneath the photic zone(which in Turkana is always less than 10 m near-

SAMPLING STATIONLOCALITIES

N

3'30' N . -

Fig. 3. Benthos sampling station localities ; 331 stations arerecorded from 1978 and 1979 surveys (from Cohen, 1984) .

shore), diurnal variations in the shelly benthos ofthis lake are insignificant . However, diurnal verticalmigrations of dipteran larvae, known to occur inother East African lakes (Burgis et al., 1973)presented an intractable problem beyond the scopeof this study.

At some shallow water stations, shingle or heavi-ly vegetated bottoms prevented the proper opera-tion of the dredge and qualitative samples were col-lected by hand . Sampling in certain shallow waterembayments was also inhibited by the considerablepopulation of Crocodylus niloticus, whose cooc-currence with ecologists is often incompatible .

Immediately upon collection, a 50 gm (approx .)surface sediment sample was removed for laterstudy, and then the remaining sample was sievedusing a U.S. sieve size #120 (125 micron) Nalgenesieve. This sieve size was small enough to retainmost ostracod instars, in addition to most othermicroinvertebrates. When the dredge sample wasundisturbed, epifauna was isolated from infaunaprior to sieving. Occasionally subsurface 02 meas-urements were also made on these undisturbedsediments prior to sieving .

Faunal samples were immediately preserved in50% ethanol or pH 9 buffered formalin. Only livedipteran larvae and molluscs were counted. Bothliving ostracods and whole, but empty carapaceswere examined . Rose Bengal has proven inconclu-sive as an indicator of soft tissues present in verysmall amounts in ostracod carapaces (K . Brassil,1979, oral commun .) and this method for recogniz-ing recently dead specimens was not used. Emptywhole ostracod carapaces were examined to indi-cate species proportions of adults where live popu-lations were extremely low. Such estimates may bepartially biased by variations in hinge complexitybetween different ostracod species. The sig-nificance of such proportions and their relation-ship to live faunal assemblage proportions will bediscussed below.

Benthic fauna of Lake Turkana - an introduction

Lake Turkana has a depauperate benthos, incomparison with most permanent lakes of its size .The appendix lists the taxa recovered to date basedon this and other studies . These include; I spongespecies, I bryozoan species, 8 gastropod species, 3

bivalve species, 17 ostracod species, 23 + insect spe-cies and several hydracarines and annelids (totalsfrom both the lake proper and the Omo Delta) . Ofthese, only a small number are found regularlyenough to be discussed further in this report .

The list clearly suggests a species dominance ofostracods among the benthic fauna . Unlike the in-sects (aside from chironomid larvae) they occur fre-quently outside of vegetated littoral areas, and areusually more abundant than molluscs orchironomids in terms of both population size anddiversity in both littoral and profundal regions .Therefore, most of this discussion will center on theostracod fauna, with mention of other taxa onlywhere appropriate.

Adequate data to assess the relationship betweenlocal water chemistry, seasonal or temperature vari-ations and benthic faunal distribution patterns donot currently exist. Substrate data however, suggestthat three broad faunal-substrate associations oc-cur in the Lake Turkana benthos :

1)

A littoral, soft bottom association.2) A littoral, rocky bottom, and aufwuchs (en-

crusting) association .3) A profundal (sensu Hutchinson, 1967), soft

bottom association .There is considerable overlap between taxa of

these three associations. In optimal areas for eachhabitat type, the associations are variable in termsof population dominance, with typical speciessometimes absent from what might seem ideal lo-calities. Population densities and zoobenthic bi-omass data for all associations are given elsewhere(Cohen, 1984) . Each association will be brieflysummarized below. The term association is used inpreference to community, because the resultspresented in this paper are principally distribution-al in nature ; preliminary data on feeding biology,competition, predation, etc., are at best circumstan-tial. Thus, to use the term `community', would bemisleading, given the current status of knowledgeof the Lake Turkana benthos .

