Characterization of tidally accumulated plastic waste and its
effect on seedling growth in sand-filled mangrove forest at Eagle
Island, Niger Delta, Nigeria
Aroloye. O. Numbere1* and Ayobami O.
Aigberua2
1Department of Animal and Environmental Biology,
University of Port Harcourt, Choba, Nigeria.
2Department of Environment, Research and Development,
Analytical Concept Ltd, Elelenwo, Port Harcourt, Rivers State.
Corresponding author:*aroloyen@yahoo.com
Running Head : Plastic waste accumulation in mangrove forest
Abstract: Plastic pollution has become a global problem with
the proliferation many plastic goods. This study thus hypothesized that
accumulated plastic waste will have adverse effect on mangrove growth.
The study was carried out at a sand-filled and deforested mangrove
forest at Eagle Island. Ten soils samples each (n =20) were
collected underneath accumulated plastic waste vertically and
horizontally. The soils were put in polythene bags and sent to the
laboratory for analysis of total hydrocarbon content (THC), and heavy
metals i.e., Zinc (Zn), Lead (Pb) and Cadmium (Cd) using the HACH DR 890
colorimeter (wavelength 420 nm) and microwave accelerated reaction
system (MARS Xpress, North Carolina) respectively. In addition, mangrove
(Rhizophora species) seedlings were also collected with soils
from the plastic waste and non-plastic waste sites (control). The result
shows that there is no significant difference in heavy metal
concentration along the profile i.e., surface, and sub-surface soils
(F1, 30 = 1.83, P = 0.186), and soil gradients
(F3, 28 = 0.60, P = 0.619) of the soil. In contrast,
there is significant difference in seedling growth between the control
and plastic soils (F4, 200 , 65.24, P<0.001).
Furthermore, microbial population showed significant difference
horizontally (F3, 11 = 3.86, P = 0.04) but not
vertically (F1, 11 = 4.60, P = 0.055) in plastic soil.
This result implies that plastic pollutants can migrate horizontally to
contaminate nearby mangroves. Thus, plastic waste should be managed to
prevent pollutants from entering the food chain to contaminate humans.
KEYWORDS: plastic waste, tides, heavy metals, sand fill, Niger
Delta, soil profile
INTRODUCTION
Plastics are petrochemical products that are manufactured for
multipurpose use such as packaging in the culinary, food, electronics,
and automobiles industries (Moshood et al. 2021). It is also used in the
manufacture of home appliances. Plastic wastes are unwanted plastics
materials that are no longer useful to the system and are thrown away
into refuse dump instead of being recycled (Moretti et al. 2020).
However, their uncontrolled disposal into the environment has generated
a nightmare for waste managers globally (Deme et al. 2022; Van et al.
2022). This is because their improper disposal into open dumps and
landfills in municipal areas have exposed them to flood and erosion
which wash them from city drainages into the aquatic environment.
The presence of plastic waste in the water body is detrimental to
aquatic organisms when they decompose into fine particles called
microplastic and are consumed (Li et al. 2022; Jiang et al. 2022).
Decomposed plastics increase the heavy metal and petrochemical contents
of the water (Marchant et al. 2022), which reduces the water quality and
affects the reproductive and physiological functions of fishes and other
aquatic organisms (Luo et al. 2022). The lodging of harmful metal in the
bodies of fish bioaccumulates in humans when they consume them (Ouyang
et al. 2022). Furthermore, plastic waste travels thousands of kilometers
from the point of disposal to the point of deposition mostly in mangrove
forest or on terrestrial sandy beaches during periods of high tides and
splashing waves (Sibaja-Cordero et al. 2022). The adventitious root of
mangrove traps some waste while the rest are carried away by tidal
currents (Silburn et al. 2022). However, the case is different for sand
filled mangrove forest because the sand barrier serves as an embankment
that prevents the retreat of the plastic waste when brought in by tidal
currents. Therefore, the deposited plastic waste on the sand remains for
long without being flushed back to the river. This situation is
different for the mangrove forest because they are right in the river
and face intense tidal flushing actions which wash away most of the
debris from the forest floor back to the river.
