Distribution of aquatic plants in relation to environmental factors in the Nile Delta

Aquatic botany ELSEVIER

Aquatic Botany 56 (1997) 75-86

Distribution of aquatic plants in relation to environmental factors in the Nile Delta Abdel Hamid A. Khedr

a,

*, Mohamed A. E1-Demerdash

b

a Department of Botany, Faculty of Science Mansoura University, Damietta, Box 34517, Egypt b Department of Botany. Faculty of Science Mansoura University, Mansoura, Egypt Accepted 8 August 1996

Abstract This paper develops a model of the relationship between aquatic macrophytes and their environment in the irrigation and drainage canals in the north-eastern part of the Nile Delta. The study area was sampled from 60 sites which were classified by two-way indicator species analysis into seven vegetation groups ( A - G ) , with the following dominant species: Phragmites australis (Cav.) Trin. ex Steud. and Typha domingensis (Pers.) Poir. ex Steud (A); Eichhornia crassipes (C. Mart.) Solms and Echinochloa stagnina (Retz.) P. Beauv. (B); Azolla filiculoides Lam. (C); Myriophyllum spicatum L. (D); Potamogeton crispus L. and Potamogeton nodosus Poir. (E); Ceratophyllum demersum L. and Potamogeton pectinatus L. (F); Ceratophyllum demersum L. (G). The first axis of a detrended correspondence analysis ordination represented a life form gradient of the aquatic vegetation. It separated vegetation types dominated by emergent species from free-floating and submerged macrophytes. Group A, consisting mainly of emergent macrophytes, had a higher Shannon diversity index. The highest species richness value was recorded in group C, dominated by Azolla filiculoides Lain. Canonical correspondence analysis was used to study species-environment relationships. The distribution of emergent and floating species was best correlated with water electrical conductivity, K + and total phosphorus content. The distribution of submerged species showed a high correlation with the increase in canal width and decreased due to shading by marginal trees.

Keywords." Canals; Classification; Drains; Diversity; Macrophytes; Ordination

* Corresponding author. Fax: + 2 057 325803. 0304-3770/97/$17.00 Copyright © 1997 Elsevier Science B.V. All rights reserved. Pll S0304-3770(96)0 1090-X

76

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1. Introduction

In Africa, the construction of irrigation and drainage canals is usually related to the amount of water needed to be carried to and from irrigation areas (Van Aart, 1985). In Egypt, the total length of canals and drains is approximately 47 000 km (Van der Bliek et al., 1982). These canals and drains are infested by aquatic weeds. The degree of infestation is affected by environment factors, including water transparency, depth of water, physico-chemical water quality, water currents and air temperature. Problems in the Egyptian irrigation canals have increased since 1975 due to the construction of the Aswan High Dam. Much of the silt load previously carried in the water began to be deposited in Lake Nasser in the front of the dam (Pieterse, 1979). E1-Gharably et al. (1982) attributed the increasing spread of aquatic weeds in the irrigation and drainage canals of the Nile Delta to some other ecological factors, e.g. increasing pollution from agricultural practices, industrial centres and human activities along canals and drains. The permanent presence of water in canals and drains all year round may create favourable and more suitable conditions for aquatic weed growth. Studies on the ecology of aquatic vegetation of the irrigation and drainage system of the Nile Delta and the northern lakes of Egypt are very few (e.g. El-Fiky, 1974; Khedr, 1989; Shaltout and El-Sheikh, 1993; Shaltout et al., 1994). These studies have described several plant communities which are comparable to those of the present study. However, some exotic weeds, e.g. Azolla filiculoides Lam. and Myriophyllum spicatum L. have since been introduced and quickly invaded the Nile Delta. In our paper, classification and ordination techniques have been used to generate hypotheses on the relationships between community composition of aquatic macrophytes in irrigation and drainage canals and the environmental factors which might regulate community composition. The aim was also to establish a long-term monitoring programme of aquatic macrophyte change in the study area.

