The study area is an abandoned sand dump located at Eagle Island (N 4˚47.317' and E 6˚58.593'), directly behind the Rivers State University (Figure 1). The area has a warm and humid climate with two seasons, dry and wet seasons [17]. The soil of the area is between sandy to muddy, and also whitish to dark brown in color [18]. The area was once a mangrove forest, but was cut down to make way for sand mining five years ago. At the end of the sand mining activity the area was later abandoned to its fate. The company that operated the sand mine left the site with heaps of white sand still on the ground surface [19]. This abandoned sand became a plat form on which a variety of weed and other plant species grow over the years. These species are plants that naturally cannot grow in swampy soil but because of the conversion of the sand from swampy to sandy soil weeds do proliferate. Moreover, the site is surrounded by a river channel which brings in sediments and seeds of plants during high tide. At the edges of the sand dump are heaps of mangrove soils placed to prevent the entry of water during high tide.

In an area measuring 3600 m² (i.e. 40 m × 90 m) eight plots were delineated from where weed and soil samples were randomly collected (Figure 2). The plants

were counted and each weed samples collected and placed in a cellophane bag. Similarly soil samples were collected beneath the plant root 5 cm below the soil with a soil augur and also placed in a cellophane bag. All the samples were put in a cooler and transported to the laboratory for physico-chemical analysis.

In other to determine the soil chemistry of the study area, physico-chemical analysis of soil were done for the following parameters: pH, conductivity, total hydrocarbon (THC), total organic content (TOC), Nickel (Ni), Lead (Pb), Chromium (Cr), Magnesium (Mg), Potassium (K), Sodium (Na), Calcium and (Ca), while for the plant sample the following were parameters were analyzed: Cd, Pb, Zn and THC using standard laboratory procedures described below.

It involved the use of spectrophotometric method using the HACH DR 890 calorimeter (wavelength 420 nm). The samples were crushed and 2 g of the crushed sample was weighed into a glass beaker and 20 ml of hexane was added, and with the aid of a glass rod, the mixture was homogenized by stirring. Afterwards, the sample was filtered in a glass funnel packed with cotton wool, silica gel and anhydrous sodium sulphate. After this, 10 ml of the filtered organic extract was transferred into a 10 ml sample curvet and inserted into the calorimeter. The detection limit for THC is 0.01 mg/l. The above method followed the procedures of [20].

Heavy metal extraction followed the example of [21]. Aliquots of 0.25 g of air dried sediment samples were weighed into a Teflon inset of a microwave digestion vessel and 2 ml concentrated (90%) nitric acid (Sigma-Aldrich, Dorset, UK) were added. The metals were extracted using a microwave accelerated reaction system (MARS Xpress, CEM Corporation, Matthews, North Carolina) at 1500 W power (100%), ramped to 175˚C in 5.5 min, held for 4.5 min, and allowed to cool down for 1 h. The cool digest solution was filtered through the Whatman 42 filter paper and made up to 100 ml in a volumetric flask by adding de-ionized water. All chemicals and reagents used were of analytical grade and of highest purity possible. Analytical blanks were prepared with each batch of the digestion set and analyzed (one blank for every set of 6 samples) in the same way as the samples. The detection limit for the three metals analyzed in mg/l i.e. Zinc, Cadmium and Lead is 0.001, 0.001 and 0.002 respectively.

The pH and conductivity were measured using Sper Scientific 86003A multi parameter meters with probes calibrated with standard solutions. Total Organic Carbon (TOC) was determined using Walkey-Black titrimetric method.

The weeds collected from the site were sent to the laboratory and identified using a handbook of West African weed by [7]. The weeds were identified based on the shape of the leaves, color, seeds, inflorescence and fruit produced.

An analysis of variance (ANOVA) was conducted to determine whether there was a significant difference in THC and heavy metal concentration in soil and plant [22]. Bar graphs were then used to illustrate the significance and difference in concentration in plant and soil. All analyses were done in [23].

The stand basal area, G, measured in m²/ha, (Equation (1)) refers to the summation of all individual basal areas per unit ground area [24]. Where, g_i (m²) is the basal area of a single stand (Equation (2)), while 25 (400) is the plot size of the 40 m × 90 m (3600 m²) main plot, and a 5 m × 5 m (25 m²) sub-plot, as the conversion factor to 1 hectare [25].

