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The Avicennia And Rhizophora Mangrove Habitats In Iran Biology Essay

This study investigates the physico-chemical variations of surface water and soil within mangrove habitat and their influence processes in the Hormozgan province, south of Iran. Point sampling method within transect was used for a period of one year started from September 21th 2008. The biweekly water sampling and seasonally soil sampling were conducted in Avicennia marina and Rhizophora mucronata habitats. The comparison of mean values using t-test indicated that there is significantly differences among all variables including ˚C, pH, EC, TSS and salinity (p<0.01), excluding DO (p>0.05). The soil tested also showed significant difference between two habitats for available potassium, organic carbon, percentage of clay and silt at depth of 0-20 cm, and organic carbon, pH, EC and percentage of sand and silt at depth of 20-40 cm (p<0.05). The results showed that water and soil characteristics are the most important environmental factors directly affecting mangrove productivity and structure. The results also illustrated that the magnitude and periodicity of the coastal system forces such as complex climatic conditions (temperature), the availability of water, physico-chemical characteristics of environment such as EC, pH and other characteristics related to water and soil, may determine the floral and faunal composition in mangrove areas and the energy signature. Furthermore, the study showed that lack of perennial flow of freshwater district may be the reason for uniformity in soil texture within mangrove forests. The sediment texture exerts strong control on other factors such as conductivity, pH, calcium carbonate, organic carbon, available phosphorus and available potassium.

Keywords: Environmental characteristics, Seasonal changes, Mangrove, Iran

INTRODUCTION

The mangroves usually occur in the intertidal zone (Naidoo, 2009) within tropical and subtropical coastal rivers, estuaries and bays of the world (Zhou et al., 2010), in sheltered saline to brackish environments (Augustinus, 1995). They act as a buffer between near shore and coastal/estuarine/bay environments with regard to the influence of salinity regime (Ramanathan, 1997). In addition, they act as a transitional zone between land and sea, where they may receive organic materials from estuarine or oceanic ecosystems (Rajkumar et al., 2009).

Since the magnitude and periodicity of the coastal system  forces such as tides, nutrients, hydro-period and stresses such as cyclones, drought and salt accumulation may mostly determine the floral and faunal composition in mangrove areas and the ‘energy signature’ (Saravanakumar et al., 2008). Thus, it has been suggested that mangrove functional and structural properties are influenced by a complex of climatic conditions, the availability of water, soil structure and some environmental physico-chemical characteristics such as temperatures, EC, pH and others related to water and soil (Augustinus, 1995).

On the other hand, distribution of nutrients and organic materials, which are mainly based on season, tidal conditions and freshwater flow, determine the fertility potential and healthiness of mangrove ecosystems (Bragadeeswaran et al., 2007; Saravanakumar et al., 2009). Therefore, it has been proposed that the magnitude changes in the microclimate matched with season are ultimately reflected in the environmental parameters (and microhabitat also), which have a direct or indirect influence over the nutrient cycle, abiotic and biotic processes in coastal environments (Bala Krishna Prasad & Ramanathan, 2009), which in turn have an important effect over the vertebrate and invertebrate population. Thus, assessment of environmental quality is an important aspect of developmental activities of the mangrove regions (Saravanakumar et al., 2008). Although a number of authors have studied the physical and chemical characteristics of some Iranian estuaries and mangroves (Danehkar, 1994; Safa, 2006), such studies have not been attempted in the Hormozgan coast of Persian Gulf and Oman Sea, and only little information is available on the physico-chemical aspects of the Hara Protected Area (HPA) and Gaz River and Hara River Deltas (GHRD) mangrove forests (Danehkar & Jalali, 2005). Hence, the present investigation attempted to record some crucial physio-chemical variables of the mangrove milieu of Hormozgan province in the HPA and GHRD.