2. Methods 2.1. Study area

Sampling sites were selected to cover the irrigation and drainage canals in eight districts of Dakahleya province in the north-east sector of the Nile Delta. Sixty stands, representing the apparent physiognomic variations in the vegetation and environmental features, were used for sampling of aquatic plants. In each stand, the floristic data of four plots (4 m 2 each) were averaged to form one sample stand. The abundance of each species in each stand (visual estimate of coverage as a percentage of the stand area) was estimated in spring and summer, 1994. Sixty sample stands (240 plots) comprising 41 species were used in subsequent analyses. Voucher specimens are kept in the herbarium of the Botany Department, Mansoura University. Species identification was according to T'~ickholm (1974) and Boulos (1995). In each stand, water temperature, water depth and canal width were measured. Field observation indicated that sites heavily shaded by bank trees (e.g. Eucalyptus sp.,

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77

Morus sp., Casuarina sp., etc.) were clear of vegetation. This observation led us to evaluate the shade effects on the aquatic vegetation in the study area. The degree of shade of the stands from marginal trees was recorded and scored on a four-point scale (none, slight, moderate and intense). Water samples were taken from each stand for chemical analysis. Water electrical conductivity was measured with a YSI Incorporated Model 33 Conductivity Meter. Water pH was determined using a combined pH meter/digital ion analyser (Model 5986-60) with a glass electrode. Total phosphatephosphorus content was determined by the colorimetric molybdenum method (Allen et al., 1974). Total nitrogen was determined using the micro-Kjeldahl method (Allen et al., 1974). Determination of Na +, K + and Ca 2+ in the water samples was carried out using a Coming 410 flame photometer. 2.2. Data analysis The species/samples data matrix was analysed using the ordination technique, detrended correspondence analysis (DCA), and the related classification procedure two-way indicator species analysis, using the computer programs DECORANA and TWINSPAN. (Hill, 1979a,b). TWINSPAN provides a hierarchical divisive classification of the data matrix (Gauch, 1982). It expresses the relationships of samples and species within the data set. Dominant species, differentiating sample groups, are identified at each level of division. The matrix consisted of 41 species and 60 stands. It was compared with another matrix with information on 11 environmental variables recorded in the same 60 samples, by canonical correspondence analysis (CANOCO; Ter Braak, 1987, 1988). The general aim of CCA is to produce an ordination diagram of plant species and environmental variables which optimally displays how community composition varies as a function of environment. One-way analysis of variance (ANOVA) was applied to assess the significance of variation in the environmental variables representing the TWlNSPAN vegetation groups. ANOVA was carried out within the package MINITAB. Tukey's studentised range test was applied as a complementary test to the ANOVA to assess the significance of differences between each pair of means. Species richness (alpha-diversity) was evaluated by counting the number of species per stand, then the species average per TWlNSPAN group was used for comparison. The Shannon-Wiener diversity index was calculated from the formula H ' =: - Y',Pi In Pi Evenness was calculated from the formula (Pielou, 1975)

E = H ' / l n (Pi) The Simpson diversity index was used to estimate species dominance within each sample s = ZP~2

A.H,A. Khedr, M.A. EI-Demerdash/ AquaticBotany56 (1997)75-86

78 3. R e s u l t s

3.1. Classification The dendrogram (Fig. 1) produced by TWlNSPAN analysis revealed eight vegetation groups at level 3; two were joined to form group A. Vegetation groups from drainage canals (A and F) were clearly separated from those in irrigation canals (D, E and G). However, groups B and C were formed from mixed stands. Group A contained 11 samples, all from drainage canals. The dominant species were the emergent species Phragmites australis and Typha domingensis. Group B comprised 15 stands, nine from drainage canals and six from irrigation canals. The dominant species of this group was the free-floating species Eichhornia crassipes and the emergent species Echinochloa stagnina. Group C comprised 15 stands, 12 in the drains and three in the irrigation canals. The dominant species was the floating aquatic fern Azolla filiculoides. Group D comprised seven stands from irrigation canals, dominated by the submerged species Myriophyllum spicatum. Group E was formed from three stands from irrigation canals. Dominant species were Potamogeton crispus and Potamogeton nodosus. Group F comprised four stands from drains, with dominant species Ceratophyllum demersum and Potamogeton pecfinatus. Group G was represented by five stands from irrigation canals. The dominant species was Ceratophyllum demersum. Table 1 shows the mean cover abundance values of ten dominant macrophytes in the study area. Phragmites australis and Typha domingensis showed similar distributions. The free-floating species Eichhornia crassipes and Azolla filiculoides and emergent species Echinochloa stagnina had a wide range of distribution in both irrigation and drainage canals. As far as the authors are aware, this is the first record of Myriophyllum

60 stands

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23 25 27 32

28 29 30

D

Cd pp

17 22 31

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E

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16 40 41 42 44

G

Fig. 1. TWlNSPAN dendrogram of the 60 sample stands. Groups (A-G) and the indicator species are abbreviated to first letter of the genus and of the species.

