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 Turkana,

Andrew S. Cohen Department of Geosciences, University of Arizona, Tuscon, AZ 85721, USA

Keywords : , benthic, invertebrates, Africa,

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 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 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 contribute impor- Lake Turkana, the largest lake in the Gregory tant data towards our comprehension of the lacus- (Eastern) Valley of E. Africa, lies in the trine ecosystem . For a wide variety of reasons such semiarid-arid northernmost part of Kenya (Fig . 1) . work has lagged behind the study of the planktonic Because of its remote location, it has been the least and nektonic elements of most lakes . Sampling studied of the . Catchment difficulties and lack of standardized methods and drainages on the east side of the lake are primarily presentation of results are only a few of the factors derived in volcanic-rift related terrains whereas the working to limit advances in our knowledge of west side of the lake drains a mixture of volcanic lacustrine benthic organisms . Not surprisingly and Precambrian metamorphic terrains . The lake is therefore, the study of the lacustrine benthos in E . a closed basin with one major perennial influent, Africa, where even planktonic is poorly un- the , two semiperennial streams, the derstood, can only be described as rudimentary. Kerio and Turkwell, and numerous seasonal and In this report I present preliminary results flash flood streams (Fig . 2) . describing the benthos of Lake Turkana, Kenya . Like most other lakes in the Eastern Rift, Lake This study provides an initial understanding of the Turkana is moderately alkaline and saline, of the distributional ecology of the Lake's invertebrate sodium chloride/sodium bicarbonate variety (Ta- fauna, as well as data on abiotic factors influencing ble 1) . Alkalinity varied between the observed distribution patterns . 17-20 .64 meq . I -1 total CO3 2 + HCO3 within the lake proper during the study period, with some-

180

Koobi Fore N

Allia Bay

0 25 Jarigole K m . 3 ° 30'N . -

Moiti Eliye Spgs :

-~Lolebe N. Sandy Bay - Turkwell River 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 Fig. 2. Bathymetric contour map for Lake Turkana . Contour in- terval is 20 m. Adapted from data from Hopson (1975) . Note lower values in the southern basin. Alkalinity of the that the place name Loyangalani, in the SE part of the map area Omo Delta region water during flood stage was appears on other maps in this paper under an older, alternative considerably more dilute (mean 7 .63 meq .1 -1 ). spelling Loiengalani. From Cohen (1984) . pH for the same intervals and localities registered 8.6-9.5 (main lake) and 7 .7 (Omo Delta) . Addi- tional details of dissolved gas concentrations, alka- between 23-32'C depending on time of day and linity and water chemistry are given in Yuretich location . (1976, 1979), Hopson (1982) and Cohen (1982, Lake Turkana water exhibits high organic and in- 1984). organic turbidity on both a seasonal and continu- Outside of some marginal embayments Lake Tur- ous basis, such that macrophyte growth is restricted kana is unstratified with respect to dissolved oxy- to less than one meter water depth in the extreme gen and temperature. Weak daily stratification cy- north. In parts of the sediment starved Southern cles develop at midday and breakdown each night . Basin this depth increases to over four meters (see The water column is usually supersaturated with re- Hopson, 1982 and Cohen, 1982 for details) . spect to oxygen, even at depths greater than 60 meters, due to strong wind activity . Even near the maximum depths of the lake, TDO averages Previous work 60-80% saturation . Water temperatures at maxi- mum depths fluctuated between 24-26.5 °C during Interest in the benthic fauna of L. Turkana dates the study interval . Surface temperatures fluctuated 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 pH Na K Ca Mg Alk . Cl So4 P0 4 Total P Si0 2 F TDS K20 study/Ref. date)

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 2584 This study (1978/1979/-) 8 .6-9 .3 - - 16 .1-21 .8 (9 .1) (19 .5)

East African Lakes in the early 1930s . Work at Tur- Severe famine in northern Kenya lead the British kana was limited by severe logistical difficulties of Government in the 1960s to institute the Lake the day, and was primarily taxonomic in nature . Rudolf Fisheries Research Project (LRFRP), in an Fish and plankton studies (Beadle, 1932 ; Worthing- effort to alleviate food shortages by introducing ton, 1932; Worthington & Ricardo, 1936) comprise fishing into the local (previously pastoral) econo- the bulk of the work published from this research . my. In addition to stimulating the first in-depth Some ostracod descriptions from collections made study of the biology of the L. Turkana fish popula- by this expedition were published by Lowndes tions, a considerable effort was expended in study- (1936). ing the benthos, as part of a routine limnological Arambourg's 1932-1933 expedition to Turkana survey of the lake . followed, from which Roger (1944) described 17 Accurate depth soundings by the LRFRP led to species of molluscs from the lake . It is clear howev- the first good bathymetric map of the lake (Hop- er, that these were actually shell collections from on son, 1975) (Fig . 2). Lake Turkana is divided into shore, representing reworked fossils and two distinct bathymetric basins (North and South) not living populations. with a maximum (South Basin) depth of approxi- Lindroth visited Lake Turkana briefly in 1948 as mately 115 m . Valuable studies of primary and part of a study of the and biogeography secondary productivity in various lake environ- of East Africa freshwater ostracods (1953) . He ments (Ferguson, 1975), identified constraints on made a number of dip-net collections in and any future estimates of energy flow into the detriti- around Ferguson's Gulf, but took no dredge or bot- vore food chain . Detailed results of the LRFRP are tom samples. presented in Hopson (1982) . Butzer (1971), in a major study of the Omo River In connection with the LRFRP, Yuretich (1976) Delta, described sedimentological and vegetational conducted a sedimentological study of the lake . regimes of the near lake and prodeltaic regions Among his results were several of significance for around the river mouth. In addition, his climatic the present study, including a) the low organic car- studies have been important in deciphering the bon content of Turkana deep water sediments, b) cause and response correlations between short-term relatively high profundal sediment accumulation lake level fluctuations and climatic changes . rates (up to 1 mm • a -'), and c) description and 1 82