Likewise, mangroves are especially adapted to survive conditions of high salinity, low soil water potential and waterlogging (Naidoo, 1985), thus, the variation in vegetation structure along environmental gradients is a common occurrence in mangrove systems (Naidoo, 2009). Two mangrove species of Avicennia marina and Rhizophora mucronata occur in the southern coast of Iran within Persian Gulf and Oman Sea (Safiari, 2002). These two kinds of mangroves display distinctive morphological, structural, physiological and floristic characteristics (Méndez Linares et al., 2007; Naidoo, 1985). Avicennia is usually the pioneer of the mangrove association and characteristically occupies the seaward portion of the swamp. While, Rhizophora occurs at mid-tidal level. Rhizophora usually occurs along the creeks and seepage channels because it requires well saturated, but not too saline soils (Naidoo, 1985). Furthermore, type of Avicennia appears to grow on land less waterlogged than does Rhizophora, however, in some areas there are mixed stands of both kinds. Thus, it is generally considered that type of mangrove might also be associated to changes in environmental condition such as water and soil parameters. It also seems clear that the changes in microhabitat due to structure of vegetations exert a controlling role on functions. Moreover, variations in microclimate of Avicennia and Rhizophora mangrove forests are of interest because both affect providing food resources, shelter, nesting and roosting sites for wide range of species and may also cause changes in relative abundance of waterbird species composition.

Since differences in structure of habitat have been attributed to a variety of environmental factors such as water parameter and soil variable (Naidoo, 2009), the aim of this investigation was to determine the differences of water and soil characteristic, separately in two types of mangrove habitat.

MATERIAL AND METHODS

Study Area. This study was conducted at two types of mangrove forest habitat in Hormozgan Province, Iran. Avicennia marina is the pure stand in the Hara Protected Area (HPA) mangrove forest, which is situated at 26˚ 23΄ – 26˚ 59΄ N and 55˚ 32΄ – 55˚ 48΄ E. Rhizophora mucronata is located in the mangrove forests of Gaz and Hara Rivers Delta (GHRD) in 26˚ 30΄ – 26˚ 50΄ N and 57˚ 00΄ – 57˚ 40΄ E.

Hara Mangrove Reserve or ‘Khouran Straits’ is located in the southern Persian Gulf between the region of the Mehran River and Kol River deltas and the island of Qeshm, within quadrant of 26˚ 23΄ – 26˚ 59΄ N and 55˚ 32΄ – 55˚ 48΄ E. Within the straits, there are 100,000 ha of low-lying islands, mangrove, mudflats and creeks which constitute much of the largest mangrove/mudflat ecosystem in Iran. The mean minimum and maximum temperature are 2˚C and 48˚C, respectively. The mean annual temperature is 27.6˚C, over a 30 year period (1975-2005) at the Qeshm meteorological station. The mean annual rainfall in the Hara Protected Area (HPA) is about 80.3 mm and mainly occurs in the winter. The mean monthly relative humidity is 83.4% and the range of high tide is 4.33 m from the Port of Shahid Rajaee, nearest to the study site.

The Gaz River and Hara River Deltas (GHRD) international wetland, with 15000 ha area, is a large area of intertidal mudflats, mangrove swamps and sandy beaches at the mouths of two rivers on the eastern shore of the Straits of Hormoz, at the entrance to the Persian Gulf (Danehkar, 1994). The mouth of the Gaz River is situated at 26°50΄N 57°40΄E while the mouths of Hara (Hivi) River is situated at 26°30΄N 57°00΄E. The mean minimum and maximum temperature are 3.5˚C and 49.6˚C respectively. The mean annual temperature is 26.5˚C, in a 30 year period (1975-2005) at the Minab meteorological station. The mean annual rainfall is about 40.6 mm and also the lowest mean monthly rainfall (0 mm) occurred over 6 months, between April and October. The highest monthly rainfall (19.6 mm) occurred in January, while the mean annual relative humidity is 77.9%.

Sampling Design. Point sampling method (within transects) were used in our study. Each site was divided by many intertidal channels. A total of three transects had been detected on the map randomly within 3 main channels. Transects were run parallel to creek at the pre-decided locations distributed in each area. A total of 35 point count stations (300 m apart from each other) were established within transect 1, 30 points in transect 2 and 32 points in transect 3, randomly in the HPA. Similar trends spread on GHRD, which 30 points were established within each transect.

The study was conducted from 22 September 2008 to 21 September 2009. Sampling was carried out over twelve months or four seasons including fall (22 September-21 December), Winter (21 December- 20 March), spring (21 March- 20 June), summer (21 June- 21 September).