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Table 1 Percentage mean cover of the dominant macrophytes at sample sites representingthe different districts in the study area Species

Mean cover (%) District a

Phragmites australis (Cav.) Trin. ex Steud. Typha domingensis (Pers.) Poir. ex Steud. Eichhornia crassipes (C. Mart.) Solms. Echinochloa stagnina (Retz.) P. Beauv. Azollafiliculoides Lam. Ceratophyllum demersum L. Potamogeton pectinatus L. Potamogeton crispus L. Potamogeton nodosus Poir. Myriophyllum ~picaturn L.

1

2

3

4

67.0 3.0 4.8 73.3 42.7 33.(I 0.0 51.3 25.2 55.8

I 1.8 0.0 12.3 18.7 0.0 45.5 42.4 30.5 6 3 . 0 15.3 31.7 14.2 5 3 . 3 100.0 4 9 . 3 22.5 20.0 5 0 . 0 60.0 0.0 20.3 13.0 27.3 17.0 0.0 50.0 41.0 67.5 24.6 0.0

5

6

7

8

25.0 17.0 52.0 46.0 82.0 24.0 0.0 0.0 0.0 0.0

21.2 57.5 74.3 54.7 48.0 2.0 0.0 2.0 0.0 0.0

3 0 . 0 50.0 14.0 0.0 2 2 . 0 50.0 9.0 9.5 9 0 . 0 60.0 33.0 0.0 0.0 0.0 33.0 0.0 0.0 0.0 0.0 0.0

a l, Mit-Ghamer:2, Mansoura; 3, Dikirnis;4, Sherbin; 5, Talkha; 6, Bilqas; 7, Manzala; 8, Simbelawin.

apicatum in the Nile Delta. It is distributed northward from Mit-Ghamer to Mansoura and Dikirnis. Potamogeton crispus and C. demersum are more represented in the samples of irrigation and drainage canals than Potamogeton nodosus and Potamogeton pectinatus. 3.2. Environmental variables Table 2 presents the mean values of the environmental and diversity variables of each T W I N S P A N vegetation group. It is clear from Table 2 that most of the environmental variables associated with the seven vegetation groups differ significantly among stands. Group A had the highest water electrical conductivity (EC) value (1.23 mS cm-~ ) and the second highest mean values of Ca 2+ (115.5 mg 1- ~). The dominant species of this group occurred in the drainage canals with the narrowest mean width (3.59 m). Group C had intermediate values for most of the environmental variables. Group D had the highest mean pH values (8.74) and occurred in the widest irrigation canals (28.8 m). It also occurred in the canals with the lowest mean EC values (0.36 mS c m - ~), and lowest Ca 2+ concentration (67.14 mg l-~). Group E occurred in irrigation canals with the lowest mean concentration of total nitrogen (0.12 mg 1-~). Group F showed the highest mean values of total nitrogen (1.07 mg 1 - i ) , Ca2+ (145,0 mg 1-~) and K + (23.75 mg I ~) concentrations. Group G showed the lowest mean values of pH (7.99). The environmental variables that correlated negatively with CCA axis 1 (Table 3) were shading ( r = - 0 . 8 8 ) , EC ( r = - 0.59), total-P ( r = - 0 . 4 1 ) , total-N ( r = - 0 . 3 3 ) , Ca 2+ ( r - - 0 . 3 5 ) and Na + ( r = - 0 . 3 3 ) . Canal width is the only variable that is correlated positively with axis 1 ( r = 0.58). Axis 2 correlates positively with K + ( r = 0 . 4 8 ) and water temperature ( r = 0 . 3 9 ) and negatively with canal width ( r = - 0.26).

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Table 2 Results o f o n e - w a y analysis o f v a r i a n c e o f environmental variables relating to 60 samples f r o m seven vegetation g r o u p s ( A - G ) o f aquatic m a c r o p h y t e s . Diversity indices are also s h o w n Aa II b 0.0/11 c

B 15 6/9

C 15 3/12

D 7 7/0.0

E 3 3/0.0

F 4 0.0/4

G 5 5/0.0

Fratio d

1.17

0.95 bd 6.4 25.2

1.47 ab 28.8 25.14 a-c 8.74 a

0.47

0.8

1.70 ns

19 25.67

13.25 27

4.55 * * 3.75 ns

8.4

8.36

0.5 cd 28.2 26.6 c 7.99

1.16 * 5.07 ns

Environmental variables W a t e r d e p t h (m) C a n a l w i d t h (m) W a t e r t e m p . (°C) pH E C ( m S c m - l) Total-P ( m g 1- i ) TotaI-N (rag 1- ~) N a + (rag 1- l) K + ( m g 1- ~) C a 2+ ( r a g 1- l)

