Diatom Research (2002), Volume 17 (1), 243-266

DISTRIBUTION AND ORIGIN OF DIATOMS IN THEBOTTOM SEDIMENTS OF THE SUEZ CANAL LAKES

AND ADJACENT AREAS, EGYPT

Abdelfattah A. Zalat

Geology Department, Faculty of Science, Tanta University, Tanta 31527, Egypt-

The diatom assemblages in the bottom sediments of Timsah Lake and adjacent sites, including theGreat Bitter Lake, the northern part of the Gulf of Suez and the Mediterranean Sea at the entrance ofthe northern canal, have been recovered and studied in detail. A total of 394 species and varietiesbelonging to 96 genera were identified. Of these, 263 diatom taxa were detected in Timsah Lakesediments, 151 species and varieties from the sediments of the Great Bitter Lake, 119 from sedimentsof the northern part of the Gulf of Suez, and 167 from the coastal sediments of the Mediterranean Sea.The distribution pattern of the diatom assemblages in these studied sites are related to a variety ofchanges in the palaeoecological conditions, including current direction, sea-level fluctuations, salinityand climatic changes. The Timsah Lake is considered to be a mixing zone characterized by theoccurrence of both autochthonous and allochthonous diatoms, as its basin floor includes taxatransported from both the Mediterranean and Great Bitter Lake through the Suez canal. Thecomposition of the diatom assemblages indicates that non-planktonic taxa dominate over theplanktonic forms. The assemblages are composed mainly of temperate littoral and shallowcosmopolitan marine species, accompanied by a considerable number of non-marine taxa, which arenormally allochthonous and probably originated from the adjacent areas.

INTRODUCTION

During recent years, the diatom composition of surface sediments has been used to explore andquantify the relationships between the diatom assemblages and a number of water chemistry variables,for instance pH, salinity and nutrient concentration, in order to evaluate the data for palaeolirnnologicalinference (e.g. Anderson et at. 1986, Birks et at. 1990, Batterbee & Renberg 1990, Bennion 1995,Hakansson et al. 1998). For the present study, the diatom species composition in the surface sedimentsofTimsah and Great Bitter Lakes, the northern part of the Gulf of Suez and the coastal sediments of theMediterranean Sea at the entrance of the Suez canal near Port Said city have been assessed in order toexpand our understanding of ecological and taphonomic factors, and to obtain more detailedinformation on the nature and origin of the diatom assemblages in both Timsah and Great Bitter Lakes.

The Suez canal area, as well as Timsah and Great Bitter Lakes, have received attention fromnumerous workers, among them EI-Sabh (1967, 1968), Morcos & Messieh (1973), Hassan & EI-Sabh(1975), Meshal (1975), Mohamed (1979), Toth & Lerman (1975), Stanley et at. (1982), Gerges &Stanley (1985). Their studies dealt with the mechanisms of water flow through the canal, the changes inwater chemistry through time, the water exchange between the Mediterranean Sea and the Gulf ofSuez, the hydrography of the canal and the dispersal of its sediments. However, the previous studies

244 A. A. ZALAT

did not discuss the micro floral characteristics of the surface sediments and the relationship between thedominant flora and the different ecological factors. Only Zalat (1997) gave a short taxonomic accountof the diatom taxa, which were recorded from samples, collected in 1993 from the bottom sediments ofTirnsah Lake.

Sites Description

The Suez canal is a 160 km long man-made structure, which connects the Mediterranean Sea at thenorth with the Gulf of Suez at the south, and passes through Tirnsah and Great Bitter Lakes. It issituated between latitudes 29° 55'-31 ° 15' N and longitudes 32° 15'-32 °35' E (Fig. 1). Along most of itslength, the canal is cut into unconsolidated Quaternary sediments and locally into older deposits ofMiocene and Pliocene age (Said 1962).

The Tirnsah Lake is one of the marine lakes connected by the Suez canal. It is located east of theNile Delta, nearly midway between the Mediterranean and the Red Sea. The lake has two lagoons, oneon its western and one on its southwestern shores and opens to the Suez Canal. It covers an area ofabout 15.5 krrr', with maximum depth of 16m, and nearly limited by latitudes 30° 30'-30° 35' Nandlongitudes 32° 15'- 32° 20' E (Fig. 1). The salinity of the lake water ranges between 38 %0 in the northand 45 %0 in the south, while the pH value varies between 7.8-8.3. The lake is bounded on the East byMiocene rocks and on the West by Quaternary sandy and clayey surface deposits (Philip et al. 1982).This lake tectonically formed as part of the "Clysmic Gulf' that represents the first phase in theopening of the Red Sea-Suez Rift during the Late Oligocene-Early Miocene (Said 1962).