Survey Design. Surface water was collected in situ from the surface, at 10 - 25 cm depth at 3 random positions per plot, 2 times in the every month (biweekly). The physico-chemical factors of surface water, including the rate of temperature (°C), acidity (pH), dissolved oxygen (DO; g L-1), electrical conductivity (EC; dS cm-1), turbidity (TSS; mg L-1) and salinity (g L-1) were measured in situ with three replication in each sampling points in each visit using a portable test Horiba (model Horiba U-10 Multi parameter Water Quality Meter) and a digital salinity meter (model Koi Medic Salinity Meter). All water samples were collected from the surface, at 10 - 25 cm depth at 3 random positions per plot. This methodology was explained by Campbell (2008), Morimoto (2010), and Omo-Irabor et al., (2008).

Sediment sampling was performed seasonally at low tide from the upper sediment layer (0 to 20 cm) and lower sediment layer (20 to 40 cm) in each point of the study areas using a core sampler of 10 cm diameter. Collected soil samples were put into closed labeled plastic bags to minimize sample contamination and put into an ice chest at low temperature during transportation and were brought to the laboratory of Azad University of Bandar Abbas for immediate processing. Samples were air-dried at room temperature and thoroughly mixed before crushing by using a mechanical grinder (A10 manufactured by 1 KA-Labor technical) and sieved through 2 mm mesh. The major physical and chemical properties of the soils were soil texture (% of silt, clay and sand), electrical conductivity (EC), acidity (pH), organic carbon (OC), calcium carbonate (CC), absorbable potassium (K+) and absorbable phosphorus (AP). The sediment sampling methodology was followed as described by Andreoni et al. (2004), Ferreira et al. (2007), and Liu et al. (2008).

Data Analysis. The range of mean, maximal, minimal and annual means (±SE) for each parameter was measured. A one-way analysis of variance (ANOVA) followed by a post hoc multiple comparison (Tukey's test) was employed to test any significant differences in all parameters between seasons. A t-student test was applied to look at any significant differences between A. marina and R. mucronata habitats. All statistical analyses were performed with SPSS version 16.0, and graphs were established using Excel 2007.

RESULT AND DISCUSSION

Water parameter. The mean values of ˚C, pH, DO, EC, TSS and Salinity were measured 26.18±0.22 °C, 7.09±0.02, 6.64±0.04 g L-1, 42.78±0.11 dScm-1, 112.28±0.66 mg L-1 and 34.72±0.27 g L-1 respectively for HPA. While for GHRD were 24.6±0.23 °C, 7.27±0.03, 6.6±0.03 g L-1, 44.38±0.14 dScm-1, 119.61±0.76 mgL-1 and 37.3±0.12 gL-1 respectively.

The results of comparing mean values of variables using t-test at HPA and GHRD showed that there was a significant difference (p<0.01) among all variables including ˚C (t= 4.91, p<0.01), pH (t= -5.48, p<0.01), EC (t= -8.97, p<0.01), TSS (t= -7.30, p<0.01) and salinity (t=-23.41, p<0.01), except DO (t= 0.86, p>0.05). Furthermore, the results of t-test showed that during non-migratory seasons, in the spring, there was a significant difference on the EC (t=-3.88, p<0.01) and TSS (t= -2.95, p<0.01), while there was no significant difference for DO (t=0.07, p>0.05) only in the summer between two sites. Moreover, during migratory seasons there was significant difference in the fall for all parameters (p<0.01), excluding EC (t=-0.82, p>0.05), while in the winter there was significant difference for all parameters (p<0.05).

On a global scale, temperature is the most significant determinant for the range of fauna and flora within habitats (Augustinus, 1995; Blasco et al., 1996). Mangroves growth in tropical and sub-tropical latitudes, where the average sea surface temperature is 24 ˚C (Hogarth, 2001), though the climax growth of mangroves has been reported only under tropical conditions where atmospheric temperature during the coldest months is greater than 20˚C and the seasonal fluctuation does not exceed 5˚C (Kathiresan & Bingham, 2001). Temporal conditions were similar in the mangrove forests of Hormozgan province.