0.99 ac 3.59 26.55 a 8.09 a

13.47 24,2 b 8.2

8.15

1.81 *

1.23 2.11

1.08 4.07

0.89 1.12

0.36 0.92

0.4 2.97

1.15 4.7

0.5 0.69

a 0.69 101.4 a 14.35 a 115.5

b 0.54 181.5 b 11.1

c 0.36 100.9 c 9.41

e 0.12 24.33 e 6.93

100

90

a-f 1.07 114.2 a-f 23.75 b 145

f 0.23 29.4 f 8.68

110.7

d 0.29 21.29 d 8.79 ab 67.14

106

2.03 *

5.80 1.26a 0.72a 0.43

5.67 0.87a 0.53a 0.55

6.07 1.07 0.61 0.45

5.43 1.02 0.59 0.49

5.33 1.03 0.77 0.31

5.20 1.02 0.74 0.39

4.50 1.01 0.73 0.41

0.39 1.89 4.42 1.80

4.61 * * 3.19 ns 8.56 * *

Diversity indices Richness Shannon (H') Evenness (J') Simpson (S)

a Vegetation group. b N u m b e r o f stands per group. c Canals/drains. d • p < 0.05; * * P < 0.01; * * * P < 0.001; ns, not significant ( P > 0.05). M e a n s sharing the s a m e letters differ significantly a c c o r d i n g to T u k e y ' s studentised range test.

Table 3 Inter-set correlations o f environmental variables with C C A axes 1 a n d 2 Variable

Axis 1

Axis 2

C a n a l w i d t h (m) W a t e r d e p t h (m) W a t e r t e m p e r a t u r e (°C) Shading EC ( m S c m - i ) pH TotaI-P ( m g 1- ~) Total-N (rag 1- i ) C a 2+ (rag 1- i) K ÷ ( m g 1- l) N a + ( m g l J)

0.58 * * * - 0.12 0.21 - 0.88 * * * - 0.59 * * * - 0.15 - 0.41 * * * - 0.33 * * -0.35 * * -0.04 -0.33 " *

- 0.26 *

* P
** P < O . O I ;

*** P < O . O O I .

- 0.18

0.39 * * 0.04 0.21 0.02 - 0.24 0.11 0.07 0.48 * * * 0.20

ns ns *** ns

A.H.A. Khedr, M.A. EI-Demerdash/ Aquatic Botany 56 (1997) 75-86

81

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Fig. 2. (a) DECORANA ordination of 6Q sample stands with TWINSPAN groups superimposed. (b) DECORANA ordination of 41 plant species with names abbreviated, see Fig. 3. 3.3. DCA ordination The T W I N S P A N groups are shown overlain on D C A axes 1 and 2 (Fig. 2(a)). Stands from group A occur on the left side o f the ordination diagram. Dominant species of this group are emergent species. Groups B and C occur in the middle of the ordination diagram• Free-floating macrophytes are the dominant species o f these groups. On the right side of the ordination diagram are groups D, E, F and G. Dominant species associated with these groups are submerged plants (Fig. 2(b)). 3.4. Species-environment relationships Fig. 3 shows a biplot ordination with species represented by points and six environmental variables represented by arrows. The emergent species Phragmites australis and