mapping of numerous textural and mineralogical shore), diurnal variations in the shelly benthos of features of the lake's deep water substrates, particu- this lake are insignificant . However, diurnal vertical larly for areas not visited by the present author. migrations of dipteran larvae, known to occur in other East African lakes (Burgis et al., 1973) presented an intractable problem beyond the scope Methods of this study. At some shallow water stations, shingle or heavi- Faunal and sediment samples were collected at ly vegetated bottoms prevented the proper opera- 331 stations throughout the lake during tion of the dredge and qualitative samples were col- July-September 1978 and July-November 1979 lected by hand . Sampling in certain shallow water (Fig. 3). Samples were taken using a modified Ek- embayments was also inhibited by the considerable 2 man Dredge with a collecting area of 225 cm and population of Crocodylus niloticus, whose cooc- a maximum sediment penetration of 50 cm . Details currence with ecologists is often incompatible . of sampling methods are given in Cohen (1984) . Immediately upon collection, a 50 gm (approx .) No systematic variations were observed between surface sediment sample was removed for later samples collected at differing times of day and it study, and then the remaining sample was sieved can be safely assumed that beneath the photic zone using a U.S. sieve size #120 (125 micron) Nalgene (which in Turkana is always less than 10 m near- 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 meas- urements were also made on these undisturbed SAMPLING STATION sediments prior to sieving. Faunal samples were immediately preserved in LOCALITIES 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 inconclu-

N sive as an indicator of soft tissues present in very small amounts in ostracod carapaces (K . Brassil, 1979, oral commun .) and this method for recogniz- ing recently dead specimens was not used. Empty whole ostracod carapaces were examined to indi- 3'30' N . - cate species proportions of adults where live popu- lations were extremely low. Such estimates may be partially biased by variations in hinge complexity between different ostracod species. The sig- nificance of such proportions and their relation- ship to live faunal assemblage proportions will be discussed below.

Benthic fauna of Lake Turkana - an introduction

Lake Turkana has a depauperate benthos, in comparison with most permanent lakes of its size . The appendix lists the taxa recovered to date based Fig. 3. Benthos sampling station localities ; 331 stations are on this and other studies . These include; I sponge recorded from 1978 and 1979 surveys (from Cohen, 1984) . species, I bryozoan species, 8 gastropod species, 3

1 83

bivalve species, 17 ostracod species, 23 + insect spe- groundwater discharge) and vegetation . The areal cies and several hydracarines and annelids (totals extent of this zone is limited by high turbidity to from both the lake proper and the Omo Delta) . Of embayments and nearshore regions along open these, only a small number are found regularly coastlines. Rare marshlands occur in the Omo Del- enough to be discussed further in this report . ta, the Kerio Turkwell Delta and in S . Central Alia The list clearly suggests a species dominance of Bay, all areas of surface or groundwater discharge. ostracods among the benthic fauna . Unlike the in- Silty mud substrates occur in most protected em- sects (aside from chironomid larvae) they occur fre- bayments on the West shore, as well as sporadically quently outside of vegetated littoral areas, and are on the East shore, north of Alia Bay. Sands and usually more abundant than molluscs or sandy silt substrates (predominately composed of chironomids in terms of both population size and quartz and volcanic fragments) occur in diversity in both littoral and profundal regions . lengthy segments along the Northwest shore, and Therefore, most of this discussion will center on the sporadically elsewhere. Details of nearshore en- ostracod fauna, with mention of other taxa only vironments are given in Cohen (1982) and Cohen et where appropriate. al. (1986). Adequate data to assess the relationship between Insects, particularly corixid and naucorid water local water chemistry, seasonal or temperature vari- bugs, and the ubiquitous ostracod Hemicypris kliei ations and benthic faunal distribution patterns do are -the most common faunal elements of this as- not currently exist. Substrate data however, suggest sociation. In extremely shallow lagoonal areas, the that three broad faunal-substrate associations oc- corixid Micronecta sp . and Hemicypris kliei are cur in the Lake Turkana benthos : usually the only macrofauna present, apparently 1) A littoral, soft bottom association. grazing on algal mats, often in great numbers. In 2) A littoral, rocky bottom, and aufwuchs (en- slightly greater depths (greater than 0 .5 m), the os- crusting) association . tracods Ilyocypris gibba, Potamocypris worthing- 3) A profundal (sensu Hutchinson, 1967), soft toni (juveniles), Cyprideis torosa and an unidenti- bottom association . fied naucorid beetle may be found . Several species There is considerable overlap between taxa of of swimming beetles (listed in the appendix) are these three associations. In optimal areas for each also associated with vegetated soft bottoms . habitat type, the associations are variable in terms Faunal densities and diversities in littoral soft of population dominance, with typical species bottom habitats are highest in areas of discontinu- sometimes absent from what might seem ideal lo- ous vegetation . They drop off slightly in areas of calities. Population densities and zoobenthic bi- continuous Cyperus and 7ypha, and almost com- omass data for all associations are given elsewhere pletely on coarser, sandy bottoms . Pulmonates, (Cohen, 1984) . Each association will be briefly which might be expected in marshy or vegetated summarized below. The term association is used in bottom habitats are conspicuously absent, except in preference to community, because the results the Omo Delta, where the large snail Pila wernei presented in this paper are principally distribution- occurs . al in nature ; preliminary data on feeding biology, competition, predation, etc., are at best circumstan- The littoral, rocky bottom association tial. Thus, to use the term `community', would be misleading, given the current status of knowledge Rocky littoral environments occur primarily in of the Lake Turkana benthos . the southern part of the lake, where - tectonic activity has been most intense during the The littoral, soft bottom association Holocene. Mixed rocky gravel and shingle sub- strates are common along the southeast shoreline The littoral zone of Lake Turkana is extremely and on the volcanic islands in the center of the lake. heterogeneous in terms of substrate texture and Extensive continuous cliffs occur in the southern- composition, sediment accumulation rate, water most regions of Lake Turkana . They are also found chemistry (due to varying degrees of evapotranspi- more sporadically around Kokoi, between Jarigole ration, photosynthetic activity, surficial and and Moiti and between Porr and El Molo Bay on

