Abstract: The present study aims at analyzing the vegetation of the wetlands alongside the fish farms in coastal deltaic region and focusing the attention towards the environmental variables that affect this type of vegetation. Classification and ordination of the data revealed seven vegetation groups (A-G) with the following dominant species: Bassia indica and Convolvulus arvensis (A), Cakile maritime (B), Phragmites australis (C), Phragmites australis and Suaeda pruinosa (D), Eichhornia crassipes and Cyperus alopecuroides (E), Amaranthus lividus, Sisymbrium irio and Amaranthus hybridus (F) and Eichhornia crassipes and Persicaria salicifolia (G). Canonical Correspondence Analysis (CCA) was applied to study the species - environment relationships. Contents of soil organic carbon, texture, pH and salinity are the main operating edaphic factors that control the distribution of this vegetation.
Introduction
The Nile Delta supports many natural and man-made types of habitats. The main natural habitats are the northern coastal dunes, salt marshes, sand sheets and brackish shallow lakes (Maruit, Idku, Borullos and Manzala). The main man-made habitats are the water courses (canals and drains), the roads and railways, the abandoned and cultivated fields and fish farms.
The vegetation of the wetlands in the Nile Delta of Egypt has been the subject of floristic and plant ecological studies for several years (e.g. El-Sheikh, 1989; Serag, 1991; Shehata, 1996; Khedr and El-Demerdash, 1997; Khedr and Serag, 1998; Khedr and Zahran, 1998; El-Kady et al., 2000 and Abd El-Aziz, 2002). The present work aims at studying the floristic features, quantitative analysis of the vegetation structure and the factors controlling the distribution of the plant communities of the wetlands habitats alongside the fish farms in the Nile delta.
Materials and Methods
The studied fish farms form a belt extending between Rosetta and Damietta near the Mediterranean sea (Fig. 1). The northern Mediterranean coast of Egypt belongs to the arid and hyperoid regions (UNESCO, 1977). The climatic conditions are warm summer (20-30oC), and mild winter (10-20oC). The aridity index (P/PET) ranges between 0.03 and 0.02 (where P is the annual precipitation and PET is the annual potential evapotranspiration). Sixty stands (size 10x10 m2 of each) with a reasonable degree of physiographic and physiognomic homogeneity were selected to represent the study area. The density and plant cover of each species have been estimated in each stand. The estimation of plant cover, has been measured by using the line-intercept method (Canfield, 1941). Relative values of density and cover of each species were calculated and summed up to give an estimate of its importance value (IV) in each stand which is out of 200. The species were classified in terms of life forms according to Raunkiaer (1934). The identification, nomenclature and floristic categories are according to Tutin et al. (1964 & 1980), Zohary (1972), Feinbrun (1978 & 1986), Tackholm (1974) and Boulos (1995, 1999 & 2000).
Soil samples were collected (one from each stand which representing profile of 50cm depth. Soil texture was determined using Bouyoucos hydrometer. The percentage of soil porosity was determined according to (1947). CaCO3 percentage was estimated by titration against 1N HCl as described by Jackson (1962). Oxidizable organic carbon was estimated using Walkely and Black rapid titration (Black, 1979). Soil salinity (E.C.) and soil reaction (pH) were estimated in 1:5 soil water extracts using the electric conductivity and pH-meters, respectively. The determination of bicarbonates and sulphates were carried out using the titration against 0.1N HCl and gravimetric with ignition of residues respectively. Na+, K+ and Ca++ were extracted using ammonium acetate solution at pH 7 and estimated using a spectrophotometer (Allen et al., 1974).
Two trends of multivariate analysis, namely: classification and ordination are commonly followed. Both trends have their merits in helping to understand the vegetational and environmental phenomena. The classification technique applied here was the Two-Way Indicator Species Analysis Using TWINSPAN-a FORTRAN Program (Hill, 1979; Hill et al., 1975). While the ordination techniques applied were the Detrended Correspondence Analysis (DCA) and the Canonical Correspondence Analysis (CCA) using CANOCO- a FORTRAN Program (Ter Breack, 1986, 1987 ). The relationships between vegetation gradients and environmental variables can be indicated on the ordination diagram produced by the Canonical Correspondence Analysis (CCA biplot), on which points represent species and arrows represent environmental variables. The statistical treatments applied in present study were according to Snedecor & Cochran (1968) and Nie et al. (1975).