The trend of changes in water parameters in the sites illustrated that surface water temperature varied from 18.1 to 32.8 ˚C at HPA and 16.7 to 31.4 ˚C at GHRD. The results of comparing mean values of temperature showed a significant difference (p<0.01) among seasons at HPA and GHRD. The mean values of temperature decrease from fall to winter, with low temperatures of 21.12±0.16 and 19.69±0.18 recorded during winter at HPA and GHRD respectively, and they gradually increase from winter to spring and summer, peaking during the summer (September in both habitats) (Fig. 1).

In general, the intensity of solar radiation, evaporation and water flow from adjacent neritic waters have an effect on surface water temperature (Govindasamy et al., 2000; Perumal et al., 2009a). In the current investigation, non-migratory troughs and especially summer peaks for air and surface water temperature were noticed. In situ observations showed a positive correlation between air and surface water temperature during seasons for all sampling points in both habitats.

Fig. 1 The trend of changes in surface water temperature during four seasons

Salinity varied from 28 to 41 g L-1 at HPA and 31 to 45 g L-1 at GHRD. There was a slightly increase in the salinity from fall to summer, while low salinity of 33.22±0.15 g L-1 and high salinity of 34.45±0.19 g L-1 were recorded during winter and summer respectively in the HPA. The recorded values in GHRD for winter and summer were 36.81±0.25 g L-1 and 37.69±0.2 g L-1 respectively (Fig. 2). The minimum salinity was probably due to the influence of rainfall and the resultant river run-off, which is a regular annual event in these areas during fall and winter (migratory season).

Salinity plays as a limiting factor in the distribution of living organisms (Perumal et al., 2009). Generally, salinity of mangrove habitats is influenced by the influx of freshwater from land and run off by tidal variations. Saravanakumar et al. (2008) reported a negative correlation with rainfall and a positive correlation with temperature. In our study, salinity in the both study areas was high during summer season and low during the winter season. Higher values during summer might be affected by high degree of evaporation or because of neritic water inflow from open sea, rainfall and the consequent freshwater inflow from the land (Perumal et al., 2009). In addition, the variations of salinity in both habitats were mainly influenced by the entry of neritic water inflow from the open sea (field obs.) and entry of freshwater from the land by Kol and Mehran rivers at HPA and the rivers of Gaz and Hara (Hivi) at GHRD, which would have moderately reduced the salinity.

Fig. 2 The trend of salinity changes during four seasons

In the present study, the Hydrogen ion concentration (pH) varied from 5.4 to 8.0 at HPA and 5.0 to 8.8 at GHRD. The seasonally mean values of pH were high (7.29±0.34) during the spring and low (6.95±0.36) during the fall and winter seasons at HPA. While it remained alkaline throughout the study period in the surface waters of GHRD, with the minimum value (7.19±0.07) occurring in the winter season and maximum values (7.30±0.05) occurring on spring and fall seasons (Fig. 3).

Generally, factors like removal of CO2 by photosynthesis through bicarbonate degradation, decomposition of organic matter and reduction of salinity and temperature are very important in the fluctuations in pH values during different seasons of the year (Perumal et al., 2009a; Saravanakumar et al., 2008), and in this study statistical analysis showed that salinity had highly significant negative correlation with the peak of photosynthesis.

Fig. 3 The trend of changes in pH during four seasons

The dissolved oxygen values were high (2.80 mg L-1) during the spring and low (8.90 mg L-1) during the summer. Fig. 4 gives the trend of DO changes during the four seasons in the HPA and GHRD mangrove forests. The higher values of DO concentration (6.92±0.07 in the HPA and 6.74±0.05 in the GHRD) were recorded during migratory seasons (fall and winter), which could be mainly due to reduced turbulence of coastal waters in the mangrove habitats. However, there was no significant difference among seasons in both habitats (p>0.05).

It is well known that dissolution of oxygen is influenced by salinity and temperature in the sea water (Saravanakumar et al., 2008; Vijayakumar et al., 2000). As can be seen, season-wide observation of DO illustrated an inverse trend against temperature, salinity and turbidity.

The current study showed no significant difference among DO between seasons for both habitats, which might be due to the cumulative effect of higher wind velocity in the summer coupled with rainfall in the winter and the resultant water mixing. Saravanakumar et al. (2008) and Perumal et al. (2009a) attributed seasonal variation of DO concentration primarily to entry of freshwater from the land and the ferruginous impact of sediments in mangroves. In addition, during a study of the mangroves in Kachchh-Gujarat in India there was no significant variation between stations and seasons (Saravanakumar et al., 2008).