Settled plastic wastes get scorched by intense sunlight and disintegrate
into finer particles (Huber et al. 2022). These microplastics are
decomposed further by soil microbes (Baig et al. 2022) and settle on and
within the sand. Chemical components of the plastics are further washed
deep down the soil profile (Nasseri and Azizi, 2022). The accumulation
and disintegration of the plastic wastes increases the heavy metal and
microbial load of the soil underneath. Release of chemicals from the
decomposed plastic materials can percolate into the soil to contaminate
the groundwater aquifer (Sajjad et al. 2022). It may also be carried
into the river by surface erosion during heavy rainfall and high tides
thereby leading to some maritime accidents (e.g., Moore, 2008;
Pattiaratchi et al. 2022). Based on the health and environmental risk
the plastic waste poses to the people that consume seafood from the
nearby river (Adeniran and Shakantu, 2022), The goal of this study,
therefore is to characterize accumulated plastic wastes, determine soil
metal composition, microbial population and the effect of plastic waste
on mangrove seedling growth in the sand filled area. plastic waste were
chosen because they are the most abundant waste type in the study area.
Similarly, the grass species Mariscus longibracteatus(Numbere, 2020) was used to test the transmission of chemicals via soil
plant pathway because they are the most dominant grass species in the
study area. The following research questions were thus, addressed: (1)
Are there differences in the accumulated plastic waste types at
different sites horizontally or vertically (2) Are there differences in
chemical and microbial properties of the soil on which the plastic waste
accumulated, (3) Are there differences in seedling growth in accumulated
plastic waste soil and soil without plastic waste (control)? And if so,
are there correlations between mangrove seedling growth and heavy metal
and microbial concentrations?
MATERIALS AND METHODS
Description of study area
The research was carried out on a section of a deforested and sand
filled mangrove forest at Eagle Island Niger Delta (Figure 1). The area
is surrounded by swampy soil that is chocolate brown in color and
borders a river course that is used for boat transportation. During high
tide the river flushes water borne plastic waste into the grassy sand
filled area, which get trapped in the grasses. The sandy area has a
combination of mangrove and non-mangrove species growing. The species
found in the area are red (Rhizophora racemosa ), black
(Laguncularia racemosa ), and white (Avicenniagerminans ) mangroves while the non-mangrove species are dominated
by grasses e.g., Mariscus longibracteatus (Numbere, 2020).
There are also some Nypa palm species (Nypa fruticans )
growing in the sand. There are also numerous fiddler crab (Ucatangeri ) burrows on the sand. The soil is slightly alkaline with
a pH of 7.5. The temperature of the soil is 26.1±0.01 and the salinity
is 1.16ppt and the TDS is 360×10 ppm. The area has two seasons, the wet
and dry seasons. Dry season occurs from November to March while the wet
season is between March to October each year (Numbere and Camilo, 2018).
Experimental design
The study used a random block design (e.g., Numbere, 2021a) in an area
measuring 55.8m × 42.6m (2, 377.08m2). Within this
area five accumulated plastic waste deposition sites were identified and
delineated (Figure 2) using a standard measuring tape at an accuracy of
0.1m. The height and circumference of the area occupied by the waste was
measured. Each plastic deposition site is made up of grassy and sandy to
muddy soils. The sandy soil is made up of 90% sand; the semi-muddy soil
is made up of a mixture of sand and mud (i.e., 30% clay and 50% sand);
and the muddy soil is made up of 90% silt. The soils were identified in
situ and classified using soil textural triangle and soil characteristic
of western Nigeria (Smyth and Montgomery, 1962). The five sites were
georeferenced with a Garmin GPS (USA) (Table 1) and photographs and
video of the sites recorded for accurate counting of the plastic waste.
Plastic waste characterization
The plastic waste is brought into the deforested mangrove forest by
tidal current. They are often deposited on the sandy coast in circular
formation at different sites close to the river. The plastic waste was
delineated into five groups, which were studied in situ. Although, there
are few scattered plastic wastes around the study area, but they don’t
form a heap. Within each group the different types of plastic wastes
were manually sorted and identified before being grouped and taken to
the lab for weighing. The circumference of each waste site was taken
using distance measurement tool in GARMIN GPS. The diameter and height
across the waste column was also taken with a measuring tape (0.1m).
Thereafter the volume of the plastic waste column at each site was
calculated using the formula (Eq. 1) and recorded in Table 2.
V=πr2h (1)
Plastic waste at each sample point were physically counted and grouped
into their different components (e.g., plates, water bottles, packaging
etc.). Pictures and videos of each waste heap were also recorded and
taken to the laboratory where they were counted in camera and in video
to derive the best estimate of the number of different plastic waste per
site. This is in addition to the physical in situ enumeration. After
grouping, one sample of plastic waste was collected and taken to the
laboratory for weighing with an Ohaus weighing machine (Model PS251).