184 M a a Na Na

a a N

O w U 'w U W W C EW 0 x m

Shingle-Hard Bottom

Gravel-Gravelly Sand

Medium-Coarse Sand

Fine-Silty Sand 19

Silt-Silty Clay 7 28

Thixotropic Inorganic Silty Clay

Gyttja 44 3

W v W W E a W m 0 N a

.y a c W E 0EW .5`O W W a n y W W (0 U L a Z a W WE 0 a 0 m c 0a 2 0 W t 0 E E 4, U 0 m c9 a

Shingle-Hard Bottom 11

24 4

23 3

Fine-Silty Sand 31 1 18

Silt-Silty Clay 18 1 38 11

60 7

Gyttja 27

1 8 5

W

Pal a

T 0

Shingle-Hard Bottom

Gravel-Gravelly Sand 4

Medium-Coarse Sand

Fine-Silty Sand 26

Silt-Silty Clay 6

Thinotropic 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 The profundal, muddy bottom association west side. The littoral, rocky bottom faunal association is The profundal zone in Lake Turkana occurs at dominated by insect larvae and molluscs which depths greater than 5 meters throughout the lake, graze or forage on the epilithic algae . The insect and shallower in the turbid North Basin. With few larvae (several unidentified species of baetid stone- exceptions sublittoral substrates are silty muds flies and taeniopterygid mayflies) are found (8-12 phi mean grain size), consisting primarily of primarily in cryptic environments such as crevices clay minerals, quartz, feldspar and calcite and poor and the undersides of rocks, whereas the gastro- in organic carbon (usually less than 1%) . Yuretich pods (Gabbiella roses, Ceratophallus natalensis (1976, 1979) described systematic variations in and ?Tomichia n. sp.) occur exposed on surface profundal substrate mineralogy throughout the aufwuchs. One, as yet unidentified spongillid lake. sponge and one leech (Placobdella fimbriata) have Chironomids (4 spp), gastropods (Melanoides also been observed on the undersides of boulders tuberculata, Cleopatra bulimoides, Gabbiella roses near Loiengalani. Ostracods are rare on both and ?Gyraulus sp.) and a variety of ostracods in- vegetated and barren littoral rocky bottoms, except habit the profundal zone on soft substrates . All of where they border on mud bottoms . these taxa are apparently detritivorous in Lake Tur- 1 86