Fig. 4 The trend of DO changes during four season in the HPA and GHRD

Moreover, it is well known that trends of EC are mainly influenced inversely by temperature and salinity (Vijayakumar et al., 2000). In the present study, the above findings are supported by the higher electrical conductivity (EC) found in water samples from HPA and GHRD. Fall season had the highest conductivity in HPA (44.51±0.27 dS cm-1), followed by winter (42.36±0.15 dS cm-1), spring (42.19±0.2 dS cm-1) and summer (42.07±0.11 dS cm-1). A similar trend with similar seasonal changes was observed in the GHRD, which recorded the highest in the fall (44.83±0.26 dS cm-1) and the lowest in the summer(43.66±0.26 dS cm-1) (Fig. 5).

Fig. 5 The trend of EC changes during four season in the HPA and GHRD

TSS varied from 79 to 148 g L-1 at HPA and 75 to 155 g L-1 at GHRD. There was a smooth fluctuation in the trends of TSS from fall to summer, where low TSS of 107.92±1.53 g L-1 for HPA and 115.87±1.49 g L-1 for GHRD were recorded in winter at both sites (Fig. 6).

The minimum TSS was probably due to the influence of rainfall and the resultant river run-off, which is a regular annual event in the areas during autumn and winter. TSS followed by salinity, DO and EC are all limiting factor in the distribution of living organisms in the water by limiting light fluency in deeper water (Perumal et al., 2009b). Generally, Saravanakumar et al. (2008) reported positive correlation between TSS and salinity/DO. In our study, salinity in both study areas was high during summer season, which might be caused by a higher rate of evaporation, and low during the winter season due to neritic water inflow from the open sea, rainfall and the consequent freshwater inflow from the land (pers. obs.) which would have moderately reduced the TSS.

Fig. 6 The seasonal changes of TSS in the water in adjacent of mangrove forest at HPA and GHRD

Soil parameter. The interactions of mangroves with soil and associated micro-organisms are a major factor in explaining why mangroves are highly productive forests (Alongi, 2002). Soil or sediment characters are known as one of the major controls on mangrove distribution (Perry & Berkeley, 2009), which must cope with a harsh, waterlogged environment. Furthermore, there has been increasing interest in developing methods which are indicators of soil health and sustainability, reflecting changes in soil properties (Wang et al., 2006).

The different physiographic positions occupied by the mangrove forests in this study appear to lead to important differences in soil composition and physicochemical conditions. This section therefore details the patterns of different component seen in soil profiles from two sites. The focus has switched from simple chemical approaches to more integrated biological approaches including effect of soil conditions on the waders food resources and so on the density/diversity of waterbirds in mangrove ecosystems, in addition to the microhabitat, which are excellent candidates to reflect changes in soil conditions.

In this study, the mean values of sediment texture in terms of silt, clay and sand were 52.7±0.88 (%), 36.42±0.68 (%) and 10.87±0.47 (%) at depth of 0-20 cm and 42.9±0.63 (%), 36.94±0.57 (%), and 16.09±0.54 (%) at depth of 20-40 cm for HPA. In addition, these were 49.73±0.72 (%), 39.46±0.79 (%) and 11.23±0.35 (%) at depth of 0-20 cm, and 50.13±0.52 (%), 37.75±0.53 (%) and 12.12±0.79 (%) at depth of 20-40 cm for GHRD. The soil texture corresponded to a Silty-Clay-Loam texture based on the USDA textural triangle class at the both sites and depths. Soil texture revealed dominance of silt in both depths of both habitats with no much variation among them, which it may be attributed to the absence of wave induced sand transport from open sea and also winnowing activity of sediment transport system. Rajkumar et al. (2009) indicated that lack of perennial flow of freshwater district may be the reason for uniformity in soil texture within mangrove forests, and Clarke and Kerrigan (2000) found that sediment texture exerts strong control on conductivity, pH, organic matter, total P, total N and total S along environmental gradients of mangroves in Northern Australia.