Sample collection
A hand-held soil augur was used to randomly collect soil samples in
October 2019 at the six accumulated plastic waste deposition sites. The
soils were collected vertically and horizontally at the waste dump. The
vertical samples were collected on the surface and in the sub-surface
(i.e., 5cm below). The horizontal samples were collected at three
gradients from the waste site namely near (low), middle (medium) and far
(high). This gives a total of 18 samples per site and in the overall 90
(16 x 5) samples. The subsurface soils were collected with soil augur
10cm below the surface. Another set of soil. The coordinates of the
sample collection points were derived with a Garmin GPS as shown in
Table 2.
Seedling growth experiment
To test the effect of plastic waste on seedling growth some seeds of the
two species of mangrove were picked up at the study site. Thirty seeds
each of Rhizophora racemose (giant seeds) and Rhizophora
mangle (dwarf seeds) were picked from the floor or plucked from the
tree, bagged, and taken to the laboratory for planting. Soil samples
were collected in situ beneath the accumulated plastic waste site and
non-accumulated plastic waste site (control) using a hand-held soil
augur. The soil samples were bagged separately and sent to the
laboratory for the seedling experiment. At the laboratory twelve plastic
containers each with height, width, and weight measuring 17.5cm × 15cm
and 2.2 kg were filled to equal level with soils collected from the
accumulated plastic and control soils. The physico-chemistry of the soil
was then taken as follows: pH (4.0-5.5), Temperature (28-29C°), and
moisture content (very wet). Similarly, the seed weight, seed length,
and bud length were measured in meters, thereafter, five seeds each were
then planted in each container i.e., plastic, and non-plastic (control)
soils to give a total of 30 seeds per species (i.e., n=5×6). The
seedling growth were then monitored for one year (March 2021-March 2022)
by measuring the number of sprouting leaves, and the seedling height.
Laboratory analysis
Total hydrocarbon (THC) and heavy metal analysis
The procedure of total hydrocarbon content involves the use of
spectrophotometric method using the calorimeter (i.e., HACH DR 890).
Samples were crushed and 2 g of it retrieved and weighed into a glass
beaker and 20 ml of hexane was added. The mixture was stirred and then
filtered in a glass funnel filled with cotton wool, silica gel and
anhydrous sodium sulphate. Thereafter, 10 ml of organic extract was put
into a 10 ml sample curvet and introduced into the calorimeter. The
detection limit for THC is 0.01 mg/l (APHA, 1995).
Heavy metal analysis: extraction of heavy metals followed the method of
Aigberua and Tarawou (2019). It involves the air drying of 0.25 g of
soil sample, which is weighed into a Teflon inset of a microwave
digestion vessel and 2 ml concentrated (90%) nitric acid
(Sigma-Aldrich,Dorset, UK) added. The metals were extracted using a
microwave accelerated reaction system (MARS Xpress, CEM Corporation,
Mathews, North Carolina). The detection limit for the three metals
analyzed in mg/l i.e., Zinc, Cadmium and Lead were 0.001, 0.001 and
0.002 respectively (Aigberua and Numbere, 2019).
Microbial analysis
Total heterotrophic bacteria (THB)
1g of soil is weighed into 9 ml sterile diluents (0.85% NaCl) under
aseptic condition. It is thoroughly shaken to homogenize and serially
diluted. Then 0.1 ml aliquot of the inoculums is collected using a
sterile pipette and inoculated on the surface of a nutrient agar (NA).
The inoculum is spread evenly with a sterile hockey stick (bent rod).
Plates are inoculated at 37⁰C for 24 hours. Thereafter, colonies are
counted to obtain colony forming unit (cfu) value per ml of the soil
sample. Distinct colonies are picked and streaked on freshly prepared
nutrient agar to obtain pure culture after 24 hours of incubation at
37⁰C. The pure culture is gram stained for microscopic examination.
Total heterotrophic fungi (THF)
One gram (1g) of soil is weighed into sterile diluents (0.85% NaCl)
under aseptic condition. It is then thoroughly shaken and serially
diluted. 0.1 ml aliquot of inoculums is inoculated on sabouraud dextrose
agar (SDA) acidified with 0.1% lactic acid to inhibit growth of
bacteria and allow for only the growth of fungi. Inoculated plates are
incubated at ambient temperature for 3-5 days. Cultural characteristics
of isolates are observed and sub cultured for purification. Microscopic
examination is done using lactophenol cotton blue stain with × 400
magnifications.