The littoral, soft bottom association

The littoral zone of Lake Turkana is extremelyheterogeneous in terms of substrate texture andcomposition, sediment accumulation rate, waterchemistry (due to varying degrees of evapotranspi-ration, photosynthetic activity, surficial and

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groundwater discharge) and vegetation . The arealextent of this zone is limited by high turbidity toembayments and nearshore regions along opencoastlines. Rare marshlands occur in the Omo Del-ta, the Kerio Turkwell Delta and in S . Central AliaBay, all areas of surface or groundwater discharge .Silty mud substrates occur in most protected em-bayments on the West shore, as well as sporadicallyon the East shore, north of Alia Bay. Sands andsandy silt substrates (predominately composed ofquartz and volcanic rock fragments) occur inlengthy segments along the Northwest shore, andsporadically elsewhere. Details of nearshore en-vironments are given in Cohen (1982) and Cohen etal . (1986) .

Insects, particularly corixid and naucorid waterbugs, and the ubiquitous ostracod Hemicypris klieiare -the most common faunal elements of this as-sociation. In extremely shallow lagoonal areas, thecorixid Micronecta sp . and Hemicypris kliei areusually the only macrofauna present, apparentlygrazing on algal mats, often in great numbers. Inslightly greater depths (greater than 0 .5 m), the os-tracods Ilyocypris gibba, Potamocypris worthing-toni (juveniles), Cyprideis torosa and an unidenti-fied naucorid beetle may be found . Several speciesof swimming beetles (listed in the appendix) arealso associated with vegetated soft bottoms .

Faunal densities and diversities in littoral softbottom habitats are highest in areas of discontinu-ous vegetation . They drop off slightly in areas ofcontinuous Cyperus and 7ypha, and almost com-pletely on coarser, sandy bottoms . Pulmonates,which might be expected in marshy or vegetatedbottom habitats are conspicuously absent, except inthe Omo Delta, where the large snail Pila werneioccurs .

The littoral, rocky bottom association

Rocky littoral environments occur primarily inthe southern part of the lake, where volcano-tectonic activity has been most intense during theHolocene. Mixed rocky gravel and shingle sub-strates are common along the southeast shorelineand on the volcanic islands in the center of the lake .Extensive continuous cliffs occur in the southern-most regions of Lake Turkana . They are also foundmore sporadically around Kokoi, between Jarigoleand Moiti and between Porr and El Molo Bay on

184

Shingle-Hard Bottom

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the east side of the lake, and near Todenyang on thewest side.

The littoral, rocky bottom faunal association isdominated by insect larvae and molluscs whichgraze or forage on the epilithic algae . The insectlarvae (several unidentified species of baetid stone-flies and taeniopterygid mayflies) are foundprimarily in cryptic environments such as crevicesand the undersides of rocks, whereas the gastro-pods (Gabbiella roses, Ceratophallus natalensisand ?Tomichia n . sp.) occur exposed on surfaceaufwuchs. One, as yet unidentified spongillidsponge and one leech (Placobdella fimbriata) havealso been observed on the undersides of bouldersnear Loiengalani. Ostracods are rare on bothvegetated and barren littoral rocky bottoms, exceptwhere they border on mud bottoms .

Pal

W

a

T

0

Fig. 4. Faunal substrate relationships . Common benthic invertebrate taxa are figured in relation to 7 important substrate textures . Barwidths and percentages to the left of the bar indicate the frequency (presence vs . absence only) at which the given species occurred inthe samples from that textural class . Most ostracod species occur frequently on a wide variety of substrates, while infaunal and aufwuchsinsects (see text) are more selective.

The profundal, muddy bottom association

The profundal zone in Lake Turkana occurs atdepths greater than 5 meters throughout the lake,and shallower in the turbid North Basin. With fewexceptions sublittoral substrates are silty muds(8-12 phi mean grain size), consisting primarily ofclay minerals, quartz, feldspar and calcite and poorin organic carbon (usually less than 1%) . Yuretich(1976, 1979) described systematic variations inprofundal substrate mineralogy throughout thelake.

Chironomids (4 spp), gastropods (Melanoidestuberculata, Cleopatra bulimoides, Gabbiella rosesand ?Gyraulus sp.) and a variety of ostracods in-habit the profundal zone on soft substrates . All ofthese taxa are apparently detritivorous in Lake Tur-

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1 86

kana, although for some (i .e. Melanoides tuber-culata, Darwinula stevensom) this is almost cer-tainly facultative. All of the gastropods and mostof the ostracods are stunted and thin shelled rela-tive to their conspecifics in other African lakes,perhaps due Jo Ca2-1- undersaturation (uncommonlittoral gastropods in Turkana are also thin shelled) .