G = ∑ i = 1 N g i × 25 ( 3600 ) (1)

g = π × ( d b h 2 ) 2 (2)

The importance value, I_v is a quantitative parameter used to show the significance of each species within a stand. It is a summation of density, frequency and dominance (Equations (3), (4), (5) and (6)). The importance value (I_v) was calculated using the formula of [24].

I_v = Relative Density ± Relative Frequency ± Relative Dominance (3)

Relative Density ( % ) = no of individuals of species ( N ⋅ ha − 1 ) total number of individuals ( N ⋅ ha − 1 ) × 100 (4)

Relative Frequency ( % ) = frequency of species ( n ) total number of species ( N ⋅ ha − 1 ) × 100 (5)

Relative Dominance ( % ) = total basal area of species Basal area of all species ( G ) × 100 (6)

To determine the species diversity, the Shannon Index (H) was used (Equation (7)), and is based on natural logarithm which considers low and high diversity species based on abundance of species. Higher values of diversity index imply higher biodiversity of that plant species and vice versa [26]. Species diversity is used for this study because it serves as an indicator of human disturbances [27].

H = ∑ I = 1 1 p i ln ( p i ) (7)

where,

H = Shannon diversity index;

∑ I = 1 1 = summation;

ln = natural logarithm.

Result (Table 1) of the physico-chemistry of the study area shows that swampy soils are more acidic (3.10 - 3.50) than sandy soil (4.20 - 4.70) that harbor the weed species. Swampy soils are also more saline and thus have higher ability to conduct electrons (8320.00 - 9880.00 µS/cm) as compared to sandy soil (4320.00 - 5650.00 µS/cm). Mangrove swamp has higher total organic carbon (TOC) (2.25% - 3.41%) than sandy soil (0.12% - 0.21%). Furthermore, swampy soil has more concentration of heavy metals i.e. Ni, Pb and Cr. In addition, swampy soils are also high in K and Na [28]. In contrast, sandy soil had more Mg content.

The results for the nine most dominant weed species (Table 2, Figure 3) indicate that there is a significant difference in THC and heavy metal concentration in soil (F_8,63 = 2.04, P < 0.05). R. indica has the highest concentration of THC in root soil. There was also a significant difference in the concentration among the parameters i.e. THC and heavy metals (P < 0.05). Zn and THC had the highest soil concentrations (Table 2). M. longibracteautus had the highest Zn (7.67 ± 0.005 mg/kg) followed by A. areum (5.79 ± 0.01) and M. ligularis (5.60 ± 0.01

mg/kg). R. indica had the overall highest THC concentration (137.1 ± 1.30 mg/kg) followed by P. vaginatum (8.84 ± 0.06 mg/kg) and M. ligularis (8.79 ± 0.01 mg/kg). Cd and Pb range between 0.001 - 0.002 mg/kg. The order of importance for the chemicals in soils is THC > Zn > Cd > Pb.

In contrast, there was no significant difference in THC and heavy metal concentration in the body of the plants (F_8,63 = 247.0, P > 0.05). However, there was a significant difference in the concentration of THC and heavy metals (P < 0.05). Zn and THC had the highest concentration in weed (Table 3). A. areum had the highest Zn concentration (52.67 ± 0.45 mg/kg) while M. ligularis has the highest THC. Cadmium had the least concentration in weed. The order of importance in weed is Zn > THC > Pb > Cd.

A total of fifteen weed species were recorded within the sand filled area of the mangrove forest (Table 4). Most of the species fall under aquatic low land and dry land weeds, which are dicotyledons and monocotyledons [2]. The result indicate that Mariscus longibracteatus had the highest diversity (−0.366) followed by Mariscus ligularis (−0.339) and Paspalum vaginatum (−0.270). Although, the total diversity (H = 1.5) is lower than the weed community in a farmland (H = 2.87) [29]. M longibracteatus is more prominent in sandy mangrove shore as revealed in previous study [30].

The result (Table 5) indicate that Mariscus longibracteatus (Family: cyperaceae) are the most abundant (n = 1570), and thus have the highest importance value in the study site (I_v = 58.24). The order of importance of weed species found in

sand fill soil include; M. longibracteatus > M. ligularis > P. vaginatum > R. indica. All species are weed except Acrostichim aureum L., which is classified as a mangrove species under the family of Pteridaceae [31]. They are however, regarded as weed in some places, for instance in Matang, Malaysia [32]. P. vaginatum are also regarded as invasive species [33].