Statistical analysis
An analysis of variance (ANOVA) was conducted to determine whether there
was a significant difference in chemical and microbial contents between
surface and subsurface soils harboring tidally accumulated plastic
waste. Similarly, an ANOVA was done to determine whether there was
significant difference in chemical and microbial contents between lower,
medium and higher gradients from plastic waste. A two-way ANOVA was also
conducted to determine whether profile and gradient have effect on
microbial and heavy metal concentration (i.e., additive effect). The
data was initially log transformed to ensure that they were normal, and
the variances were equal (Logan, 2010). Later bar graphs were then used
to illustrate the results (Quinn and Keough, 2002). Similarly, a
post-hoc Tukey’s HSD test was done to investigate pair wise mean
differences between groups. Pearson’s product-moment correlation was
done to compare whether there was any significant difference between
heavy metal concentration and mangrove seedling growth. All analysis was
performed in R statistical environment (R Development Core Team, 2013).
RESULTS
Plastic waste characterization
In all 13 different plastic waste types were identified in the five
accumulated plastic waste sites (Table 1). In each category several
plastic waste components of all sizes were identified ranging from
syringes with their needles still intact, medicine, cream, perfume,
mouth wash, kitchen detergent, soda, and water containers. The
individual weight ranges of the plastic waste were from 6.7-350.1g. The
waste was found on either grassy or sandy areas. Grassy areas have the
highest accumulated waste.
Furthermore, site 5 has the highest waste volume (4.50kg) in terms of
diameter, radius, and height of waste column (Table 2). However, site 3
has the highest total plastic weight per mass (974.4kg) followed by site
1 (535.7kg) and site 2 (323.2kg). Nevertheless, Site 1 has the highest
number of waste types (19) followed by site 3 (17) and Site 2 (13).
The most dominant individual waste is bottled drinks (41.51%) followed
by bottled water (19.16%) and food pack (14.01%) (Table 3).
Seedling growth
experiment
Plastic vs. control :
The t-test result showed that there is no significant difference between
growth in control and plastic soil (t =1.25, df =192.45,P =0.21). However, there was slightly higher mean growth in height
(cm) in control soil (6.19cm) than in plastic soil (5.64cm) (Figure 3a).
Weekly to one year growth of seedlings (height) :
The ANOVA result showed that there is no significant difference in
seedling growth between control and plastic soils (F1,
325, 1.09, P=0.30). However, mangrove seedlings have higher bud growth
in control soil initially after planting, but at the
5th-6th week the seedlings in
plastic soil grew taller, then at the 7th week to on
year seedling in control grew taller. This pattern shows a fluctuating
growth (Figure 3b).
Number of leaves:
The ANOVA result showed that there is no significant difference in the
number of leaves of seedlings between control and plastic soils
(F1, 325, 1.27, P=0.26). There was also an increase in
the number of leaves from 2 to 10 within one year period. Seedlings in
the control soil retained their leaves longer than seedlings in the
plastic soil (Figure 3c).
Inter species comparison (R. Racemosa vs. R. mangle) :
The ANOVA result showed that there is significant difference in growth
between the two species (F1, 285, 9.66,
P<0.002; Figure 3d). Rhizophora mangle seedling grew
faster than Rhizophora racemosa in control soil compared to
plastic soil. This shows that pollutants from plastic waste can impactR. mangle growth in deforested mangrove forest (Figure 5).
Total hydrocarbon and heavy metal concentration
Total hydrocarbon content :
The ANOVA result showed that there is no significant difference along
soil profile (i.e., surface, and sub-surface) (F1, 30 =
1.83, P = 0.186, Table 4). But there is significant difference between
the metals (F3, 28 = 5.18, P = 0.0057). Total
hydrocarbon content (THC) has the highest concentration followed by zinc
and lead (Table 4). For THC surface and sub-surface concentrations are
40.85±32.52mg/kg and 4.26±2.16kg/mg respectively while for zinc the
concentrations are 2.14±0.53kg/mg and 0.37±0.01kg/mg for surface and
sub-surface respectively. The same result is replicated in soil
gradients.
Heavy metals :
The ANOVA result indicates that there is no significant difference in
metal concentration along soil gradient (F3, 28 = 0.60,
P = 0.619, Table 4). Furthermore, a two-way ANOVA indicates that the
profile and gradient have no effect on metal concentration
(P>0.05).
3.4. Microbial analysis (Conc. vs microbe)
The one-way ANOVA indicates that there is no significant difference in
bacteria and fungi population between soil profile (F1,
14 = 3.89, P = 0.0687, Figure 4). Similarly, there is no significant
difference in microbial concentration along soil profile
(F1, 14 = 2.85, P = 0.11) and soil gradient
(F3, 12 = 2.97, P = 0.08). However, a two-way ANOVA
(interaction of gradient × profile) indicates that the microbial
population is significantly different in soil gradient
(F3, 11 = 3.86, P = 0.04), but not significantly
different in soil profile (F1, 11 = 4.60, P = 0.055) in
the plastic accumulation sites (Figure 4). Bacteria (THB) have higher
population than fungi (THF).