kana, although for some (i.e. Melanoides tuber- tracods associated with them for the simple reasons culata, Darwinula stevensom) this is almost cer- that; 1) the ostracods cannot remain in position on tainly facultative. All of the gastropods and most the bottom for long enough to grasp their food, of the ostracods are stunted and thin shelled rela- and 2) most food particles of a size range and qual- tive to their conspecifics in other African lakes, ity appropriate for ostracods are winnowed out of perhaps due Jo Ca2-1- undersaturation (uncommon areas with strong wave or current activity. On the littoral gastropods in Turkana are also thin shelled) . other hand, where macrophytes have been able to Population densities tend to be low (total profun- stabilize such areas (usually quite restricted `toe- dal invertebrate dry weight standing crops range holds') or where logs have been deposited, crawlers from 10 to 150 mg • m-2), reflecting the general like Hemicypris kliei may occur in abundance (even absence of detritus below 20 m (Cohen, 1984). The in areas that are otherwise barren of ostracods), proportions of individual taxa in this association having a firm surface to cling to during the near are more uniform between localities than for the continuous water motion . These abrupt faunal dis- other two. continuities do not correlate with significant water chemistry changes, but do suggest that substrate is Faunal-substrate associations an important factor for ostracod distribution in this instance. Figure 4 shows the frequency of association for Chironomids which make shallow burrows have, each abundant species with the most common sub- not surprisingly, a closer relationship with substrate strate types for the lake. Most ostracod taxa show texture than is the case with the epifaunal ostra- only weak and irregular associations with a partic- codes. Chironomid sp . A and B tolerate a wide va- ular substrate, being found instead on a wide varie- riety of predominately silty and often organic rich ty of bottom types. Aquarium studies of several substrates at medium depths . The less common, species of ostracods from Lake Turkana shed some deeper water species C and D, were found exclusive- light on this subject . ly on very fine grained bottoms (either inorganic or Four species studied in detail to date organic in the case of C, but only inorganic for D), (Plesiocypridopsis newtoni, Hemicypris interme- where they are often found in small (less than 1 cm dia, Darwinula stevensoni, and Sclerocypris cf. long), fragile, vertical tubes . The tubes are aggluti- clavularis) in my aquarium show one of two consis- nated from clay flocs, pellets, and a muccilaginous tent locomotion patterns. Crawling is restricted to binder. Like chironomid tubes elsewhere (Pennak, firm, usually vegetated surfaces, particularly on 1978) they are probably used to assist the organism macrophyte leaves and stems . Where ostracods oc- in water filtration. Species A and B occur in silt, cur over soft, unvegetated substrates, they almost and apparently do not construct tubes . perpetually hover over the sediment-water interface The mayfly larva Povilla sp. was found burrow- (excluding nonswimmers like Darwinula or infre- ing in the mud at 10 meters depth by the LRFRP, quent swimmers like Ilyocypris), usually between but was not recorded in this study. Corixids are as- 1-10 cm above the bottom . They will alight on the sociated with algal mats, which themselves develop substrate only occasionally (presumably to grasp a on a variety of underlying sedimentary textures . particle of food) and remain on the bottom for The gastropods Melanoides tuberculata and only a few seconds . None of the ostracod species Cleopatra bulimoides were primarily restricted to examined so far in my aquarium are infaunal, and soft, mud bottoms of various types, with only an no live dredge haul specimens have been observed occasional specimen found on coarse substrates . in the substrate, despite numerous searches . I con- Melanoides tuberculata is a shallow burrower, clude therefore, that most of the Lake Turkana os- while Cleopatra bulimoides may be found both in- tracods are epifaunal. (However, related species of and epifaunally. Gabbiella rosea is found in rocky Ilyocypris, Darwinula, and Cyprideis are infaunal areas as well as on soft bottoms, but always epi- elsewhere; R. Forrester, written commun. 1985; P. faunally. ?Gyraulus sp. may prefer sandier bot- DeDeckker, written commun . 1985.) Thus, their tie toms, but its rarity makes any generalization dubi- to any specific substrate is considerably reduced . ous at this time . Ceratophallus natalensis was also Sandy, high energy substrates have almost no os- found only rarely in this study but sampling of the 1 87 rocky south end of the lake and South Island was Hemicypris kliei and Ilyocypris gibba are more minimal and A. Hopson (pers . commun., 1979) in- restricted in their distributions . Since shallow, forms me that they are very abundant on rocky vegetated areas are most common in the north shorelines of that area . (from Loelia north on the west side, and from Alia Bay north, on the east side), their distributions re- flect this habitat variance. However, it can be seen Geographic distribution of benthic invertebrates that in the few localities in the South Basin (i.e., at Loyangalani Bay) where vegetated habitats do oc- The separation of the lake into distinct physio- graphic basins and water masses inevitably leads to the question of whether faunal `regions' exist, iso- lated by geography in addition to substrate type. For example, variations in benthic faunas from different parts of lake basins have been observed for (ostracods and gastropods- Cohen and Johnston, unpub.), (deca- pods, 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 distribu- tions 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 con- centric 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 inhabit- ed by this species. Many of the areas with large A numbers of C. torosa are adjacent to regions of sig- nificant Na+ and K+ enriched groundwater dis- . charge (for example the regions immediately north Fig. 5. Geographic ranges for selected common ostracod taxa Percentages refer to 076 of total live ostracod fauna for each sam- and south of Ferguson's Gulf and the area near the ple station (100 individuals counted at each station) . Turkwell Delta) . Na-1- concentration and ground- 5a) Hemicypris kliei. water discharge areas have been found to be impor- 5b) Ilyocypris gibba. tant in regulating the distribution of this species 5c) Gomphocythere angulata. 5d) Darwinula stevensoni. elsewhere (Cohen et a l., 1983; P. DeDeckker, writ- 5e) Cyprideis torosa. ten common. 1985 ; R. Forrester, oral commun . 5f) Sclerocypris cf . clavularis. 1985). 5g) Hemicypris intermedia. 1 88

36E. /Ilyocypris gibba

5-25% X25% low

cur, both of these species may be fo llusc, Ceratophallus natalen- The distributions of Gomphocyt lacobdella JObrAW and was, stevensoni are more puz he only 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 these taxa north of , despite many distribution patterns are regulated by groundwater searches on appropriate rocky habitats . These spe- seepage patterns (R . Forrester, written common . cies may be limited by the increasing alkalinity of 1985), but insufficient data exists at present on lo- the North Basin (Hart & Fuller, 1974). Certainly cal water chemi the vagility of the leech Placobdella (which is a sibility. temporary parasite of fish) would be adequate to JbhunoylvI wo spread it throughout the lake, were its distribution d a number of not being regulated by some environmental factor. to were on the w Tomichia? n, sp. also appears to be restricted to e lake. Specimens recovered from the east calm water, western inlets, north of Ferguson's were all juveniles . Gulf, where it occurs on small cobbles and plants, 189