The Great Bitter Lake is a large high-evaporation basin located south of Timsah Lake, betweenlatitudes 30° 10'-30° 25' N and longitudes 32° 18'-32° 35' E, and has a maximal depth of about 20m. Itholds the largest amount of water in the entire canal system, and it is a major trap for sediments, mostof which are transported from the south (Stanley et at. 1982). The aridity of the subtropical area;evaporation from the large water-surface area and the dissolution of old evaporites underlying largeportions of the lake have led to a high lake salinity (Meshal 1975, Hassan & El-Sabh 1975). Duringsampling, the pH value of the Great Bitter Lake water ranged between 8.1 and 8.4 and the salinity washigher than 45 %0, reaching values as high as 70 %0 in some places.

The Gulf of Suez (extending for about 300 km) is a closed arm of the Red Sea. It forms a linearzone of subsidence and has been the site of an immense accumulation of sediments throughout itsgeological history. Samples for diatom analysis were collected from the northern part of the Gulf, southof Port Taufiq city, where the water has a pH value of 8.0-8.3 and a salinity rate of about 42-43 %0. Samples from the Mediterranean coast were collected at the entrance of the Suez canal near thePort Said city, in the area located between latitudes 31° 14'-31 ° 18' N and longitudes 32° 15'-32° 25' E.The pH values in this area ranged between 8.1 and 8.4 and the salinity was 38 %0.

MA TERIAL AND METHODS

The present work is based on the study of 56 grab samples collected from bottom sediments in fourlocalities as follows: 1) 28 samples were taken from Timsah Lake. The depth of sampling ranged from0.30 m near the shore to 16m in the central part of the lake. The samples were obtained from the upper10 cm of bottom sediments. The shore samples are mainly composed of yellowish to white, fine tomedium sands with abundant Mollusca shells. The samples from the interior of the lake consist mainlyof dark blue greyey sandy clay and grayish green mud with few Molluscan shells. 2) 8 samples weretaken from the coastal sediments of the Mediterranean Sea near the northern canal terminus at Port Saidcity. These samples consist mainly of micaceous sand, silt and mud. 3) 8 samples were taken from thebottom sediments of the western area of Great Bitter Lake. These samples consist mainly of yellowishsand and olive-gray mud. 4) 12 samples were collected from the northern part of the Gulf of Suez.

DISTRIBUTION AND ORIGIN OF DIATOMS OF THE SUEZ CANAL LAKES 245

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246 A. A. ZALAT

These samples consist of a mixture of fine to medium-grained sand, mud and a carbonate fraction withminor Mollusca shells.

Diatoms were extracted from 5g of dry sediment using the following technique: the samples weretreated with 30% HCI, heated to boiling point to remove the carbonate fraction. The samples were thenrinsed several times with distilled water until pH neutral. The procedure was repeated with H202 toeliminate the organic matter. The coarse particles were removed by further decantation. Finally, onedrop of the final suspension was dried on a cover glass and mounted with ephrax.

Microscopic examination used phase contrast illumination, normally using a Zeiss microscopewith a 100x oil immersion objective. Diatom counts were made on the basis of the first 500 diatomvalves per sample at 630x magnification. The recorded diatom taxa and their relative frequency ratingin the studied localities are shown in Appendix as follows: R=Rare, F=Frequent, C=Common andA=Abundant. The average of the relative abundance of each taxon in the examined samples for eachlocality was used to determine relative frequency. The most dominant species, those that have a relativefrequencies above 5% of the total diatom valves (TDV%) in at least three samples were analyzed usingthe classification program Twinspan (Hill 1979) to distinguish the diatom assemblages. Euclideandistance measures and the average clustering method (Carney 1982) produced the cluster analysis ofthe investigated samples. Identification of the diatom taxa to species level or variety was done based onthe works of Hustedt (1930-1966), Hendey (1951, 1964, 1970), Andrews (1976,1978), Gerloff &Natour (1982), Krammer & Lange-Bertalot (1986,1988,1991), Williams & Round (1986), Round etat. (1990) and Hartley et al. (1996).

DIATOM RESULTS

Altogether, 394 species and varieties belonging to 96 genera were identified. A complete list ofthese taxa with their relative abundances is presented in the appendix. The distribution pattern of theidentified assemblages varies significantly between the studied localities, and indicates changes in thefloristic composition based on the salinity, pH-gradient, water depth and nutrient concentration.Application of Twinspan analysis on the 130 most common diatom taxa in the examined 56 samplesled to recognition of eight diatom assemblages.