Correlation between plant species abundance and metal
concentration
There was no correlation between mangrove seedling growth and heavy
metal concentration (t = 0.856, df = 28, p-value =
0.399; cor = 0.1596273; Figure 5a). Similarly, there was no
correlation between seedling growth and microbial concentration
(t = 0.596, df = 14, p-value = 0.56043, cor =
0.1574088; Figure 5b).
DISCUSSION
The identified wastes have floating ability that is why they are easily
carried by tidal currents into the sand filled area. More waste was
found in the grassy area because grass serves as a trap to the waste
when brought in by tides. The dominant grass species that does the
trapping is Mariscus longibracteatus (Numbere, 2020). Site
3 has the highest accumulated waste because they are trapped by the
grasses, which prevent them from being carried back to the sea. The
proliferation of bottled water in this region is because of lack of
provision of public water facility, which has made many to resort to
buying bottled water from stores. The waste bottle is thrown away into
the environment after the content is consumed because there is no strict
enforcement and punishment of offenders for dumping waste in the public
space. Waste dumped into the open is carried by flooded water into
public drainage system during heavy rainfall. Subsequently, the plastic
waste is eventually carried into the river from the municipal drainage
system which are often connected to the water body (Rakib et al. 2022).
Boat travelers and people living in shanties along at the coastal areas
dump their waste directly into the river because of poor waste
management.
Mangrove seeds were planted in soils collected from accumulated plastic
waste dump site to show the impact of plastic pollution on seedling
growth. Seedlings in control soil grew taller than those in plastic soil
probably because of low plastic-mediated metallic content (Table 4). The
entire study area is polluted from oiling activities around the creeks
and these pollutants along with plastic and other hazardous waste are
continuous circulated around the sand filled area during high tides and
during flooding and erosion. Nevertheless, it was observed that sites
with accumulated plastic waste have higher metallic content compared to
sites without accumulated plastic waste because of the decomposition of
the plastic material by solar radiation and microbes. The decomposed
materials therefore percolates into the soil underneath (Lin et al.
2022).
The growth in height of mangrove seeds in control and plastic soil has
fluctuating pattern (Figure 3b). This is because during the first four
weeks of planting the seeds in control soil grew taller than the seeds
in plastic soil (i.e., bud length). This pattern reversed at the
6-7th week where seeds in plastic soil grew taller.
But from the 7th week to on year the seedlings in
control soil continueed to grow taller than seedlings in plastic soil.
Even though no correlation (R=0.16 ) was found between seedling
growth and the chemical and microbial composition of the soil (Figure
5). The result implies that the decomposed plastic is harmful
and thus slows the growth of the seedlings.
The seedlings of R. mangle (dwarf seeds) grew taller than the
seedlings of R. racemosa (giant seeds) in both control and
plastic soil because of their small size and low nutrient utilization
ability, which enabled them to grow in poor soil. This shows that they
have the ability of surviving better in polluted soil than the R.
racemose . This result was supported by the stud of lamparelli et al.
(1997), which revealed that R. mangle has lesser defoliation in
soil polluted soil compared to Laguncularia racemosa andAvicennia schaueriana. Since R. mangle has high ability to
grow faster in polluted soil compared to the giant seed that require
higher nutrients for growth, the dwarf mangrove seeds can be used more
often to restore remediated sites as nursery seedling recruits
There was no difference between THC concentration between surface and
sub-surface soils. This means total hydrocarbon contents from plastic
waste percolates into the sub-surface quickly after decomposition to
contaminate the ground water aquifer. The result of no significance of
heavy metals and THC across soil gradients show that soil pollutants
from accumulated plastic waste sites can migrate and spread outwardly or
circumferentially to contaminate neighboring soils (Jin et al. 2022).
This can be harmful to organisms around the plastic waste dump sites
such as the fiddler crabs (Uca tangeri ), west African red
mangrove crab (Goniopsis pelii ), mud skipper and tilapia species,
which are captured and eaten by the local people.
Soil microbes are found everywhere and helps in the breakdown of plastic
waste into fine materials leading to the release of heavy metals into
the soil (Byrne et al. 2022; Joshi et al. 2022). Microbial distribution
varies horizontally across the plastic waste dump site (gradients)
compared to the vertical distribution at surface and sub-surface soils.
This result agrees with the study of Numbere (2021b), who revealed that
leaf litter decomposition by microbes was cumulatively higher in surface
than in sub-surface soil.