Marine Pollution Bulletin 78 (2014) 252–257

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Baseline

Plastic debris retention and exportation by a mangrove forest patch Juliana A. Ivar do Sul ⇑, Monica F. Costa, Jacqueline S. Silva-Cavalcanti 1, Maria Christina B. Araújo 2 Laboratory of Ecology and Management of Estuarine and Coastal Ecosystems, Departamento de Oceanografia, Universidade Federal de Pernambuco, Av. Arquitetura, Recife, PE CEP 50740-550, Brazil

a r t i c l e

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Keywords: Mangrove forests Coastal ecosystems Environmental conservation Marine debris Fishers communities

a b s t r a c t An experiment observed the behavior of selected tagged plastic items deliberately released in different habitats of a tropical mangrove forest in NE Brazil in late rainy (September) and late dry (March) seasons. Significant differences were not reported among seasons. However, marine debris retention varied among habitats, according to characteristics such as hydrodynamic (i.e., flow rates and volume transported) and relative vegetation (Rhizophora mangle) height and density. The highest grounds retained significantly more items when compared to the borders of the river and the tidal creek. Among the used tagged items, PET bottles were more observed and margarine tubs were less observed, being easily transported to adjacent habitats. Plastic bags were the items most retained near the releasing site. The balance between items retained and items lost was positive, demonstrating that mangrove forests tend to retain plastic marine debris for long periods (months-years). Ó 2013 Elsevier Ltd. All rights reserved.

The historical occupation of estuarine and mangrove areas put them at great risk of pollution, including contamination with plastic debris. Considering the social, economic and ecological importance of mangroves, relatively few works on plastic pollution have been conducted in these habitats worldwide, and Brazil is no exception (e.g., Cordeiro and Costa, 2010; Costa et al., 2011). However, such baseline studies are an essential first step in the conservation of estuarine areas and mangrove forests worldwide, guaranteeing the integrity of the river-estuary-ocean gradient. Plastics are widely spread all over the marine environment (Moore, 2008), and once in the sea they are easily dispersed to adjacent areas by surface currents, winds and tides (Wilber, 1987; Kubota et al., 2005; Ivar do Sul et al., 2009), where it accumulates. Plastic cargoes accidentally spilled in the ocean have been used to the benefit of the ocean sciences such as opportunistic experiments like other Lagrangian floaters (drift bottles, cards and more recently ALPS), to help provide insight into paths of contaminants and sea current patterns in the North Pacific Ocean. In coastal areas, the deliberate experimental release of marine debris to trace their trajectory has been used in previous studies focused

⇑ Corresponding author. Tel./fax: +55 (021 81) 2126 7223. E-mail address: [email protected] (J.A. Ivar do Sul). Present address: Universidade Federal Rural de Pernambuco, Unidade Acadêmica de Serra Talhada, Fazenda Saco, Serra Talhada, PE CEP 56900-000, Brazil. 2 Present address: Universidade Federal do Rio Grande do Norte, Departamento de Oceanografia e Limnologia, Via Costeira, Natal, RN CEP 59014-100, Brazil. 1

0025-326X/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.marpolbul.2013.11.011

on tagged items as tracers of plastic movements on beaches and estuarine environments (Williams and Simmons, 1996; Wilson and Randall, 2005). In this context, we released and tracked tagged items in a mangrove forest patch at the Goiana Estuary, an estuary in the Northeast coast of Brazil (Fig. 1a). The estuary (4700 ha) is an important source of natural resources and services to traditional communities of a Marine Conservation Unit (Barletta and Costa, 2009; Silva-Cavalcanti and Costa, 2009). The occurrence of plastic debris was reported on downstream estuarine beaches throughout the year (Ivar do Sul and Costa, 2013). The site chosen for our release and track experiment was an island with the river (Goiana river) at one side and a tidal creek on the other side (Fig. 1b). Both sides are subject to tide and river flow (Table 1). The higher ground is a relatively high and flat area, flooded only during extreme tidal events. The habitats considered in the experiment (river, higher ground and tidal creek) (Fig. 1c) have different hydrodynamic characteristics and mud/sand sediment ratio, resulting in different plastic debris retention potential. The forest (Rizhophora mangle) density and height were also expected to influence plastic debris trapping at each habitat (Table 1 and Fig. 1c). On each habitat, three identical tagged items (Table 2 and Fig. 1d, e) were put in areas of 20 m2 delimited by zebra-tape for reference during work. The experimental areas were cleared from all plastic debris before starting the experiment. Three items of each type were painted yellow (March – early rainy season) and pink (September – early dry season) (Barletta and Costa, 2009)