Assemblage I is composed mainly of polyhalobous tychoplanktonic taxa such as Plagiogrammapulchellum, P. pulchellum var. pygmaea, P. staurophorum, Rhaphoneis nitida, R. parvula,

R. amphiceros var. tetragona, Delphineis surirella with frequent occurrence of Dimeregramma fulvum,

D. furcigerum, Fragilaria schulzii. This assemblage is characteristic of the northern part of the Gulf ofSuez.

Assemblage II, represents the Mediterranean samples and contains a high abundance ofpolyhalobous taxa; Petroneis monilifera, Rhaphoneis rhombica, R. amphiceros, Cymatosiralorenziana, Gyrosigma strigile, Biddulphia alternans, Podosira stelligera along with a frequentoccurrence of Amphora montana, Eupodiscus radiatus, Licmophora abbreviata, L. ehrenbergii,Mastogloia pumila, Synedra crystallina. The mesohalobous species, Campylodiscus echeneis, andAchnanthes brevipes, were also common in the Mediterranean sediments, but are infrequently in somesamples from the other sites.

Assemblage III is dominated by benthic, temperate and subtropical coastal species. Members ofgenus Mastogloia are most common, including Mastogloia arabica, M angulata, M cribrosa,M dissimilis, M fimbriata, M. lanceolata, M. ovulum, M. pseudoparadoxa, M. punctifera,M. splendida and M. subaffirmata associated with common to frequent occurrences of Diploneis

mediterranea, D. suborbicularis, Pleurosigma strigosum, Licmophora gracilis, L. remulus andTryblinella punctata. This assemblage characterizes the Great Bitter Lake sediments and certain taxawere frequent to rare in other samples from other sites.

Table I. Diatom results of the studied localities.

Dominant species Life-form groups pH - groups Salinity groups No. of taxa No. of Localities(%) (%) (%) identified samples

Achnanthes brevipes, Biddulphia alternans, Non-planktonic 82% Alkaliphilous 79% Polyhalobous 67.5% 167 8 MediterraneanCampylodiscus echeneis, Coscinodiscus Planktonic 18% Alkalibiontic 6% Mesohalobous 15.5% Seaeccentricus, C. lineatus, Cymatosira lorenziana, Neutral & indifferent 13% Oligohalobous 17%Odontella aurita, Pleurosira laevis, Acidophilus 2%Rhaphoneis amphiceros and Triceratium favus.

Amphora angusta, A. cofJeaeformis, Non-planktonic 71% Alkaliphilous 74% Polyhalobous 58.5% 263 28 Timsah LakeAulacoseira granulata, Azpeitia nodulifer, Planktonic 29% Alkalibiontic 5% Mesohalobous 13%Biddulphia biddulphiana, Coscinodiscus Neutral & indifferent 21% Oligohalobous 28.5%cen tra lis, C. eccentricus, C. lineatus, Cyclolellastylorum, Gyrosigma balticum, Nitzschiascalaris, N. sigma, Pleurosira laevis,Rhopalodia gibberula, R. musculus,Triceratium favus, Tryblionella granulata,T. levidensis and Tryblionella navicularis

Amphora ostrearia, Cyclotella stylorum, Non-planktonic 74% Alkaliphilous 51 % Polyhalobous 87% 151 8 Great BitterGyrosigma balticum, Mastogloia arabica, Planktonic 26% Alkalibiontic 4.2% Mesohalobous 9% LakeM. cribrosa, M. dissimilis, M. fimbriata. Neutral & indifferent 43% Oligohalobous 4%M. lanceolata, M. ovulum, M. pseudoparadoxa, Acidophilus 1.8%M. punciifera, M. splendida, M. subaffirmata,Nitzschia scalaris, N. sigma, N. sigmoidea,Paralia sulcata, Pleurosigma strigosum,Tryblinella gracilis and Tryblinella punctata.

Odontella aurita, Plagiogramma pulchellum, Non-planktonic 63% Alkaliphilous 73% Polyhalobous 88.5% 119 12 Gulf of SuezP. pulchellum var. pygmaea, P. staurophorum, Planktonic 37% Alkalibiontic 4.5% Mesohalobous 8%Rhaphoneis nitida, R. parvula, R. amphiceros, Neutral & indifferent 21% Oligohalobous 3.5%R. amphiceros var. tetragona, Delphineis Acidophilus 1.5%surirella.

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248 A. A. ZALA T

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DISTRIBUTION AND ORIGIN OF DIATOMS OF THE SUEZ CANAL LAKES 249

Assemblage IV, found only in Timsah Lake sediments, is characterised by a predominance ofoligohalobous and mesohalobous taxa, including Aulacoseira italica, A. varians, A. juergensii,Nitzschia amphibia, N. angularis, Anomoeoneis sphaerophora and Tryblionella levidensis, besideAulacoseira granulata, Amphora o valis, Gomphonema parvulum, Navicula cryptotenella, Nitzschiafrustulum, N. perminuta, Hantzschia amphioxys, H. virgata, Tryblionella circumsuta, Hyalodiscuslaevis and Pleurosira laevis that are common to frequent in both Timsah Lake and Mediterraneansediments.