Population densities tend to be low (total profun-dal invertebrate dry weight standing crops rangefrom 10 to 150 mg • m-2), reflecting the generalabsence of detritus below 20 m (Cohen, 1984) . Theproportions of individual taxa in this associationare more uniform between localities than for theother two.

Faunal-substrate associations

Figure 4 shows the frequency of association foreach abundant species with the most common sub-strate types for the lake. Most ostracod taxa showonly weak and irregular associations with a partic-ular substrate, being found instead on a wide varie-ty of bottom types. Aquarium studies of severalspecies of ostracods from Lake Turkana shed somelight on this subject .Four species studied in detail to date

(Plesiocypridopsis newtoni, Hemicypris interme-dia, Darwinula stevensoni, and Sclerocypris cf.clavularis) in my aquarium show one of two consis-tent locomotion patterns. Crawling is restricted tofirm, usually vegetated surfaces, particularly onmacrophyte leaves and stems . Where ostracods oc-cur over soft, unvegetated substrates, they almostperpetually hover over the sediment-water interface(excluding nonswimmers like Darwinula or infre-quent swimmers like Ilyocypris), usually between1-10 cm above the bottom . They will alight on thesubstrate only occasionally (presumably to grasp aparticle of food) and remain on the bottom foronly a few seconds . None of the ostracod speciesexamined so far in my aquarium are infaunal, andno live dredge haul specimens have been observedin the substrate, despite numerous searches . I con-clude therefore, that most of the Lake Turkana os-tracods are epifaunal. (However, related species ofIlyocypris, Darwinula, and Cyprideis are infaunalelsewhere; R. Forrester, written commun. 1985; P.DeDeckker, written commun . 1985.) Thus, their tieto any specific substrate is considerably reduced .

Sandy, high energy substrates have almost no os-

tracods associated with them for the simple reasonsthat; 1) the ostracods cannot remain in position onthe bottom for long enough to grasp their food,and 2) most food particles of a size range and qual-ity appropriate for ostracods are winnowed out ofareas with strong wave or current activity. On theother hand, where macrophytes have been able tostabilize such areas (usually quite restricted `toe-holds') or where logs have been deposited, crawlerslike Hemicypris kliei may occur in abundance (evenin areas that are otherwise barren of ostracods),having a firm surface to cling to during the nearcontinuous water motion . These abrupt faunal dis-continuities do not correlate with significant waterchemistry changes, but do suggest that substrate isan important factor for ostracod distribution inthis instance.

Chironomids which make shallow burrows have,not surprisingly, a closer relationship with substratetexture than is the case with the epifaunal ostra-codes. Chironomid sp . A and B tolerate a wide va-riety of predominately silty and often organic richsubstrates at medium depths . The less common,deeper water species C and D, were found exclusive-ly on very fine grained bottoms (either inorganic ororganic in the case of C, but only inorganic for D),where they are often found in small (less than 1 cmlong), fragile, vertical tubes . The tubes are aggluti-nated from clay flocs, pellets, and a muccilaginousbinder. Like chironomid tubes elsewhere (Pennak,1978) they are probably used to assist the organismin water filtration. Species A and B occur in silt,and apparently do not construct tubes .

The mayfly larva Povilla sp. was found burrow-ing in the mud at 10 meters depth by the LRFRP,but was not recorded in this study. Corixids are as-sociated with algal mats, which themselves developon a variety of underlying sedimentary textures .

The gastropods Melanoides tuberculata andCleopatra bulimoides were primarily restricted tosoft, mud bottoms of various types, with only anoccasional specimen found on coarse substrates .Melanoides tuberculata is a shallow burrower,while Cleopatra bulimoides may be found both in-and epifaunally. Gabbiella rosea is found in rockyareas as well as on soft bottoms, but always epi-faunally. ?Gyraulus sp. may prefer sandier bot-toms, but its rarity makes any generalization dubi-ous at this time . Ceratophallus natalensis was alsofound only rarely in this study but sampling of the

rocky south end of the lake and South Island wasminimal and A. Hopson (pers . commun., 1979) in-forms me that they are very abundant on rockyshorelines of that area .