Based on the information obtained in the HPA, the mean values of pH, EC, calcium carbonate, organic carbon, available phosphorus and available potassium in the depths of 0-20 cm were measured 7.09±0.04, 47.26±0.57, 37.3±0.3, 1.83±0.24, 207.41±3.24 and 8.72±0.35 respectively, and in the depths of 20-40 cm were measured 7.2±0.05, 49.28±0.62, 37.37±0.48, 1.72±0.03, 204.07±3.02 and 9.14±0.3 respectively. While, the results of soil profiles collected from GHRD showed 6.99±0.05, 45.49±0.72, 37.53±0.42, 1.51±0.04, 204.42±4.09 and 9.74±0.37 in the depths of 0-20 cm, and 6.97±0.06, 43.6±0.51, 37.59±0.36, 1.33±0.05, 205.87±2.51 and 9.35±0.23 in the depths of 20-40 cm respectively (Fig 7).

The soil tested showed significantly difference between the two sites study sites for available potassium (t= -2, p<0.05), organic carbon (t=5.74, p<0.01), clay (t= -2.88, p<0.01), silt (t=2.6, p<0.05) at depths of 0-20 cm, and organic carbon (t=6.69, p<0.01), pH (t=3, p<0.01), EC (t=6.87, p<0.01), sand (t=4.18, p<0.01) and silt (t=-3.87, p<0.01) at depths of 20-40 cm. Moreover, the soil tested showed significant difference (p<0.05) among seasons only for absorbable potassium at a depth of 20-40 cm (F3,48=3.23, p<0.05), and EC in the same depth (F3,48=3.6, p<0.05).

Mangrove sediments are in general relatively rich in organic carbon (Kristensen et al., 2008). Since most mangrove forests occur along sedimentary coastlines in large estuaries and deltas, large quantities of suspended organic carbon brought in by tides or rivers are deposited along with local mangrove detritus (Victor et al., 2004). The organic carbon content at depth of 0-20 cm varied from 1.78 (in the spring) to 1.89 (in the fall) in the HPA, and from 1.48 (in the spring) to 1.55 (in the winter) in the GHRD. The finding of this study showed the highest rate of organic carbon occurred in the migratory seasons (fall for HPA and winter for GHRD). Additionally, the findings showed how the distribution of organic carbon (especially in the depths of 0-20 cm) closely followed the distribution of sediment type (i.e., sediment low in clay content was low in the organic carbon and as the clay content increased, the organic carbon content also increased) as reported by Saravanakumar et al. (2009).

In the present study the highest mean value of organic carbon at depths of 0-20 cm was recorded in the migratory seasons (fall and winter) in both sites, however in the HPA was significantly higher than GHRD during all seasons.

The results showed low rate of organic carbon concentration during non-migratory seasons (spring and summer) and high during migratory season (fall and winter). Sverdrup et al. (1942) reported that an abundant supply of organic matter in the water column, relatively rapid rate of accumulation of fine grained inorganic matter and low O2 content of the water immediately above the bottom sediments would favor high organic matter and organic carbon in the bottom sediments. Looking into reports by Bouillon et al. (2003) and Kristensen et al. (2008) based on the isotope composition of sediment organic carbon from mangrove systems where significant amounts of C4 vegetation occurs in the catchment areas, illustrates the potential importance of riverine-transported terrestrial material in mangrove systems. The available global estimates of carbon accumulation are mainly calculated by difference using litter fall, export and consumption rates (Jennerjahn & Ittekkot, 2002), however, Kristensen et al. (2008) indicated that a representative global estimate of carbon content is likely to be close to 2.2%.

A one way ANOVA test on the results showed significant differences in mean value of organic carbon among seasons in the HPA (F(3,60)= 3.56, p=0.02), while there was no significantly difference in the mean value of OC among seasons in the GHRD (F(3,60)= 0.13, p=0.94). It is difficult to interpret, however it may be affected by lack of neritic water inflow from the open sea in the GHRD rather than HPA, or may reflect a variable contribution by other carbon sources. Also, it is now recognized that foraging and feeding activities of mangrove fauna can influence the properties and availability of organic carbon (Kristensen et al., 2008). Furthermore, nutrients are considered to be the major determinants of mangrove environment, influencing distribution, growth, reproduction and metabolic activities of biotic components. The nutrients distribution within mangroves is mainly influenced by season, tidal conditions and freshwater flow from land source (Saravanakumar et al., 2009).