Assemblage V includes mesohalobous taxa combined with polyhalobous forms. The mostdominant species are Navicula phyllepta, N. cincta, Diploneis bombus, D. crabro, Nitzschia lorenziana,Psammodictyon panduriforme, P. panduriforme var. delicatula, P. panduriforme var. lata, Rhopalodiaacuminata, Tryblionella apiculata, T gracilis and Tropidoneis lepidoptera. This assemblage wasobserved in both Timsah, Great Bitter Lakes and Mediterranean sediments but not found in the Gulf ofSuez.

Assemblage VI is characterised by the common occurrence of polyhalobous taxa with a limitednumber of polyhalobous-mesohalobous forms. Amphora angusta, Gyrosigma balticum, Nitzschiasigma, Rhopalodia gibberula, Triceratium favus and Tryblionella granulata are most common inTimsah Lake sediments but infrequently in some samples from the other three sites. Actinocyclusoctonarius, Amphora crassa, A. cymbifera, A. ocellata, Cocconeis scutellum, Coscinodiscus radiatus,C. eccentricus, Hemidiscus cuneiformis, Diploneis smithii, Lyrella lyra, L. lyroides, Odontella aurita,Trachyneis aspera, Thalassionema nitzschioides and Triceratium antediluvianum are frequent indifferent samples from the four sites.

Assemblage VII contains abundant polyhalobous taxa associated with a small amount ofpolyhalobous-mesohalobous forms. Taxa from this assemblage are distributed throughout the foursites, but with variation in their relative abundances. Three associations were observed within thisassemblage. The first association, which includes Amphora ostrearia, Azpeitia nodulifer, Cyclotellastylorum, Diploneis chersonensis, Navicula carinifera, Nitzschia bilobata, Okedenia inflexa, Opephoraangusta, Petroneis humerosa, Planothidium delicatulum and Triceratium robertsianum, are common inthe Timsah, Great Bitter Lakes and Gulf of Suez, however, absent in the Mediterranean samples. Thesecond association of Cyclotella striata, Nitzschia scalaris, N. sigmoidea, Paralia sulcata, Surirella

fastuosa, Synedra formosa and Triceratium broeckii are represented in Timsah, Great Bitter Lakes andMediterranean samples but also absent in the sediments of the Gulf of Suez. The third association iscomposed mainly of polyhalobous taxa with a few mesohalobous-oligohalobous forms, such asActinoptychus senarius, A. splendens, Amphora ostrearia var. vitrea, Biddulphia tuomeyi, Lyrellaclavata, L. praetexta, Navicula inflexa, Pleurosigma rigidum, Tropidoneis vitrea, Caloneis liber,Placoneis clem en tis, Tryblionella lanceola and Tryblionella navicularis. This association was observedonly in Timsah and Great Bitter Lakes.

Assemblage VIII is dominated by polyhalobous taxa and contains two associations. The first one ischaracteristic of only Timsah Lake sediments and includes Achnanthes angustata, Actinoptychusminutus, A. nicobaricus, Amphora graeffei, Coscinodiscus centra lis, C. gigas var. diorama,C. hirosakiensis, C. stellaris, Gyrosigma fasciola, G. obscurum, Odontella obtusa, Pleurosigmadelicatulum and Thalassiothrix longissima. The second association contains taxa, which are common tofrequent in both Timsah Lake and Mediterranean samples, such as Amphora coffea eform is,A. grannulata, Biddulphia biddulphiana, Coscinodiscus lineatus, Diploneis weissjlogii,Grammatophora angulosa, G. marina, G. oceanica, Gyrosigma sea lpro ides, Pleurosigma angulatum,Odontella rhombus, Rhopalodia musculus, Surirella striatula and Synedra tabulata.

Moreover, the cluster analysis has classified the 56 samples into seven zones with different speciescomposition. Zone I represents the coastal sediments of the Mediterranean Sea (samples 29-36),characterised by a predominance of species belonging to Assemblage II. The diversity of theencountered taxa in this zone is relatively high, and their relative abundances varied from common torare. The zone contains a high proportion of benthic diatoms and species which are most abundant in

250 A. A. ZALA T

the near shore zone. The planktonic taxa are present in low frequencies. The most common specieswere A chnanthes brevipes, Biddulphia alternans, Campylodiscus echeneis, Coscinodiscus eccentricus,C. lineatus, Cymatosira lorenziana, Odontella aurita, Pleurosira laevis, Rhaphoneis amphiceros andTriceratium favus. This zone is also dominated by taxa that prefer slightly alkaline water and belong tothe alkaliphilous group, while the neutral, alkalibiontic and acidic forms are very limited. Based ontheir tolerance to salinity, the main constituent of the diatom assemblage was assigned to the marinepolyhalobous group. The mesohalobous taxa which can tolerate a wide range of salinity from brackishto saline water are represented by a limited number of species; the most frequent were Achnanthesbrevipes, A. longipes, Pleurosira laevis, Campylodiscus clypeus, C. echeneis, Hyalodiscus laevis,Tryblionella granulata, Psammodictyon panduriforme, Nitzschia sigma, Rhopalodia gibberula,R. musculus and Surirella striatula. Oligohalobous diatoms were also frequently observed; the mostimportant taxa were Aulacoseira granulata, Cyclotella meneghiniana, C. ocellata, Amphora ovalis,Cocconeis placentula, Encyonema silesiacum, Gomphonema gracilis, G. insigne, Navicula cincta,N. radiosa, N. tenelloides and Synedra ulna.