Geographic distribution of benthic invertebrates

The separation of the lake into distinct physio-graphic basins and water masses inevitably leads tothe question of whether faunal `regions' exist, iso-lated by geography in addition to substrate type.For example, variations in benthic faunas fromdifferent parts of lake basins have been observedfor Lake Tanganyika (ostracods and gastropods-Cohen and Johnston, unpub.), Naivasha (deca-pods, chironomids-Litterick et al., 1979) and Chad(molluscs, chironomids-Dejoux et al., 1971) . Majorenvironmental variations between parts of eachlake (in particular, water chemistry, major substratechanges and vegetation) can usually be called uponto explain these faunal boundaries .

In an effort to test this proposition, the distribu-tions of seven common ostracod taxa were plottedon maps of the lake, with contouring expressed asa percentage of the total ostracod fauna countedfor each station . The results are shown inFigures 5a-5g .

In these maps there is little to suggest any majorgeographic zonation within the lake as a whole .Two taxa (Sclerocypris cf. clavularis andHemicypris intermedia) show clearly defined con-centric distribution patterns which approximatelyfollow depth contours, simply becoming moreabundant (as a percentage) in deeper water (andgenerally on finer substrates) . Cyprideis torosa,which reaches its maximum percentage abundanceat 5-10 meters depth, clearly shows this on themap, but again, both basins of the lake are inhabit-ed by this species. Many of the areas with largenumbers of C. torosa are adjacent to regions of sig-nificant Na+ and K+ enriched groundwater dis-charge (for example the regions immediately northand south of Ferguson's Gulf and the area near theTurkwell Delta). Na -1- concentration and ground-water discharge areas have been found to be impor-tant in regulating the distribution of this specieselsewhere (Cohen et a l., 1983; P. DeDeckker, writ-ten common. 1985; R. Forrester, oral commun .1985) .

Hemicypris kliei and Ilyocypris gibba are morerestricted in their distributions . Since shallow,vegetated areas are most common in the north(from Loelia north on the west side, and from AliaBay north, on the east side), their distributions re-flect this habitat variance. However, it can be seenthat in the few localities in the South Basin (i .e., atLoyangalani Bay) where vegetated habitats do oc-

1 87

A

Fig. 5. Geographic ranges for selected common ostracod taxa .Percentages refer to 076 of total live ostracod fauna for each sam-ple 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.

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36E. /Ilyocypris gibba

5-25%X25% low

cur, both of these species may be foThe distributions of Gomphocyt

andwas, stevensoni are more puzare anomalously rare in some partswhile common elsewhere on very similar sand no physiographic features correspond to eitherof these distributions, Many lacustrine ostracoddistribution patterns are regulated by groundwaterseepage patterns (R. Forrester, written common .1985), but insufficient data exists at present on lo-cal water chemisibility.JbhunoylvI wo

d a number ofto were on the w

e lake. Specimens recovered from the eastwere all juveniles .

llusc, Ceratophallus natalen-lacobdella JObrAW

he only

these taxa north of Central Island, despite manysearches on appropriate rocky habitats . These spe-cies may be limited by the increasing alkalinity ofthe North Basin (Hart & Fuller, 1974). Certainlythe vagility of the leech Placobdella (which is atemporary parasite of fish) would be adequate tospread it throughout the lake, were its distributionnot being regulated by some environmental factor.Tomichia? n, sp. also appears to be restricted tocalm water, western inlets, north of Ferguson'sGulf, where it occurs on small cobbles and plants,

although its water chemistry and temperature toler-ances are unknown. The remaining molluscan spe-cies of the lake proper are all widespread . The OmoDelta-Sanderson's Gulf fauna, indicated by aster-isks on the faunal list, is not found elsewhere, butnone of these species are truly lacustrine.

The reasons for the apparently widespread na-ture of the Lake Turkana benthos are not difficultto understand . Despite some geographic barriers atshallow depths, the profundal zone provides aneasy corridor for passive dispersal of the few,vagile, cosmopolitan, deeper water species whichare present. Kornicker and Sohn (1971) have shownthat ostracod eggs can be transported alive in thedigestive systems of fish .