82

A.H.A. Khedr, M.A. El-Demerdash / Aquatic Botany 56 (1997) 75-86 2.5

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Fig. 3. CCA ordination diagram with environmental variables represented by arrows and species by points• The species names are: As, Alternanthera sessilis (L.) DC; Aq, Aster squmatus (Spreng.) Heiron.; Af, Azolla filiculoides Lam; Cd, Ceratophyllurn demersum L.; Cm, Chenopodium ambrosioides L.; Cy, Cynodon dactylon (L.) Pers.; Ca, Cyperus alopecuroides Rottb.; Ecg, Echinochloa crusgalli (L.) P. Beauv.; Es, Echinochloa stagnina (Retz.) P. Beauv; Ea, Eclipta alba (L.) Hassk; Ec, Eichhornia crassipes (C. Mart.) Solms; El, Elodea canadensis Michx.; Ic, lpomoea carnea Jack.; Lg, Lemna gibba L.; Ls, Ludwigia stolonifera Guill. & Perr•; Lh, Leersia hexanda Sw; Ma, Marsilea aegyptiaca Willd.; Ms, Myriophyllum spicatum L.; Mm, Mentha microphylla C. Koch; NI, Nymphaea lotus L.; Oa, Onelia alismoides L. Pers.; Pr, Panicum repens L.; Po, Paspalum distichum L.; Ps, Persicaria salicifolia (Willd.) Assenov.; Pa, Phragmites australis (Cav.) Trin. ex Steud; Pd, Pluchea dioscoridis (L.) DC; Pm, Polypogon rnonspeliensis (L.) DesK; Pc, Potamogeton crispus L.; Pn, Potamogeton nodosus Poir.; Pp, Potamogeton pectinatus L.; Rs, Ranunculus sceleratus L.; Rr, Ranunculus rionii Lagger; Rp, Rorippa palustris (L.) Besser; Rd, Rumex dentatus L•; Ss, Saccharum spontaneum L.; SI, Scirpus littoralis Schrad.; Sm, Scirpus maritimus L.; Sp, Spirodela polyrhiza (L.) Schleiden.; Sn, Solanum nigrum L.; Td, Typha domingensis (Pers•) Poir ex Steud; Va, Veronica anagallis-aquatica L.

Typha domingensis of group A are located in the top left quadrant of the biplot. The cover of these species was positively correlated with electrical conductivity (EC) and narrower drains• The floating plant commonly represented in the present study, Eichhornia crassipes, and emergent species Echinochloa stagnina showed no significant correlations with any of the studied environmental variables. The submerged species Myriophyllum spicatum, Ceratophyllum demersum, Potamogeton nodosus, Potamogeton pectinatus and Potamogeton crispus were correlated with gradient of wider canals and the lowest degree of shading. Fig. 3 shows the distribution of species along the shade gradient. The floating-leaved species Nymphaea lotus and Ottelia alismoides and the submerged species Myriophyllure spicatum, Potamogeton crispus, Potamogeton nodosus, Potamogeton pectinatus and C. demersum were associated with the wide canals with no shading• The free-floating species Eichhornia crassipes, Azolla filiculoides, Ludwigia stolonifera and Lemna gibba showed intermediate location along the shade gradient• The emergent species, T.

A.H.A. Khedr, M.A. EI-Demerdash / Aquatic Botany 56 (1997) 75-86

83

domingensis, Scirpus maritimus and Echinochloa crusgalli were associated with more shaded canals.

3.5. Species diversity Emergent and floating macrophyte groups had high species richness values. The highest mean value was recorded in group C, with the dominant species Azolla filiculoides (6.1 species per stand). However, the lowest value (4.5 species per stand) is recorded in group G, which is dominated by C. demersum. H' was highest in emergent macrophytes, group A ( H ' = 1.26), with the second highest wllue in the free-floating species, group C ( H ' = 1.07) and the lowest value in group B ( H ' = 0.87), representing floating macrophytes (Table 2). H' was positively correlated with species richness ( r = 0 . 6 1 6 , P < 0.0001) and evenness ( r = 0.433, P < 0.001) and negatively correlated with the concentration of dominance ( r = -0.754. P < 0.0001). Group E, with dominant species Potamogeton erispus and Potamogeton nodosus in the irrigation canals, showed the highest mean evenness value ( J ' = 0.77). The lowest mean evenness value ( J ' = 0.53) was recorded in group B with dominant species Eichhornia crassipes and Echinochloa stagnina. The Simpson index was highest in group B (S = 0.55). The floating macrophytes of this group had high cover abundance values in the districts under investigation (Table 1). The submerged species of group E had the lowest mean Simpson index (S = 0.31). Evenness associated with the seven vegetation groups (Table 2) was the only diversity index to differ significantly between stands.