although its water chemistry and temperature toler- habitat barriers. However, La ances are unknown. The remaining molluscan spe- portant migratory water cies of the lake proper are all widespread . The Omo known to be significant dispersal agents for os- Delta-Sanderson's Gulf fauna, indicated by aster- tracods (Klie, 1939 ; Sandberg, 1964; McKenzie, isks on the faunal list, is not found elsewhere, but 1970) . Thus, there is probably a regular transport none of these species are truly lacustrine. of shallow water species between all coastal areas The reasons for the apparently widespread na- of the lake, populations being at least potentially ture of the Lake Turkana benthos are not difficult established wherever the habitat is appropriate . to understand . Despite some geographic barriers at There is very little endemism displayed by the shallow depths, the profundal zone provides an Turkana benthic invertebrate fauna. Except for a easy corridor for passive dispersal of the few, small number of endemics (eg . Hemicypris kliei) vagile, cosmopolitan, deeper water species which most species of benthic invertebrates in Lake Tur- are present . Kornicker and Sohn (1971) have shown kana have widespread geographic ranges beyond that ostracod eggs can be transported alive in the the lake, and some of them are truly cosmopolitan digestive systems of fish . on a global scale (i.e., Darwinula stevensoni, Shallow water populations are more isolated by Melanoides tuberculata) . 1 90

Ostracod depth ranges working . Furthermore, the deepest water ratios do not change any conclusions which could not other- Figure 6 illustrates the depth ranges of the 9 wise be gained from only examining the evidence to most common ostracod species found in Lake Tur- a depth of 50 meters . The faunal composition of kana, as well as their mean abundance for each the shallowest part of the lake (less than 5 meters) depth range (expressed as a percentage of the total is quite distinct from greater depths, being domi- fauna) . Below 50 meters, extremely small popula- nated by Hemicypris kliei to the near exclusion of tion sizes (rarely more than 1 live individual per other species. Ilyocypris gibba and Cyprideis toro- dredge haul) occurred . Thus it was necessary to sa are found in most samples, but in relatively small supplement the live ostracod ratios (for the greater numbers . Potamocypris worthingtoni was found than 50 meter depth range) with adult dead valve primarily in juvenile (instars II -IV) forms in shal- ratios. The great similarity between; 1) live ostracod low water in 1979, but during the more restricted ratios from the 20-50 meter range ; 2) somewhat sampling season of 1978 (E . Turkana, from Alia deeper (max. sampled depth 84 meters) dead valve Bay to the Omo Delta only) it was quite rare. ratios, and 3) rare, live specimen ratios from deep Plesiocypridopsis newtoni (not shown in Fig . 6) water suggest that this is a valid approach, and that may be locally abundant in shallow water, but was the data are not significantly skewed by valve re- not found nearly as prolifically as apparently oc-

1 9 1

E . E m m m n~ 0 c p o~ a m m b m xl C U R m c E .U. m T ql m T Z 9 m o 5 oI E n m 6' 0 m x T U U 0 T 3 o > c 0 0 E O E E E m 0 S 0 DEPTH a RANGE m (m.) 1 83 10 4 2

1-2 77 10 2 6 11 1I 1I i 2-5 79 1 15

5-10 37 7 4 4 1 18 16 1

10-20 12 1 2 1 8 19 26 33

20-50 2 1 2 2 30 60

50 1 2 2 11 34 63 i

minimum 0 0 0 0 .5 1 .5 1 .5 1 .5 recorded depth

maximum 3.5 64 34 85 44 60 72 85 85 recorded depth

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 Fergu- represented largely by adults, unlike its shallow wa- son's Gulf area. ter occurrences, but the implications of this pecu- At moderate depths (5-20 meters), Hemicypris liar distribution pattern are unclear . It may be sig- kliei disappears and Cyprideis torosa becomes nificant in this regard that Lindroth collected at much more abundant . The species that dominate Ferguson's Gulf between 15-23 March (1948) dur- the deeper water assemblages (Hemicypris interme- ing the height of the rainy season, whereas my col- dia and Sclerocypris cf. clavularis) appear abun- lections were made during the dry seasons . dantly for the first time . Gomphocythere angulata Limnocythere africana occurs commonly, but at is most abundant at this depth range . low frequencies, in the 1-20 meter depth range . In There is little evidence of a `dominant' ostracod many East African alkaline lakes L. africana is species in the 5-20 meter depth range . While quite abundant in littoral and sublittoral waters . Cyprideis torosa is most common from Cohen et al. (1983) and Nielsen (1984) however, sug- 5-10 meters and Sclerocypris cf. clavularis for the gest that this species may persist in Lake Turkana 10-20 meter interval, the variance on these statis- near the lower limit of its alkalinity range . tics are large, and in any given sample in the Below 20 meters the two species Hemicypris in- 5-20 meter range, any one of several species (in- termedia and S. cf . clavularis occur in far greater cluding Gomphocythere angulata, Potamocypris proportions than any others. Below 65 meters, worthingtoni and Hemicypris intermedia in addi- valve assemblages usually contain only these two tion to the above named species) may be most species, with the occasional Darwinula stevensoni abundant . P worthingtoni at this depth range is and Gomphocythere angulata . The latter of these