Shallow water populations are more isolated by

189

habitat barriers. However, Laportant migratory waterknown to be significant dispersal agents for os-tracods (Klie, 1939; Sandberg, 1964; McKenzie,1970). Thus, there is probably a regular transportof shallow water species between all coastal areasof the lake, populations being at least potentiallyestablished wherever the habitat is appropriate .

There is very little endemism displayed by theTurkana benthic invertebrate fauna. Except for asmall number of endemics (eg . Hemicypris kliei)most species of benthic invertebrates in Lake Tur-kana have widespread geographic ranges beyondthe lake, and some of them are truly cosmopolitanon a global scale (i .e., Darwinula stevensoni,Melanoides tuberculata) .

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Ostracod depth ranges

Figure 6 illustrates the depth ranges of the 9most common ostracod species found in Lake Tur-kana, as well as their mean abundance for eachdepth range (expressed as a percentage of the totalfauna) . Below 50 meters, extremely small popula-tion sizes (rarely more than 1 live individual perdredge haul) occurred . Thus it was necessary tosupplement the live ostracod ratios (for the greaterthan 50 meter depth range) with adult dead valveratios . The great similarity between; 1) live ostracodratios from the 20-50 meter range ; 2) somewhatdeeper (max. sampled depth 84 meters) dead valveratios, and 3) rare, live specimen ratios from deepwater suggest that this is a valid approach, and thatthe data are not significantly skewed by valve re-

working . Furthermore, the deepest water ratios donot change any conclusions which could not other-wise be gained from only examining the evidence toa depth of 50 meters . The faunal composition ofthe shallowest part of the lake (less than 5 meters)is quite distinct from greater depths, being domi-nated by Hemicypris kliei to the near exclusion ofother species . Ilyocypris gibba and Cyprideis toro-sa are found in most samples, but in relatively smallnumbers . Potamocypris worthingtoni was foundprimarily in juvenile (instars II -IV) forms in shal-low water in 1979, but during the more restrictedsampling season of 1978 (E . Turkana, from AliaBay to the Omo Delta only) it was quite rare .Plesiocypridopsis newtoni (not shown in Fig . 6)may be locally abundant in shallow water, but wasnot found nearly as prolifically as apparently oc-

curred when Lindroth (1953) sampled in the Fergu-son's Gulf area.At moderate depths (5-20 meters), Hemicypris

kliei disappears and Cyprideis torosa becomesmuch more abundant . The species that dominatethe deeper water assemblages (Hemicypris interme-dia and Sclerocypris cf. clavularis) appear abun-dantly for the first time . Gomphocythere angulatais most abundant at this depth range .

There is little evidence of a `dominant' ostracodspecies in the 5-20 meter depth range . WhileCyprideis torosa is most common from5-10 meters and Sclerocypris cf. clavularis for the10-20 meter interval, the variance on these statis-tics are large, and in any given sample in the5-20 meter range, any one of several species (in-cluding Gomphocythere angulata, Potamocyprisworthingtoni and Hemicypris intermedia in addi-tion to the above named species) may be mostabundant . P worthingtoni at this depth range is

1 9 1

Fig. 6. Ostracode depth ranges. Nine common taxa are shown . Values to the left of the bars indicate mean percentage of each samplemade up by the taxa in question (n = 100) . Minimum and maximum depths for which each species has been recorded live are shownbelow each column. Hemicypris kliei is exclusively shallow water, while H. intermedia and Sclerocypris cf . clavularis are most frequentin profundal environments .

represented largely by adults, unlike its shallow wa-ter occurrences, but the implications of this pecu-liar distribution pattern are unclear. It may be sig-nificant in this regard that Lindroth collected atFerguson's Gulf between 15-23 March (1948) dur-ing the height of the rainy season, whereas my col-lections were made during the dry seasons .

Limnocythere africana occurs commonly, but atlow frequencies, in the 1-20 meter depth range. Inmany East African alkaline lakes L. africana isquite abundant in littoral and sublittoral waters .Cohen et al. (1983) and Nielsen (1984) however, sug-gest that this species may persist in Lake Turkananear the lower limit of its alkalinity range .