Results
Sixty eight species belonging to 57 genera and 24 families were recorded in the study area. These species included 35 annuals (51.5%) and 33 perennials (48.5%). The recorded species included 19 monocot, and 49 dicots, Chenopodiaceae, Compositae, Cruciferae and Polygonaceae were the most represented families (60.3%). The life-form spectrum analysis indicated that the therophytes were highly represented (50%) followed by cryptophytes (geophytes – helophytes + hydrophytes = 22%) and chamaeophytes (13.2%). The geographical affinities of the recorded species reveal that, they are mostly the Mediterranean taxa. These taxa are either Pluriregional (22 species), Biregional (14 species) and Monoregional (2 species) [see Appendix].
The application of TWINSPAN classification on the importance values of 68 species recorded in 60 sampled stands led to the recognition of 7 vegetation groups (Fig. 2). The vegetation types are presented in synoptic Table 1. Group A comprises three stands codominated by Bassia indica (IV = 14.4%) and Convolvulus arvensis (IV = 12%). Other important species which attain relatively high IV are Sencio glaucus (10.9%) and Cakile maritime (IV = 9.1%). The indicator species identified by TWINSPAN classification in this group is Bassia indica (IV = 28.3%). Group B comprises three stands dominated by Cakile maritime which also identified as indicator species. The most common species in this group are Phragmites australis (IV = 15.8%) and Senecio glaucus (13.5%). Group C comprises 37 stands dominated by Phragmites australis (IV = 20.4%) which is also identified as indicator species for this group.
| Table 1: | Percentages mean of the importance values (out of 200) of the common weed species in different TWINSPAN groups. |
| Appendix: | Life span, life form and chorotype of plant species recorded in the study area. |
| Table 2: | Mean and standard error of different soil variables that characterize the stands of different TWINSPAN groups. |
| Table 3: | Pearson-moment correlation (r) between different soil variables in the stands of study area. [Por. = Porosity, O.C. = Organic carbon, E.C. = Electric Conductivity'*. Significant at P≤0.05 ** Significant at P≤0.01 *** Significant at P≤0.001. |
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| Table 4: | Comparison between some floristic variables in the present study and some previous related studies. |
The important species were Arthrocnemum macrostachyum (IV = 7.5%), Inula crithmoides (IV = 7%) and Juncus rigidus (IV = 6.1%). Group D comprises 8 stands codominated by Phragmites australis (IV = 17.6%) and Suaeda pruinosa (indicator species IV = 15.9%). Polypogon monspeliensis (IV = 10.1%), Bassia indica (IV = 7.2%) and Tamarix nilotica (IV = 7%) are the most common species of this group. Eichhornia crassipes (IV = 23.7) and Cyperus alopecuroides (IV = 19.2) are the codominant species of group E which comprises four stands. The indicator species was Eichhornia crassipes while Spergularia marina, Phylla nodiflora, Typha domingensis and Phragmites australis are the most important members of this group. Group F comprises two stands and co-dominated by Amaranthus lividus (IV = 17.4%), Sisymbrium irio (IV = 15.8) and Amaranthus hybridus (IV = 13.9%). Other important species are Portulara oleracea and Spergularia marina while the indicator species for this group is Sisymbrium irio. Group G comprises three stands and is codominated by Eichhornia crassipes (IV = 25.5%) and Persicaria salicifolia (IV = 21.6%). Typha domingensis (IV = 8.8%) and Cyperus rotundus (IV = 5%) are the most common species. Imperata cylindrical (IV = 13.6%) is the indicator species identified by TWINSPAN classification in this group.