1 92

has not yet been recovered either alive or as empty Analysis of faunal associations valves from depths greater than 72 meters . Unfor- tunately it has not yet been possible to sample be- In order to assess the degree of co-occurrence low 85 meters . Such depths (85-115 meters) how- among members of the soft bottom benthic faunas ever represent only about 1% of the total lake of the lake, an association matrix (Fig . 7) was de- bottom area . veloped for the 15 most common taxa . Jaccard's Note the apparent differentiation in ranges be- coefficient was used in the determinations of tween the two species of Hemicypris in Lake Tur- species-species co-occurrences for this matrix . The kana, H. kliei being found strictly on or near coefficient is expressed as: vegetated substrates while H. intermedia is almost always profundal. The two samples containing live C H. intermedia from shallow water were both from NI +N2 -C nonvegetated bottoms in turbid water . Lindroth (1953) described H. intermedia from swampy where C iss the number of samples in which the two habitats in the of southern Kenya (al- species . being compared co-occur, N l is the total though he gives no specific environment) . number of occurrences of sp . #1 and N2 the num-

a 0 • 7 O V • Q1 U U E N ¢ m m 3 C • • ¢ Ce • • m a 0 • a • Q) • • O • V N x 0 • Q) 0 0 V) 2 • U Gabbiella rosea .10 .10®®® .00 .02 .07 .00 .00 .05 .00 0o 0 I I Sclerocypris cf . clavularis . , .9 0 ®®®M®E®® .08 .03 .04 .00 HemicvDria intermedia ,M®EM®®®® .07 .03 .00 .00 .01- .15 gvorideig toros a M 09 .04 .01 Gomphocvthere anqulata . . . 04 .05 .02 .00 .16- .30 Potamocypris worthinptoni , 08 .10 .04 .03 parwinula ~tevensoni . . . ®~®~® .05 .02 02 .31-.45 Cleopatra bulimoides M,E®®®®05 .04 .00 .00 .00 Melanoides tuberculata. . . . ®, 08 .04 .03 .04 .00 .46-.60 ∎ Limnocvthere africana . . No M® ,.30®M® .00 icbba 04 > .60 )IVOCVDri§ E∎ ® ∎U,®.20 ∎ chirono mid sp . A ®, E .30 .03 chironomid sp . B 20 .05 Hemicvorig Klil:i NEON NONE. :.'"..EM11 M corixid sp . A ∎∎. ∎ 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 associ- ation values is discussed in the text. 193 ber of occurrences of sp . #2. For this analysis, 264 Conclusions soft bottom samples from both the 1978 and 1979 field seasons were used . The Jaccard coefficient is A two year study of Lake Turkana, Kenya was used here in preference to other indices of associa- conducted to provide data on the distributional tion because of its conservatism and symmetry ecology its benthic invertebrates . Lake Turkana is a properties . Valentine (1973) has suggested that Jac- large alkaline lake with internal drainage. Ekman card's coefficient be used where sampling is as- dredge hauls at 331 sampling localities, shoreline sumed to be relatively complete and few elements surveying and 02, alkalinity, water temperature, of a local fauna are missing from any given sample, pH and Secchi measurements form the primary conditions largely met by this study. The data were data base for this study. compiled into a best fit matrix, with association Substrate variability is very high in shallow "zones" clustering around nuclei of maximum as- waters, typical of large, tectonic lake basins . Much sociation. of the lake's shoreline is sand or rock-shingle bot- Two association zones are apparent from this toms, particularly on the south and west sides . analysis, one of which is both larger in number of Muddy and vegetated shallows are more restricted. associations and stronger in depth of associations Deep water substrates are almost entirely fine than the other. The larger and stronger zone centers grained silty muds. around the mutual associations between Oxygen and temperature data show that the lake Sclerocypris cf. clavularis, Hemicypris intermedia, is holomictic except in a few shallow silled embay- Gomphocythere angulata and Cyprideis torosa. ments. 02 content is almost always well above Hemicypris intermedia (with 86 occurrences) and saturation . Sclerocypris cf. clavularis (with 93 occurrences) Three benthic faunal associations have been were found together 85 times and these consti- identified for Lake Turkana: tute the strongest element of this association . 1) A littoral, soft bottom association, dominated Potamocypris worthingtoni and Darwinula steven- by the ostracod Hemicypris kliei and the corixid soni are also grouped into this zone, but at a some- Micronecta sp. This association is found through- what lower level of association. This first associa- out the basin in water depths less than 2 m . Most tion arises from the numerous co-occurrences of all lakeside sloughs and lagoons contain these two spe- of these taxa at depths ranging from about cies exclusively. 7 -10 meters (see Fig. 6). 2) A littoral, rocky bottom association, com- The smaller and looser association occurs posed of stonefly and mayfly larvae, gastropods, a around the taxa Ilyocypris gibba, Hemicypris kliei, leech and a sponge . This association is mostly chironomid sp. A and -Limnocythere africana. This found in the southern part of the lake, where hard is the core of the shallow water (less than 5 meters bottoms are common. water depth), soft bottom assemblage . 3) A profundal, muddy bottom association, Cyprideis torosa crosses over with strong associ- composed of stunted gastropods, chironomids and ations to both zones. Limnocythere africana and Il- ostracods. This association occurs throughout the yocypris gibba are frequently associated with Gom- basin at depths below 2-5 meters. phocythere angulata in the transition Sandy bottoms are generally devoid of benthos (5-10 meters) between the two assemblages . at all water depths . Infaunal invertebrates, particu- The relatively infrequent occurrence of the re- larly bivalves, which frequent high energy sandy maining insect (chironomid sp. B, corixid sp. A bottoms in other African lakes, are absent from (= Micronecta sp.) and the three molluscan taxa Lake Turkana. Epifaunal ostracods are prevented listed) keep them from forming strong associations from feeding on shifting sandy substrates, and with any of the other taxa . It is clear from their macrophytes also have difficulty in colonizing relative frequencies of association however, that the them. molluscs all belong with the deep water association Geographic distribution of benthic invertebrates and the insects with the shallow water association . 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 Chancellor's Patent Fund and an ARCO Student local substrate, water chemical and feeding condi- Research Grant . Analytical field gear was provided tions are appropriate. Most of the invertebrate spe- by Jere Lipps and Charles Goldman, University of cies present in the lake benthos have adaptations California-Davis . I am particularly indebted to my for long range, passive dispersal . field assistants, Karen Higgins and Nancy Dickin- Depth range and faunal association studies of son for all their help. The staff of the Kenya the common invertebrate taxa show two associa- Department of Fisheries and Wildlife, particularly tions which can be related to water depth and Messrs. P. C. Kongere and B. Ogilio provided me which parallel the two soft bottom associations with tremendous logistical support, without which mentioned above. Most probably, these associa- this research would have been impossible. Thanks tions are only secondarily correlated with water also go to Mr. E. K. Ruchiami of the Office of the depth, being principally regulated by food resource President, Government of Kenya, for his assistance. availability. Many of the ideas presented here arose from con- versations with Leo Laporte, Hilde Schwartz and . Leo Laporte, Richard Cowen, Acknowledgements Peter Ward, Rick Forester, Patrick DeDeckker and Mary Burgis read early versions of the manuscript, I would like to thank Leo Laporte and Kay and Koen Martens and Dirk Van Damme provided Behrensmeyer and the University of California- invaluable assistance with the and mol- Santa Cruz for financial support of this project . luscan taxonomy, though of course, all errors are Funding was provided by grants from NSF my own. (#EAR77-2349), the University of California-Davis