Below 20 meters the two species Hemicypris in-termedia and S. cf . clavularis occur in far greaterproportions than any others. Below 65 meters,valve assemblages usually contain only these twospecies, with the occasional Darwinula stevensoniand Gomphocythere angulata . The latter of these

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has not yet been recovered either alive or as emptyvalves from depths greater than 72 meters . Unfor-tunately it has not yet been possible to sample be-low 85 meters . Such depths (85-115 meters) how-ever represent only about 1% of the total lakebottom area .

Note the apparent differentiation in ranges be-tween the two species of Hemicypris in Lake Tur-kana, H. kliei being found strictly on or nearvegetated substrates while H. intermedia is almostalways profundal. The two samples containing liveH. intermedia from shallow water were both fromnonvegetated bottoms in turbid water . Lindroth(1953) described H. intermedia from swampyhabitats in the Ngong Hills of southern Kenya (al-though he gives no specific environment) .

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In order to assess the degree of co-occurrenceamong members of the soft bottom benthic faunasof the lake, an association matrix (Fig . 7) was de-veloped for the 15 most common taxa . Jaccard'scoefficient was used in the determinations ofspecies-species co-occurrences for this matrix . Thecoefficient is expressed as :

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ber of occurrences of sp . #2. For this analysis, 264soft bottom samples from both the 1978 and 1979field seasons were used . The Jaccard coefficient isused here in preference to other indices of associa-tion because of its conservatism and symmetryproperties . Valentine (1973) has suggested that Jac-card's coefficient be used where sampling is as-sumed to be relatively complete and few elementsof a local fauna are missing from any given sample,conditions largely met by this study. The data werecompiled into a best fit matrix, with association"zones" clustering around nuclei of maximum as-sociation .

Two association zones are apparent from thisanalysis, one of which is both larger in number ofassociations and stronger in depth of associationsthan the other. The larger and stronger zone centersaround the mutual associations betweenSclerocypris cf . clavularis, Hemicypris intermedia,Gomphocythere angulata and Cyprideis torosa.Hemicypris intermedia (with 86 occurrences) andSclerocypris cf . clavularis (with 93 occurrences)were found together 85 times and these consti-tute the strongest element of this association .Potamocypris worthingtoni and Darwinula steven-soni are also grouped into this zone, but at a some-what lower level of association. This first associa-tion arises from the numerous co-occurrences of allof these taxa at depths ranging from about7 -10 meters (see Fig . 6) .The smaller and looser association occurs

around the taxa Ilyocypris gibba, Hemicypris kliei,chironomid sp . A and -Limnocythere africana. Thisis the core of the shallow water (less than 5 meterswater depth), soft bottom assemblage .

Cyprideis torosa crosses over with strong associ-ations to both zones . Limnocythere africana and Il-yocypris gibba are frequently associated with Gom-phocythere angulata in the transition(5-10 meters) between the two assemblages .

The relatively infrequent occurrence of the re-maining insect (chironomid sp. B, corixid sp. A(= Micronecta sp.) and the three molluscan taxalisted) keep them from forming strong associationswith any of the other taxa . It is clear from theirrelative frequencies of association however, that themolluscs all belong with the deep water associationand the insects with the shallow water association .

Conclusions

A two year study of Lake Turkana, Kenya wasconducted to provide data on the distributionalecology its benthic invertebrates . Lake Turkana is alarge alkaline lake with internal drainage. Ekmandredge hauls at 331 sampling localities, shorelinesurveying and 02 , alkalinity, water temperature,pH and Secchi measurements form the primarydata base for this study.

Substrate variability is very high in shallowwaters, typical of large, tectonic lake basins . Muchof the lake's shoreline is sand or rock-shingle bot-toms, particularly on the south and west sides .Muddy and vegetated shallows are more restricted.Deep water substrates are almost entirely finegrained silty muds .

Oxygen and temperature data show that the lakeis holomictic except in a few shallow silled embay-ments . 02 content is almost always well abovesaturation .Three benthic faunal associations have been

identified for Lake Turkana :1) A littoral, soft bottom association, dominated

by the ostracod Hemicypris kliei and the corixidMicronecta sp. This association is found through-out the basin in water depths less than 2 m . Mostlakeside sloughs and lagoons contain these two spe-cies exclusively.