4. Discussion

Phytosociologists have classified and described the various types of macrophyte communities (Gauch, 1982; Springuel and Murphy, 1991) and used ordination techniques to simplify distribution patteFns along the gradients of environmental variables (Grillas, 1990; Spink, 1992). The TWINSPAN dendrogram showed the kseparation of the macrophytic communities in the irrigation and drainage canals into emergent, floating and submerged communities. Canal width seems to be the most important environmental factor affecting species distribution. Communities with submerged species occurred in the wide canals, floating species were associated with canals of medium width and emergent and wetland species characterised the narrower canals. This distribution pattern would be expected, based on the distribution and succession models of Moeller (1985), and Wilcox and Simonin (1987). Recent ecological studies on the Nile Delta (Serag, 1991; Zahran and Willis, 1992; Shaltout and El-Sheikh, 1993, Shaltout et al., 1994) have described macrophytic communities in water and along canal banks. Some of the described communities are similar to those recorded in our study. These included Phragmites australis, Typha

domingensis, Eichhornia crassipes, Echinochloa stagnina, Potamogeton pectinatus, Potamogeton crispus and Ceratophyllum demersum.

84

A.H A. Khedr, M.4. EI-Demerdash/ Aquatic Botany56 (1997) 75-86

Myriophyllum spicatum and Azolla filiculoides were abundant in the sites surveyed in the present study but neither are listed in recent studies dealing with the aquatic vegetation of the Nile Delta (Serag, 1991; Shaltout and El-Sheikh, 1993; Shaltout et al., 1994). They are exotic species, recently introduced to Egypt, with high spreading capacity. Myriophyllurn spicatum was reported as a rare species in Egypt (T~ickholm, 1974). A1-Khouly (1989) reported a dense population of this species in upper Egypt and its absence in the Nile Delta. The escape of M. spicatum from upper Egypt (upstream) to the Nile Delta may be enhanced by normal stream flow and/or boating activities carrying its fragments downstream. Azolla filiculoides is usually found growing in calm water, replacing the native Lemna species in the Nile Delta (Boulos, 1995). It spreads in brackish water and highly polluted drains, indicating its wide ecological niche. The middle position of this species in relation to environment variables (Fig. 3) confirms its high invasion capacity. The aquatic plants recorded in our study have certain features in common, e.g. vegetative reproduction and relatively rapid growth (Murphy et al., 1990). Others may tolerate physical disturbance by being strong and flexible (Spink, 1992). The association of emergent and floating macrophytes in narrow canals and drains could be attributed to mechanical control, which is usually performed mainly in the wide canals and drains. However, the occurrence of submerged macrophytes in the wider canals and drains may be related to the role of shading by trees and emergent species along the banks of narrow canals. The use of trees to create a half-shade effect along streams and rivers has been introduced as an effective and economic technique to control aquatic plants (Dawson and Kern-Hansen, 1979). Shading by banks, trees, etc. has been shown to reduce photon irradiance by 35-95%, depending on tree leaf structure (Owens and Edwards, 1961). The present study indicated that the shade effect was inversely related to canal width. Shading had a greater influence on the submerged vegetation than the emergent and floating vegetation. This study supported the use of shading as an alternative management technique for macrophyte control, in at least narrow canals and drains. The CCA ordination diagram indicated a close association between the dominant species of the macrophyte groups and certain environmental variables. The highly significant environmental variables associated with the dominant species on the CCA diagram were: shade, canal width, EC and potassium ion concentration. These results are supported by those detailed in the literature (Barko et al., 1991; Haslam et al., 1991). The high species richness and diversity index ( H ' ) of the emergent and free floating macrophyte groups (A and C) may be related to their habitat heterogeneity. This supports the view that increasing habitat heterogeneity increases species diversity (Nilsson et al., 1989). Group C, dominated by Azolla, had the highest species richness among the other groups. This may be attributed to the thick mat growth pattern of this floating fern (about 5 cm), which provides a habitat for other species, e.g. Panicum repens, Ranunculus sceleratus, Polypogon monspeliensis, Ludwigia stolonifera, and Alternanthera sessilis. The present study indicates that the vegetation groups of submerged macrophytes (G, F, E and D) are less diverse than other groups, since they have lower species richness and lower concentration of dominance (Simpson index). This may be attributed to the

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high specificity between the species forming these groups and their habitats, water pollution and the stability of their habitats resulting in a slow rate of species succession (Shaltout et al., 1994). Other environmental factors, such as fluctuation of the water level, clearing practices and excessive waste discharge are also important (Khattab and E1-Gharably, 1984). To understand how the present pattern of diversity has arisen, more information related to canal management, species adaptation, migration mechanism and geographical origin of the species present in the Nile Delta should be considered. We recommend that future studies should consider the effects of canal bridges, sediment texture/composition and factors affecting the rate of water flow on the embankment vegetation. Monitoring canal and drain ecosystems should be undertaken to establish control management priorities.

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