Appendix - Checklist of benthic macroinvertebrates recorded from Lake Turkana

Reference* Phylum Porifera F . Spongillidae This report sp . inident . Phylum Bryozoa F . and sp, inident . (statoblasts only) Harbott (pers . commun ., 1980) Phylum CI . Sub . Cl . Prosobranchia Ord . Mesogastropoda F. Thiaridae Melanoides tuberculata Cleopatra bulimoides F. Potamiopsidae Tomichia? n . sp . This report F . Gabbiella rosea F . Ampullariidae Pila wernei** Sub . Cl . Pulmonata Ord . Basommatophora F. Planorbidae Gyraulus? sp . Ceratophallus natelensis Verdcourt, 1960 Segmentorbis angustus Cl . Bivalvia Ord . Eulamellibranchia F . Mutelidae 195

Appendix (continued) .

Spathopsis wahlbergi hartmanni** Caelatura aegyptiaca** F. Etheriidae Etheria elliptica** Phylum Annelida Cl. Oligochaeta F. and sp . inident . LRFRP-Ferguson, 1975 Cl. Hirudinea F. Glossiphonidae Placobdella fimbriata Phylum Arthropoda Cl. Crustacea Sub. Cl. Ostracoda Ord. F. Cyprididae Hemicypris fossulatus Klie, 1939 Hemicypris kliei Hemicypris intermedia Oncocypris worthingtoni Lowndes, 1936 Oncocypris sp . This report Potamocypris mastigophora This report Potamocypris worthingtoni Plesiocypridopsis newtoni Sclerocypris cf. clavularis Sclerocypris bicornis This report Strandesia minuta F. Ilyocyprididae Ilyocypris gibba F. Darwinulidae Darwinula stevensoni F. Cytheridae Cyprideis torosa Gomphocythere angulata This report 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. This report Ord. Ephemeroptera F. Baetidae sp . inident. This report F. Polymitarchidae Povilla sp . LRFRP-Prog . Rept ., 1974 Ord. Odonata F. and sp . inident . This report Ord. Hemiptera F. Corixidae Micronecta ras Micronecta sp . This report sp . A This report sp . B This report 196

Appendix (continued).