2) A littoral, rocky bottom association, com-posed of stonefly and mayfly larvae, gastropods, aleech and a sponge . This association is mostlyfound in the southern part of the lake, where hardbottoms are common.

3) A profundal, muddy bottom association,composed of stunted gastropods, chironomids andostracods. This association occurs throughout thebasin at depths below 2-5 meters .

Sandy bottoms are generally devoid of benthosat all water depths . Infaunal invertebrates, particu-larly bivalves, which frequent high energy sandybottoms in other African lakes, are absent fromLake Turkana. Epifaunal ostracods are preventedfrom feeding on shifting sandy substrates, andmacrophytes also have difficulty in colonizingthem .

Geographic distribution of benthic invertebrateswithin the lake mostly follows habitat variationswith depth . With the exception of some of therocky bottom species from the South Basin, all

193

1 94

common taxa occur throughout the lake whereverlocal substrate, water chemical and feeding condi-tions are appropriate. Most of the invertebrate spe-cies present in the lake benthos have adaptationsfor long range, passive dispersal .

Depth range and faunal association studies ofthe common invertebrate taxa show two associa-tions which can be related to water depth andwhich parallel the two soft bottom associationsmentioned above. Most probably, these associa-tions are only secondarily correlated with waterdepth, being principally regulated by food resourceavailability.

Acknowledgements

I would like to thank Leo Laporte and KayBehrensmeyer and the University of California-Santa Cruz for financial support of this project .Funding was provided by grants from NSF(#EAR77-2349), the University of California-Davis

Phylum PoriferaF. Spongillidae

sp . inident .Phylum Bryozoa

F. and sp, inident . (statoblasts only)

Phylum MolluscaCI. GastropodaSub . Cl . ProsobranchiaOrd. MesogastropodaF. Thiaridae

Melanoides tuberculataCleopatra bulimoides

F. PotamiopsidaeTomichia? n. sp .

F. BithyniidaeGabbiella rosea

F. AmpullariidaePila wernei**

Sub . Cl. PulmonataOrd. BasommatophoraF. Planorbidae

Gyraulus? sp .Ceratophallus natelensisSegmentorbis angustus

Cl . BivalviaOrd . EulamellibranchiaF. Mutelidae

Chancellor's Patent Fund and an ARCO StudentResearch Grant . Analytical field gear was providedby Jere Lipps and Charles Goldman, University ofCalifornia-Davis . I am particularly indebted to myfield assistants, Karen Higgins and Nancy Dickin-son for all their help. The staff of the KenyaDepartment of Fisheries and Wildlife, particularlyMessrs. P. C. Kongere and B. Ogilio provided mewith tremendous logistical support, without whichthis research would have been impossible. Thanksalso go to Mr. E. K. Ruchiami of the Office of thePresident, Government of Kenya, for his assistance .Many of the ideas presented here arose from con-versations with Leo Laporte, Hilde Schwartz andKay Behrensmeyer. Leo Laporte, Richard Cowen,Peter Ward, Rick Forester, Patrick DeDeckker andMary Burgis read early versions of the manuscript,and Koen Martens and Dirk Van Damme providedinvaluable assistance with the crustacean and mol-luscan taxonomy, though of course, all errors aremy own.

Appendix - Checklist of benthic macroinvertebrates recorded from Lake Turkana

Reference*

This report

Harbott (pers .commun., 1980)

This report

Verdcourt, 1960

Appendix (continued) .

Spathopsis wahlbergi hartmanni**Caelatura aegyptiaca**

F. EtheriidaeEtheria elliptica**

Phylum AnnelidaCl. OligochaetaF. and sp . inident .

Cl. HirudineaF. Glossiphonidae

Placobdella fimbriataPhylum Arthropoda

Cl. CrustaceaSub. Cl. OstracodaOrd. PodocopidaF. Cyprididae

Hemicypris fossulatusHemicypris klieiHemicypris intermediaOncocypris worthingtoniOncocypris sp .Potamocypris mastigophoraPotamocypris worthingtoniPlesiocypridopsis newtoniSclerocypris cf. clavularisSclerocypris bicornisStrandesia minuta

F. IlyocyprididaeIlyocypris gibba

F. DarwinulidaeDarwinula stevensoni

F. CytheridaeCyprideis torosaGomphocythere angulataLimnocythere africana minorLimnocythere africana africana

Cl. ArachnidaOrd. HydracarinaF. and sp . inident .