Zone II is characteristic of the northwestern region of Timsah Lake (samples 14-18,20-21). Thisregion has the highest diatom abundance and diversity of any of the others zones. The non-planktonictaxa reach maximum abundance, while the planktonic forms are frequent. Most of the recorded diatomsbelong to the alkaliphilous group, but a considerable amount of the neutral and alkalibiontic taxa arepresent. The water salinity is about 38 %0. The polyhalobous taxa are common, while the oligohalobousand mesohalobous species, characteristic of nutrient loading, increase in relative abundance. The mostdominant species in this zone are Amphora angusta, A. cofJeaeformis, Biddulphia biddulphiana,Cocconeis scutellum, Coscinodiscus radiatus, C. eccentricus, Diploneis smithii, Gyrosigma balticum,Hemidiscus cuneiform is, Nitzschia scalaris, N. sigma, Odontella aurita, Rhopalodia gibberula,Tryblionella apiculata, T. levidensis, T. granulata, Triceratium antediluvianum and Triceratium favus.Thirty different non-marine diatoms were identified in this region, including some taxa that were notobserved in other parts of the lake. The zone includes frequent to rare, fresh and brackish, moderatelyto well preserved diatom taxa. The most frequent taxa are Aulacoseira granulata, A. italica, Cyclotellakutzingiana, C. meneghiniana, C. ocellata, Stephanodiscus rotula, Cyclostephanos dubius, Martyanamartyi, Staurosira construens, Staurosirella pinnata, Synedra ulna, Cocconeis placentula, Naviculacincta, Placoneis clementis, Sellaphora pupula, N. similis, N. viridula, Gomphonema gracilis,G. affine, G. parvulum, Amphora libyca, A. o valis, Cymbella aspera, C. cistula, C. ehrenbergii,Encyonema silesiacum, Epithemia adnata, E. smithii, E. sorex, Rhopalodia gibba, Nitzschia frustulum,Cymatpleura elliptica, C. solea and Surirella ova lis.

Zone 1lI (samples 8-13) is located in the middle of Timsah Lake and the north eastern area of thelake (samples 19, 22, 23). This zone is characterised by large increases in abundance of polyhalobousplanktonic diatoms and those taxa associated with nutrient enriched environments. The mesohalobousand oligohalobous forms are frequent, particularly in the samples 19,22,23 of the north eastern region.The most dominant species are Amphora cofJeaeformis, Actinoptychus senarius, A. splendens, Azpeitianodulifer, Biddulphia biddulphiana, Coscinodiscus cen tra lis, C. eccentricus, C. gigas, C. lineatus,C. radiatus, Cyclotella stylorum, Diploneis chersonensis, Hemidiscus cuneiformis, Lyrella clavata,Navicula carinifera, Odontella obtusa, Triceratium robertsianum and Triceratium favus. Theplanktonic diatom taxa have. maximal abundance in this zone and the most common forms belong tothe genera Actinoptychus, Coscinodiscus, Biddulphia and Triceratium. The salinity of water variesbetween 38 to 42 %0 and increased relatively toward the eastern side of the Lake. The majority of theidentified taxa belong to the alkaliphilous group. The neutral and alkalibiontic components are presentsporadically.

Zone IV covers the eastern region of Timsah Lake (samples 24-28). Many taxa, particularly thepolyhalobous, eurytopic eurythermal species, begin to increase in abundance in this region, withmarked increase in water salinity. The non-planktonic diatoms dominate over the planktonic forms.The alkaliphilous group dominates the zone with very few alkalibiontic forms, and fresh and brackish

DISTRIBUTION AND ORIGIN OF DIATOMS OF THE SUEZ CANAL LAKES 251

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water species are absent. Among the common diatom taxa are Amphora coffeaeformis, Biddulphiabiddulphiana, Coscinodiscus centralis, C. eccentricus, C. lineatus, Cyclotella stylorum, Diploneischersonensis, Gyrosigma balticum, Lyrella clavata, Odontella aurita, Paralia sulcata and Triceratium

favus.Zone V (samples 1-7) characterize the southern region of Timsah Lake. This is a region of low

diatom diversity and increasing salinity which reaches a maxima of 45 %0. The non-planktonic diatomspredominate. Most of the diatoms belong to the alkaliphilous group, while the neutral and alkalibionticelements are rare. Many diatom taxa that were observed in the Great Bitter Lake sediments are foundhere. Polyhalobous taxa are abundant and the most common are Amphora angusta, A. coffeaeformis,A. ostrearia, Cyclotella stylorum, Diploneis chersonensis, D. smithii, Gyrosigma balticum, Lyrellaclavata, Nitzschia lorenziana, N. sigma, Okedenia inflexa, Paralia sulcata, Pleurosigma rigidum,Rhopalodia gibberula, Triceratium favus and Tryblionella granulata. However, there is a sporadicoccurrence of mesohalobous and oligohalobous forms.