The ordination diagram given by Detrended Correspondence Analysis (DCA) is shown in Fig. 3(a & b) for stands and the species respectively. The DCA ordination of stands and species are indicated on the plane of the first and second DCA axes. It is clear that the vegetation groups yielded by TWINSPAN classification are markedly distinguishable and having a clear pattern of segregation on the ordination planes. Group A (codominated by Bassia indica and Convolvulus arvensis) and group B (dominated by Cakile maritime) are separated at the upper left side. However, group C (dominated by Phragmites australis) is separated at the lower left side. Group D (codominated by Phragmites australis and Suaeda pruinosa) is separated at the middle left side.
| Fig. 1: | Location map showing the study area. |
| Fig. 2: | TWINSPAN Dendrogram of stands based on the importance values of 68 species in the wetlands alongside the fish farms in the Nile Delta. The indicator species are abbreviated to the first letters of the genus name and first three letters of species name. |
| Fig. 3a: | DCA ordination of 60 stands with TWINSPAN group. |
| Fig. 3b: | DCA ordination of the species with names abbreviated into the first letters of the genus and species. For complete name see the Appendix |
| Fig. 4: | Canonical correspondence analysis (CCA) ordination of the species and soil variables (arrows) in the wetlands alongside the fish farms in the Nile Delta. The indicator and preferential species are indicated by the first letters of the genus name and the first three letters of species name. |
Group E (codominated by Eichhornia crassipes and Cyperus alopercuroides) is separated at the lower right side, while, group F (codominated by Amaranthus lividus, Sisymbrium irio and Amaranthus hybridus)5 and group G (codominated by Eichhornia crassipes and Persicaria salicifolia) are segregated at the upper right side of the DCA diagram.
The soil variables of the seven groups of stands derived from TWINSPAN classification are presented in Table 2. The soil texture of all the vegetation groups is formed mainly of sand and partly of silt and clay. The percentage of the soil porosity is obviously comparable in all groups. Organic carbon content attained the highest mean value in the soil of group G (2.76%) and the lowest in that of group A (1.43%). The highest content of calcium carbonates (5.53%) was attained in the soil of group A, but the lowest (2.50%) in that of group F. The pH values indicate that the soil reaction is slightly alkaline in all groups. The electric conductivity (E.C.) attained its highest mean value (7899 μmhos/cm) in the soil of group C and the lowest in that of group G (880 μmhos/cm). The soil of group A had highest mean bicarbonates (0.38%), while that of group F had the lowest (0.08%). The soil of groups F and G attained the highest mean of sulphates (0.27%), while that of group B attained the lowest (0.13%). The concentrations of extractable cations, [sodium, potassium and calcium] were higher in the soils of group C than the other groups.
The correlation coefficient between different soil variables in the sampled stands are shown in Table 3. Several edaphic variables are significantly correlated with each other such as sand, silt, clay, organic carbon, pH, electric conductivity and extractable cations (Na+, K+ and Ca++). However, porosity, calcium carbonates, bicarbonates and sulphates contents have no correlations with any of the other soil variables (Table 3).
The correlation between vegetation and soil is indicated on the ordination diagram produced by Canonical Correspondence Analysis (CCA) of the biplot of species-soil is shown in Fig. 4. The length and direction of an arrow representing a given environmental variable provide an indication of the importance and direction of the gradient of environmental change for that variable. The angle between an arrow and each axis is a reflection of its degree of correlation with the axis. The most effective environmental variables which have high significant correlation with the distribution of the recorded species are organic carbon, clay, pH value, bicarbonates and salinity.
Discussion
The wetlands are considered as homes to many different types of species; their high plant productivity supports large numbers of animal species; many migratory birds stop on wetlands on the way to their winter or summer homes; act as natural water purification systems removing sediment, nutrients and toxins form flowing water; stabilize shorelines and reduce the damage caused by storm surges; important for recreation and reduce the effect of flooding.
Fish farms form a living system with its distinctive composition, structure, environmental relations, development and functions; it is entirely a man-made system. Man create this ecosystem artificially for his own purpose favouring the use of cultured fish production.
The present study show that the wetlands alongside the fish farms in the coastal Nile Delta, support a relatively low number of perennial and annual species. Sixty eight species constitute the bulk of its flora. In general the unstable conditions along the banks of water bodies (e.g. erosion, cleaning particles, repeatable change in the water level and excessive human disturbance) may inhibit the establishment of plants from seed, but favour the colonization by creeping growth (Roberts & Ludwig, 1991).