Cl. HexapodaSub. Cl. InsectaOrd. PlecopteraF. Taeniopterygidae

several sp, inident.Ord. Ephemeroptera

F. Baetidaesp . inident.

F. PolymitarchidaePovilla sp .

Ord. OdonataF. and sp . inident .

Ord. HemipteraF. CorixidaeMicronecta rasMicronecta sp .sp . Asp . B

LRFRP-Ferguson, 1975

Klie, 1939

Lowndes, 1936This reportThis report

This report

This report

This report

This report

LRFRP-Prog .Rept., 1974

This report

This reportThis reportThis report

195

196

Appendix (continued) .

References

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Burgis, M . J., P. E. Darlington, I . G. Dunn, G. G. Ganf, J . J .Gwahaba & L. M. McGowan, 1973 . The biomass and distri-bution 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 ofthe Rift Valley Lakes of East Africa . Ph.D. Diss . Univ.California-Davis, 314 pp.

Cohen, A. S ., 1984. Effect of Zoobenthic standing crop on lami-nae preservation in tropical lake sediment, Lake Turkana, E .Africa . J. Paleontol . 58: 499-510 .

Cohen, A. S ., R . Dussinger & J. Richardson, 1983 . Lacustrinepaleochemical interpretations based on Eastern and SouthernAfrican ostracodes . Paleogeogr. Paleoclimatol . Paleoecol . 43 :129-151 .

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F. Naucoridaesp. Asp . B

F. NotonectidaeAnisops worthingtoniAnisops balcis

Ord. DipteraF. Chironomidae

sp. Asp . Bsp . Csp. D

Ord. ColeopteraF. Dyticidae

Eretes sticticusEretes sp .Canthydrus biguttatusLaccophilus umbrinusCybister tripunctatus

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This reportThis report

Worthington, 1930

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Worthington, 1930Worthington, 1930Worthington, 1930Worthington, 1930Worthington, 1930

Worthington, 1930

* References are listed as This report, if collected in Lake Turkana for the first time during this survey . Unreferenced species werecollected in this survey and by earlier workers . Referenced species were not collected during this survey, but were recorded by thereferenced author .

** Collected in the Omo River Delta only .

Cah. O.R.S .T.O.M. Ser. Hydrobiol . 5 : 213 -223 .Ferguson, A. J. D ., 1975. Invertebrate production in Lake Tur-

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McKenzie, K. G ., 1971 . Paleozoogeography of freshwater os-tracoda . Bull . Cent . Res . PAU-SNPA 5: 207-237 .

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Pennak, R. W., 1978 . Freshwater Invertebrates of the UnitedStates, 2nd Edn. John Wiley & Sods, NY., 803 pp .

Roger, J ., 1944 . Mollusques fossiles et subfossiles du Bassin duLac Rudolphe. In C. Arambourg (ed .), Mission ScientifiqueDe LOmo (1932-1933). Mus. Natn . Hist. Nat., Paris 2 :119-155 .

Rome, D. R., 1962 . Ostracodes . In Exploration Hydrobiolo-giques Du Lac Tanganika (1946-47) . Inst . r. Sci . nat . Belg . 3,305 pp .

Sandberg, P. A., 1964. The ostracode genus Cyprideis in theAmericas . Stockholm Contr. Geol. 12: 1-178 .

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197

Verdcourt, B., 1960 . Some further records of molusca from N .Kenya, Ethiopia, Somaliland and Arabia, mostly from aridareas . Rev. Zool . Bot . Aft. 61: 221-265 .

Worthington, E . B,, 1932. A report on the fisheries of Ugandainvestigated by the Cambridge Expedition to the East Africanlakes 1932-33. Crown Ag. Colon ., 88 pp.

Worthington, E . B . & C . K. Ricardo, 1936 . Scientific results ofthe 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 geologi-cal significance of modern sediments in Lake Rudolf (Lakelurkana), Eastern Rift Valley, Kenya . Ph.D. Diss ., PrincetonUniv., Princeton, 305 pp .

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Received 10 July 1985; in revised form 31 January 1986 ; accept-ed 26 March 1986.