F . Naucoridae sp . A This report sp . B This report F . Notonectidae Anisops worthingtoni Anisops balcis Worthington, 1930 Ord . Diptera F . Chironomidae sp . A This report sp . B This report sp . C This report sp . D This report Ord . Coleoptera F . Dyticidae Eretes sticticus Worthington, 1930 Eretes sp . Worthington, 1930 Canthydrus biguttatus Worthington, 1930 Laccophilus umbrinus Worthington, 1930 Cybister tripunctatus Worthington, 1930 F . Hydrophilidae Coleostoma sp . 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 Cah . O .R.S .T.O.M . Ser. Hydrobiol . 5 : 213 -223 . Ferguson, A. J. D ., 1975. Invertebrate production in Lake Tur- Beadle, L . C ., 1932 . Scientific results of the Cambridge expedi- kana . Symp. on the hydrobiology and fisheries of Lake tion to east African lakes 1930-1931 . The waters of some Turkana-Molo. Lake Rudolph Fish . Res . Proj . 13 pp. East African lakes in relation to their fauna and flora . Zool. Hart, C . W. & S. L. H . Fuller (eds), 1974 . Pollution Ecology of J. linn . Soc. 38 : 157-211 . Freshwater Invertebrates . Academic Press, N.Y., 389 pp. Burgis, M . J., P. E. Darlington, I . G. Dunn, G. G. Ganf, J . J . Hopson, A . J. (ed.), 1975 . Lake Rudolf Fisheries Research Pro- Gwahaba & L. M. McGowan, 1973 . The biomass and distri- ject Progress Report . 14 pp . (unpub.) . bution of organisms in Lake George, . Proc. R . Soc . Hopson, A . J . (ed .), 1982 . Lake Turkana: A report on the find- Lond. B 184 : 271-298 . ings of the Lake Turkana Project . Univ. Stirling, 1605 pp. Butzer, K. W., 1971 . Recent History of an Ethiopian Delta . Univ. Hutchinson, G. E., 1967 . A Treatise on Limnology 2. Introduc- Chicago, Dep. of Geogr. Res . Pap. 136, 184 pp . tion to Lake Biology and Limnoplankton . Wiley & Sons Inc ., Cohen, A . S., 1982 . Ecological and Paleoecological Aspects of N.Y., 1115 pp . the of East Africa . Ph.D. Diss . Univ. Jonasson, P. M ., 1969. Bottom fauna and eutrophication . In California-Davis, 314 pp. Eutrophication : Causes, Consequences, Correctives . Natn . Cohen, A. S ., 1984. Effect of Zoobenthic standing crop on lami- Acad . Sci., Wash . D.C . : 274-305 .' nae preservation in tropical lake sediment, Lake Turkana, E . Klie, W., 1939 . Ostracoden aus dem Kenia-Gebeit, vorhehmilich Africa . J. Paleontol . 58 : 499-510 . von dessen Hochgebirgen . Int . Revue ges. Hydrobiol . 39 : Cohen, A. S ., R . Dussinger & J. Richardson, 1983 . Lacustrine 99-161 . paleochemical interpretations based on Eastern and Southern Kornicker, L . S. & I . G. Sohn, 1971 . Viability of ostracod eggs African ostracodes . Paleogeogr. Paleoclimatol . Paleoecol . 43 : egested by fish and effects of digestive fluids on ostracode 129-151 . shells : ecologic and paleoecologic implications. Paleoecologie Cohen, A . S ., D. Ferguson, P. Gram, S. Hubler & K . Sims, 1986 Ostracodes . Pau 1970 . Bull . Cent . Res . Pau-SNPA 5 : (in press) . The distribution of coarse grained sediments in 125-135 . modern Lake Turkana : - Implications for clastic sedimenta- Lindroth, S ., 1953 . Taxonomic and zoogeographic studies of the tion models of Rift Lakes . Geol . Soc. London, Symp. ostracode fauna in the inland waters of East Africa . Zool. Sedimentation Afr. Rift System . Blackwell Publishing Co ., Bidr. Uppsala Univ. 30 : 43-156 . Lond . Litterick, M ., J. Gaudet, J . Kalff & J. Melack, 1979 . The limnol- Dejoux, C ., L . Lauzanne & C . Leveque, 1971 . Nature des fonds ogy of an African Lake, , Kenya . Soc. Int . Lim- et repartition des organismes benthique dans la region do Bol . nol . Wkshop Afr. Lakes, 73 pp. 197

Lowndes, A . G., 1936. Scientific results of the Cambridge Expe- Verdcourt, B., 1960 . Some further records of molusca from N . dition to the East African lakes 1930-1931, 16 . The smaller Kenya, , Somaliland and Arabia, mostly from arid crustacea . Zool . J. linn . Soc . 40, 31 pp . areas . Rev. Zool . Bot . Aft. 61 : 221-265 . McKenzie, K . G ., 1971 . Paleozoogeography of freshwater os- Worthington, E . B,, 1932. A report on the fisheries of Uganda tracoda . Bull . Cent . Res . PAU-SNPA 5 : 207-237 . investigated by the Cambridge Expedition to the East African Nielsen, C ., 1984 . Ostracods as paleochemical indicators at Lake lakes 1932-33. Crown Ag. Colon ., 88 pp. Elmenteita, Kenya. Am . Quart. Ass . Bienn. meeting . Boulder Worthington, E . B . & C . K. Ricardo, 1936 . Scientific results of Co., USA . Prog . with abstract 8 : 94 . the Cambridge Expedition to the East African lakes, Pennak, R . W., 1978 . Freshwater Invertebrates of the United 1930-31, 15 . The fish of Lake Rudolf and . States, 2nd Edn. John Wiley & Sods, NY., 803 pp . Zool . J . linn . Soc. 39 : 353-389 . Roger, J ., 1944 . Mollusques fossiles et subfossiles du Bassin du Yuretich, R . F., 1976 . Sedimentology, geochemistry and geologi- Lac Rudolphe. In C . Arambourg (ed .), Mission Scientifique cal significance of modern sediments in Lake Rudolf (Lake De LOmo (1932-1933) . Mus . Natn . Hist. Nat., Paris 2 : lurkana), Eastern Rift Valley, Kenya . Ph.D. Diss ., Princeton 119-155 . Univ., Princeton, 305 pp . Rome, D. R., 1962 . Ostracodes . In Exploration Hydrobiolo- Yuretich, R . F., 1979 . Modern sediments and sedimentary giques Du Lac Tanganika (1946-47) . Inst . r. Sci . nat . Belg . 3, processes in Lake Rudolf (Lake Turkana), Eastern Rift Valley, 305 pp . Kenya. Sedimentology 26 : 313-331 . 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 Received 10 July 1985 ; in revised form 31 January 1986 ; accept- Biosphere. Prentice Hall Inc . . Englewood Cliffs, N .J., 511 pp . ed 26 March 1986.