Available Phosphorus (AP) concentrations in the soil samples taken at depths of 0 to 20 and 20-40 cm in both habitats confirmed some interesting results as well. A one way ANOVA test showed that there was no significant difference in mean value of available phosphorus contents among seasons in the HPA for depths of 0-20 cm (F(3,60)= 0.35, p>0.05) and 20-40cm (F(3,60)= 0.28, p>0.05) and in the GHRD for depths of 0-20 cm (F(3,60)= 0.15, p>0.05) and 20-40 cm (F(3,60)= 0.2, p>0.05). In addition, a t-student test on the results of soil samples taken at HPA and GHRD showed no significant differences among habitats in the depth of 0-20 cm (t= 0.58, p>0.05) and 20-40 cm (t= -0.44, p>0.05).

In the HPA, available Phosphorus was highest at winter (211.25 ± 9.11 m-equiv L-1), followed by spring and fall (208.81±5.49 and 207.56±3.26 m-equiv L-1, respectively) in the soil samples taken at a depth of 0 to 20cm. In contrast, high value of available Phosphorus in soil samples taken at depth of 20-40 cm in the winter and spring in the sites could be attributed to the regeneration and release of phosphorus from bottom mud into the surface layer, which measured at 208.19±3.77 m-equiv L-1 and 204.31±5.17 m-equiv L-1 in the winter and spring respectively. Similar trends are observed in the soil of GHRD. The variation may be caused by the various processes, such as phosphorus adsorption-desorption processes and/or buffering action of sediments in mangrove habitats under unstable environmental conditions, as defined by Rajasegar (2003).

In the present study, total available Potassium ranged between 6.20 and 18.3 m-equiv L-1. In the HPA at depth of 0 to 20 cm, K+ was lowest in the fall (8.44±0.63 m-equiv L-1) and highest in summer (10.78±0.66 m-equiv L-1). The rate of K+ at winter and spring were 9.19±0.71 and 10.57±0.83 m-equiv L-1 respectively. Moreover, the amount of available Potassium in the soil samples taken at the HPA at depths of 20-40 cm during fall, winter, spring and summer were measured at 9.48±0.59, 7.94±0.29, 10.38±0.87 and 8.74±0.32 m-equiv L-1 respectively. Likewise, at the GHRD at depths of 0 to 20cm, K+ was lowest in the fall (8.44±0.63 m-equiv L-1) and highest in summer (10.78±0.66 m-equiv L-1) followed by 9.19±0.71 m-equiv L-1 and 10.57±0.83 m-equiv L-1 in winter and spring respectively. Furthermore, the amounts obtained at depth of 20-40 cm during fall, winter, spring and summer were given as 8.62±0.35, 8.71±0.42, 9.94±0.6 and 10.12±0.34 m-equiv L-1 respectively. Therefore, there was a gradual increase in the rate of available potassium (K+) near the surface in sites from autumn to summer. However, there was a fluctuation in the concentration of K+ in the depth range of 20-40 in the HPA. A higher concentration of potassium was observed during the summer season in the 0-20 cm layer at both habitats, which may be due to dead organic matter from the top layer and low values observed in the autumn may be related to removal of top layer sediments by heavy floods.

Calcium carbonate (CaCO3) is one of the key factors influencing the concentration of organisms such as molluscs in the mangroves. The percentage of CaCO3 in the HPA at 0-20 was a little bit higher in summer (37.69±0.5 %), with values ranging between 34.6 and 42.3%. CaCO3 was also measured at 36.85±0.85%, 37.14±0.44 % and 37.51±0.53 % during fall, winter and spring respectively. The CaCO3 contents at 20-40 cm were also very different, with the highest values corresponding to winter (38.51±0.74%), followed by spring (37.4±0.78 %), fall (37.31±1.07 %) and summer (36.23±1.21 %). The CaCO3 contents of GHRD soils at 0-20 cm were also higher in the summer (38.25±0.73 %) than others (37.82±0.62 %, 36.68±1.04 % and 37.4±0.93 % during fall, winter and spring respectively). The contents at 20-40 cm were calculated with values ranging between 37.25±0.98 % (in the fall) and 38.48±0.46 % (in the summer). Perry & Berkeley (2009) reported that the loss of calcium carbonate from intertidal sediments largely results from the acidic pore waters associated with organic-matter mineralization. One hypothesis for the clear lack of carbonate dissolution in the intertidal sediments is the continual production of carbonate and/or ambient sedimentary carbonate levels currently remaining sufficiently high to buffer acid generated during microbial decomposition of organic-matter, thus enhancing bioclast preservation (Barbieri, 2001). An alternative hypothesis is that acid production is limited by poor sediment oxygenation (Perry & Berkeley, 2009).