Zone VI (samples 37-44) come from the surface sediments of the western coast of Great BitterLake. Diatom species diversity is variable and characterized by a large number of benthic taxa and arelatively moderate number of planktonic ones. The majority of the diatoms from this zone occur inslightly alkaline to pH-neutral waters. The occurrence of alkalibiontic taxa is sporadic, while speciesliving in acidic waters are very rare. Great Bitter Lake water has high salinity values, reaching to morethan 45 %0. The lake sediments consist of sand at the shore and olive-gray mud further into the lake.The diatom community is composed mainly of marine polyhalobous species, while the eurythermaltropical and subtropical coastal species reach their maximum abundance here. Among of the mostcommon taxa are Amphora ostrearia, Cyclotella stylorum, Gyrosigma balticum, Mastogloia arabica,M cribrosa, M. dissimilis, M. fimbriata, M. lanceolata, M ovulum, M. pseudo paradoxa, M. punctifera,M. splendida, M. subaffirmata, Nitzschia scalaris, N. sigma, N. sigmoidea, Paralia sulcata, P. sulcatavar. coronata, Pleurosigma strigosum, Tryblinella gracilis and Tryblinella punctata. However, themesohalobous species are represented in a limited number, and diatoms known only from freshwaterhabitats occur in minor amounts in some shore samples.

252 A. A. ZALA T

90 r---------------------------------------------~8070605040302010o

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Fig. 4. Percent distribution of diatom pH-categories in the studied localities.

Zone VII (samples 45-56) come from northern part of the Gulf of Suez. Here, the non-planktonictaxa dominate over the planktonic forms, particularly in the marginal, shallow depth sites. However,the relative abundance of the planktonic forms increases with depth for the entire Gulf samples. Mostof the encountered diatom taxa belong to the alkaliphilous group. The neutral and indifferent elementsoccur frequently, while the alkalibiontic and acidophilous taxa are less represented. One of the mostinteresting characteristics is the absolute abundance of Rhaphoneis spp., rare in zones I and VI, andabsent in ZOnes II-V. This zone can be differentiated into two subzones. Subzone A (samples 50-56) isdominated by marine polyhalobous taxa with a few mesohalobous forms. The dominant species areOdontella aurita, Plagiogramma pulchellum, P. pulchellum var. pygmaea, P. staurophorum,Rhaphoneis nitida, R. parvula, R. amphiceros, R. amphiceros var. tetragona, Delphineis surirella,Fragilaria schulzii and Coscinodiscus cernulatus. Subzone B (samples 45-49), located close to thenorth western coast, is dominated by marine polyhalobous, temperate coastal species with significantnumbers of mesohalobous and oligohalobous taxa. Beside the dominant taxa in subzone A, thefollowing species are frequently present, Achnanthes brevipes, Amphora coffeaeformis A. cymbifera,A. decussata, Azpeitia nodulifer, Dimeregramma fulvum, D. furcigerum, Diploneis smith ii,Lithodesmium undulatum, Opephora angusta, 0. marina, 0. olsenii and Triceratium formosum.

DISCUSSION

The results gained from the Twinspan analysis of the diatom assemblages and the cluster analysisof the investigated samples indicate that the distributional pattern of diatoms is influenced by a varietyof environmental factors including water salinity, depth, nutrient supply and type of substrate. Waterdepth was the most important variable influencing the diatom assemblages composition within thestudies samples, since decreasing depth is often correlated with an increased proportion of benthic andepiphytic habitats. The encountered diatom assemblages were largely dominated by non-planktonicforms, this is attributed to the fact that samples were collected from the shallow coastal areas.

The Mediterranean diatom assemblage (Zone I) is dominated by marine polyhalobous diatom staxa, with a significant numbers of fresh and brackish water diatom species. The relative abundance ofthe freshwater taxa is relatively high in the shore samples, but decreases gradually further from theshore. The recognized taxa are similar to those reported in the Nile Delta sediments (Zalat 1995). Manyof the species are allochthonous, originating in the Nile delta region and being transported into theMediterranean Sea with the Nile River sediments.