With regard to the life form spectrum of the present study, the therophytes (50%) is the most common life form followed by cryptophytes (22%). For the Egyptian flora as a whole, the percentage of these two life forms are 59 and 16.2% respectively (Hassib, 1951). For eastern part of the north Delta, the values are 55.6 and 20.5% (Mashaly, 1987) and for the water courses in the middle are 56.2% and 25.8% respectively (Shaltout et al., 1994). The nature of the prevailing climate in the study area, water availability and the sandy nature of this habitat help therophytes to predominate during the favourable seasons. On other hand the high frequency of cryptophytes in the study area may be according to McKee and Seneca (1982) due to the resistance of their rhizome to decomposition under constant submergence than other life forms. The low percentage of hydrophytes may be due to the regular change of water in the fish farms (see Appendix).
This study indicates the presence of Mediterranean phytogeographic belt in Egypt and this belt represents a transition between the pure Mediterranean and the Saharo-Arabian territory. This conclusion is based on the richness of the Mediterranean taxa (55.9%) in the present study area and also the bioclimatic features and life form spectrum. In contrast Zohary (1972) concluded that there is a gap in the Mediterranean territory between southern Palestine and Libya in which the Saharo-Arabian belt closely approach the Mediterranean coast.Kershaw and Looney (1985) stressed the importance of a comprehensive description of vegetation, to build a mental picture of an area and its vegetation and to allow a comparison as well as the ultimate classification of different vegetational units. These have sought to reduce the complexity of a set of field data either by classification and/or ordination based on floristic data. The results of vegetation analyses have been related to environmental data. Alternatively, vegetation-habitat relationships have been derived from a single analysis of combined floristic and environmental variables (Ter Break, 1987). The classification of the vegetation of the wetlands alongside the fish farms in the coastal Nile Delta following TWINSPAN analysis identified seven distinct vegetational groups. The dominant species of these groups are: Bassia indica, Convolvulus arvensis, Cakile maritime, Phragmites australis, Suaeda pruinosa, Eichhornia crassipes, Cyperus alopecuroides, Amaranthus lividus, Sisymbrium irio, Amaranthus hybridus and Persicaria salicifolia. In the Nile Delta, several ecological studies have been made, such studies have enumerated several plant communities, many of which are comparable to those recognized in the present study (Table 4).
The analysis of the relationships between the vegetation composition and environmental variables indicated that the species distribution is mainly controlled by soil texture, organic carbon, pH and salinity.
Comparing the recorded species in the present study (68 species) with those of the previous related studies on the other wetlands habitats (Table 1), it was found that 76% of them were recorded in the study of Khedr and Zahran (1998) in the two Mediterranean Deltaic Lakes (Lakes Manzala and Burollos). Along irrigation canals and drains, 55.9% and 53% of them recorded in the study of Al-Sodany (1998) on the north Nile Delta and El-Sheikh (1989) on the middle Delta, respectively. In salt marshes habitat, 36.8, 30.9 and 30.9% of them were recorded in the study of Mashaly (1987) in the coastal region of dakahleya and Damietta province; El-Demerdash et al. (1990) in the coastal region of Damietta and Port-Said province and El-Kady et al. (2000) in the coastal region of Kafr El-Sheikh and Behyra province, respectively. In aquatic habitats, 20.6, 19.1 and 14.7% of them were recorded in the study of Khedr and El-Demerdash (1997) in dakahleya province, Khedr and Serage (1998) in Damietta province and Shehata (1996) in Qalubia, Monoufia and El-Gharbia province, respectively. In rice fields habitat, 13.2, 7.4 and 3.0% of them were recorded in the study of Imam and Kosinova (1972) in Egypt, abd El-Aziz (2002) in Damietta province and Zaki and Mashaly (1992) in Dakahleya province, respectively.
From this comparison, we can conclude that, the vegetation of the wetlands habitat alongside the fish farms is related to lakes and canal banks habitat more than the other wetlands habitats in the north Nile Delta.
Acknowledgments
The author sincerely thank Prof. Dr. K.H. Shaltout, Faculty of Science, Tanta University for revising the manuscript. I wish to thank also Dr. A. Khedr for his kind help in the multivariate data analysis.