The pH in the upper layer sediments at HPA stations was about 6.99±0.14 (spring) to 7.2±0.11 (summer) whereas at GHRD the recorded pH was 6.87±0.1 in the fall to 7.08±0.09 in the spring. The highest pH values were 8.10 (at depth of 0-20 cm) and 8.7 (at depth of 20-40 cm) in HPA and at GHRD were 7.6 (at depth of 0-20 cm) and 7.8 (at depth of 20-40 cm). Since mangrove soils are typically waterlogged, then changes in soil pH are closely related to changes in surface water pH, while pore water acidity may increase as a function of mangrove age (Marchand et al., 2004). The acidity of soil influences the chemical transformation of most nutrients and their availability to plants (Kristensen et al., 2008). Most mangrove soils are well buffered, having a pH in the range of 6 to 7. Wang et al. (2006) demonstrated that biological activities in soil were sensitive to pH change. They reported how pH was the most important factor influencing soil biological activities by testing ranges of pH from pH 4.74 to 6.88 or 7.27 for high or low metal soil respectively. In addition, they tested to see whether reducing pH had significant negative impact on soil microbial activity, such as biological activities in rhizosphere enzymes, such as the enzyme of soil alkaline phosphatase which removes a phosphate group from soil into a phosphate ion, or nitrification potential which significantly reduces after soil acidification.

The EC content of soil was consistent with the results obtained for the EC of water. The total EC contents ranged from 39.40 to 68.30 at depth of 0-20 cm and 42.60 to 69 at depth of 0-20 cm at HPA, whereas in GHRD the figures were 33.10 to 60 and 30.60 to 54.2 respectively. The measurement of EC has been used as a rapid means of assessing the potential impact of additional organic input to marine sediment (Pearson and Stanley, 1979). Reliable measurements of EC require great care to minimize exposure of the soil sample to air (English et al., 1997). The findings of this study showed how the EC of soil was influenced by the EC of water (likely with pH) and aeration of a tidal creek.

Fig. 7 Seasonal changes of CaCO3, Organic Carbon, and pH in the mangrove forest at HPA and GHRD

Fig. 8 Seasonal changes of Available Potassium and phosphorus and EC in the mangrove forest at HPA and GHRD

CONCLUSION

Therefore, this study illustrated that microclimate variables are the major control on mangrove structure, distribution and spatial extent. For instance, salinity has a significant effect on the growth and zonation of mangrove forests. To evaluate ecosystems is in terms of health and productivity, it is necessary to monitor of soil and as well as surrounding water sources. Several literatures have come out with contradictory findings about importance of water and soil in mangrove forests (Kathiresan & Bingham, 2001; Saravanakumar et al., 2009). However, far too little attention has been paid to the mangroves of Iran (Danehkar & Jalali, 2005). The present study was designed to determine the environmental condition due to four seasons of year in the A. marina and R. mucronata mangrove forest in Iran. These forests are flooded twice daily under natural conditions with salt water that result in radical changes in the soil characteristics. This aspect of the subject has already been argued by Tomlinson (1999) and Saravanakumar et al. (2009). Soil characteristics are one of the most important environmental factors directly affecting mangrove productivity and structure. Therefore, this study illustrated that water and soil variables are the major control on mangrove structure, distribution and spatial extent. For instance, salinity has a significant effect on the growth and zonation of mangrove forests.

On the other hand, this study suggested that mangrove loss will also reduce coastal water quality, reduce biodiversity and nursery habitat. Fertility and healthiness of mangrove environment is reflected in productivity of plankton communities, including phyto/zoo as primary/secondary producers, which they are the key players in controlling food webs in mangrove waters and they are reflected in abundance of molluscs and fishes.

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