DISTRIBUTION AND ORIGIN OF DIATOMS OF THE SUEZ CANAL LAKES 253

In Great Bitter Lake sediments (zone VI), a few freshwater taxa were also observed. Theseallochthonous fOnTISmay have been derived from the adjacent areas and transported into the lake bywater discharge from the habitation areas along the shore. No living cells were found in the water ofthe lake. Most of the marine taxa are considered to be of autochthonous origin. However, some of themmay be transported into the lake by marine currents which entered the canal from the Suez Gulf,particularly during the high sea level season in the Suez Gulf. This is illustrated by the occurrence of50% of the Gulf assemblage in the Great Bitter Lake sediments, such as species found in the firstassociation of the Twinspan assemblage VII.

Unlike the Mediterranean, the Great Bitter Lake is characterized by a high level of temperate andsubtropical coastal species belonging to the genera Mastogloia and Paralia. The genus Mastogloia ispredominant, particularly towards the south, and is represented by the common occurrence of 14 taxa.The heavy and numerous valves of Paralia sulcata in excellent state of preservation make it acandidate for palaeoecological studies (McQuoid & Hobson 1998). This species is one of the mostcommon diatom species in coastal deposits; it usually lives on the bottom but is easily lifted into theplankton (McIntire & Moore 1977, Crawford 1979). Although it is generally described as a marinediatom (salinity 25-35 %0, Hendey 1964, Hasle & Syvertsen 1996), it has also been characterized aspleio-euryhaline (salinity 5-35 %0, Simonsen 1962). Some authors have found a correlation betweenParalia sulcata valve diameter and environmental factors such as salinity and temperature (Margalef1969, Roelofs 1984). The dominance of a large valve size in the lake sediments together along withsubtropical Mastogloia species and the decrease in centric planktonic species are obviously related tothe increase in water salinity reaching more than 65 %0 southward. This agrees well with Noel'sobservations (1986) that hypersaline waters are characterized by the lack or the scarcity of centricdiatoms, and Roelofs (1984) who indicated that there may be a linear correlation between the size ofParalia sulcata and salinity.

The main constituents of the Suez Gulf diatom assemblage are autochthonous marine, temperatecoastal species. Many of the Suez Gulf species are found in the bottom sediments of Great Bitter Lake,and some of them have been found in the Timsah and Mediterranean samples. The genus Nitzschia,with a large numbers of taxa in both the Timsah and Mediterranean samples, has only a scattered andvery rare occurrence in the Suez Gulf. The Gulf diatom assemblage is generally well preserved withonly slight dissolution of some delicate forms.

Within Timsah Lake (depending on limnological, hydrodynamic and other environmentalconditions), the diatom communities varied considerably over a short distance. This small shallowwater body (like Great Bitter Lake) has the Suez Canal running through it. Therefore, the salinity of thelake is subject to seasonal fluctuations. The salinity of lake water increases relatively toward the south;its ranges from 38 %0 in the northwestern part to 45 %0 in the southern region.

Cluster analysis and Twinspan have differentiated four distinct zones (II-V) within the TimsahLake sediments.

Zone 11,covering the northwestern part of the Timsah Lake, is represented by a high abundance offresh and brackish water diatom taxa, caused by the influx of freshwater and sewage to this region.Moreover, the occurrence of many eutrophic and mesosaprobic taxa records the increase in seweringdue to human population growth. The dominance of small diatom valves, particularly benthic forms, isattributed to decreasing salinity caused by freshwater discharge from the Ismailia canal. This appears toconfirm the results of Kuylenstierna (1990), who indicated that the diatom valves of estuarine andmarine species tend to be smaller in lower salinity water. The reduction in valve size is probably due tothe increased nutrient content of the lake which is more conducive to increased vegetative proliferationof cells, with less sexual reproduction which would be accompanied by larger valves and less restingspore formation (Sanders & Cibik 1985, French & Hargraves 1985).

A salinity gradient was observed, with the greatest freshwater influences in the northern part of thelake. Zone II samples and samples 22 and 23 of zone III, contain a considerable amount of fresh andbrackish water diatoms, reflecting freshwater increase towards the north while marine fOnTISdominated

254 A. A. ZALAT

zones IV and V with decreasing numbers of non-marine taxa reflecting increased salinity toward theeastern and southern regions.

The oligohalobous freshwater forms in the Timsah Lake are normally allochthonous and they aremixed in the sediments with the marine diatoms of allochthonous and autochthonous origin. Thismixture may be due to the ability of tides to transport cells into the lake on a diurnal basis, with highspring tides or storm surges being especially proficient at mixing species from different environments(Hemphill-Haley 1995). Also the changes in circulation pattern may have resulted in a supply ofallochthonous diatom valves brought in from the shallow areas.

In general, the common occurrence of non-marine diatoms in the Timsah Lake sediments (zonesII, Ill) indicates a two-component transport system with supply from both northerly and westerly tradewinds (Zalat 1997), and transport of freshwater diatoms on one hand from the Ismailia canal andIsmailia city, and on the other hand, fluviatile deposits from the Wadi EI-Tomilat (El Shazly et al.(975). Philip et al. (1982) showed that the lake sediments were similar to that of the River Nilesediments. It is therefore believed that the lake was once connected with one of the old Nile branches(Pelusiac branch). They also suggested that the lake has experienced alternating conditions of salineand freshwater throughout its history. Furthermore, the observed oligohalobous freshwater diatoms inthe coastal sediments of the Mediterranean Sea, north of Port Said city are also represented in the lakesediments. It is assumed that these freshwater taxa are carried with the silt and clay from the Nile intothe canal during storm periods. The occurrence of freshwater diatoms and phytoliths in the lakesediments is believed to reflect climatic changes in the adjacent area.

The variability in life forms between the diatom assemblages is mainly related to water depth andsediment composition. The mud and organic-rich sediments found in the deep parts of Timsah Lakehave the maximum complement of planktonic diatoms (Zone III), while in the marginal areas ofshallow and intermediate water depths (zones II, IV, V), the non-planktonic forms dominate. All theencountered taxa are considered to be characteristic of eutrophic waters; some of them are classified asindicators of anthropogenic pollution. Particularly, in zones Ill-V, the shallow coastal, littoral marinecosmopolitan species are predominant. The mass occurrence of these taxa seems to reflect high diatomproductivity, which in turn requires waters rich in nutrients, a large supply of silica such as provided inupwelling zones and changing circulation patterns. .

A comparison of taxa from the Mediterranean, Great Bitter Lake and Suez Gulf shows that about35% of the diatoms found in Timsah Lake sediments were observed in Mediterranean sediments, 31%noted in Great Bitter Lake, and 23% in the Suez Bay. About 42% of the total diatoms areautochthonous in origin, since they are not recorded in the other localities. The common occurrence ofMediterranean diatoms compared to Red Sea diatoms in Timsah Lake sediments suggests that most ofthe time there is an open access to marine waters from the Mediterranean Sea throughout the Suezcanal. This may be attributed to the absence of ecological barriers along the canal between Port Saidand Timsah Lake. Therefore, the shallow water components of the diatom flora are transportedsouthward to Timsah Lake. El-Sabh (1968) indicated that the Mediterranean and Nile-derivedsediments can be traced southward to the vicinity of Ballah Bypass north of the Timsah Lake. Thisreflects the annual summer (August-October) net southerly flow to the Bitter lake resulting from aslightly higher sea-level in the eastern Mediterranean, due to storm winds, and from lower water levelsin the Great Bitter Lake owing to higher evaporation during summer.

The relatively low abundance of Red Sea diatoms in Timsah Lake sediments was probably due tothe presence of the Great Bitter Lake which acts as an ecological barrier. However, the occurrence ofmany Great Bitter Lake taxa in Timsah sediments, particularly in the southern region (zone V) reflectsa northward transport mechanisms. This movement of sediments results from a net northward watertransport from October through June driven by a higher sea-level in the Gulf of Suez during thisseason, and stronger tidal flood currents in the southern canal (Morcos & Messieh 1973, Hassan & EI-Sabh 1975). Therefore, Timsah Lake is considered as a zone of mixing between southerly dispersal ofMediterranean and Nile sediments and materials transported northward from Great Bitter Lake.

DISTRIBUTION AND ORIGIN OF DIATOMS OF THE SUEZ CANAL LAKES 255

908070

6050

4030

20

10

o

Fig. 5. Percent distribution of diatom life-form grouping in the studied localities.

CONCLUSIONS

13 PlanktonicEINon-planktonic

A detailed study of 56 samples collected from the bottom sediments of Timsah Lake, the westerncoast of Great Bitter Lake, the northern part of the Gulf of Suez and the Mediterranean Sea at theentrance of the northern canal identified 8 assemblages and 7 zones. Each zone is characterised by acertain diatom assemblage that reflects a distinctive environmental condition.

The distribution pattern of diatom taxa in the bottom sediments of Timsah Lake indicates changesin the floristic composition which are controlled by several factors, mainly salinity, current regime, andwater depth. The differences in the relative abundances of diatom taxa reflect changes in theenvironmental conditions during the history of the lake. The presences of shallow water components ofthe Mediterranean and Great Bitter Lake in the Timsah Lake sediments reflects south- and northwardscurrent movements which are probably related to sea-level fluctuations and climatic changes during theyear.

The identified assemblages are composed mainly of temperate littoral and shallow cosmopolitanmarine species, associated with a significant numbers of oligohalobous freshwater taxa. Theencountered freshwater diatoms are normally allochthonous and probably derived from several sourcessuch as the adjacent Nile Delta region, freshwater discharge and habitation.

ACKNOWLEDGEMENTS

I would like to thank an anonymous reviewers for the critical reading of the manuscript and theirvaluable suggestions and comments. I am also very grateful to Prof. Karen K. Serieyssol for readingthe manuscript and a linguistic review of the paper.

256 A. A. ZALAT

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