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Fig. 1. (a) Location of the Goiana Estuary in the Northeast of Brazil; (b) the studied island (300.000 m2) and transect I-II, from the tidal creek to the river; (c) schematic representation of the monitored habitats according to the transect I-II. The tagged items were painted (d) yellow for March and (e) pink for September. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Table 1 Detailed dynamic characteristics of the studied habitats. Goiana river Physical characteristics Width Depth Sediment

Bank slopes Winds Hydrodynamic characteristics Volume transported Flow rates Waves Tides Mangrove forest (Rhizophora mangle)

River: 10 m–1.7 km from the upper to the lower portions of the estuary (400 m around the studied area) Tidal creek: 20 m River: 6–8 m Tidal creek: 3 m River: fine sand Tidal creek: mud Higher grounds: sandy mud River: 160° from the water surface Tidal creek: 110° from the water surface Prevailing winds E-S-SE (130° Az); high percentages E (70–92° Az). Wind speed 0–6 m s 1 (higher concentration 3–4 m s

1

)

River: March – 1.749,888 m3 7 days 1 River: September – 12.703,824 m3 7 days 1 River: 11 m3 s 1 (0.5–25 m3 s 1) Tidal creek: 0.005 m3 s 1 No waves inside the estuary; high incidence of frontal waves on the coast 0.0–2.5 m (semi-diurnal)

River: 0.7 ind. m 2 Tidal creek: 1 ind. m 2 High grounds: 0.4 ind. m

2

and identified with numbers and letters, which corresponded to each area (Fig. 1d and e). A total of 189 tagged items were released in the mangrove forest in each experiment. Both experimental periods corresponded to equinoctial tides.

In the first day of each experiment (0 h), the tagged items (n = 189) were randomly thrown into their respective areas within each habitat. During the following 6 days, the items remaining in the forest were counted every 24 h (24, 48, 72, 96, 120 e 144 h),

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Table 2 Physical proprieties of materials used in the experiment.

= wind;

= Plastic items;

= positively buoyant;

= negatively buoyant;

= neutraly buoyant.

Tagged items used during the experiment

Dimension

Polymer type

Buoyancy in seawater (polymer)

Cap-closed bottles

2.5 l/300 cm2

Polyethylene terephthalate (PET)

Yes

Open bottles

2.5 l/300 cm2

PET

Yes

Open and squashed bottles

2.5 l/300 cm2

PET

Yes

Plastic bags

900 cm2

Low-density polyethylene (LDPE)

Yes

200 ml plastic cups

44 cm2

High-density polyethylene (HDPE)

Yes

Polystyrene blocks

200 cm3

Polystyrene (PS)

Yes

Open margarine tubs

500 cm3

HDPE

Yes

during the diurnal low tide. The nocturnal low tide was not used. Items were counted inside, near (10 m) their original areas. Concomitantly, adjacent estuarine beaches (Fig. 1b) and waters were searched for the tagged items. At the end of each experiment, all remaining items were recovered and adequately disposed off. The general behavior of the tagged items used in the experiment was similar (Student t-test; p  0.05) in both seasons (Fig. 2a). From the 189 items used to contaminate the forest in the first day (0 h), 51 (27%) and 48 (25.4%) were recovered in the last day of the experiment (144 h later) in March and September, respectively. Movement patterns of the tagged items were also similar between seasons. After the contamination (0–48 h), the majority of the tagged items were no longer in their original areas (Fig. 2a and b). The remaining items were slowly, but continually, transported along the following 5 days (48–144 h). In the Goiana Estuary, tides are apparently the main driving force controlling plastic debris movement patterns (Ramos et al., 2011) as previously reported by Wilson and Randall (2005) in Sidney, Australia. In South Wales (UK) floods, and not tides, were the main responsible for large-scale movements of plastics (Williams and Simmons, 1996). The tagged items were significantly (Factorial Analysis of Variance – ANOVA; p  0.05) more retained on the higher grounds when compared with the margins of the river and the tidal creek (Figs. 2b and 3a). At the higher and flat area (Fig. 1c), in addition

Surface exposed to wind or currents/ buoyance in seawater (item)

to weaker tidal currents, vegetation height and density contributed to marine debris retention within the mangrove forest. Tagged items firstly circled around the areas for different periods before being finally carried through the river and probably further to the sea (Ivar do Sul and Costa, 2013). The importance of vegetation in trapping debris was highlighted on the backshore of sandy beaches (e.g., Thornton and Jackson, 1998; Portz et al., 2011) and riparian vegetation (Williams and Simmons, 1996; Wilson and Randall, 2005). Both rigid and soft plastics were trapped, but plastic bags entangled in the vegetation resulting in a ‘Christmas tree’ effect (Williams and Simmons, 1996). In addition, the low hydrodynamics caused the burying of items, which apparently remain buried for long periods (Williams and Tudor, 2001; Costa et al., 2011). During the present study, a mangrove land crab was observed pulling a plastic bag to its hole, a behavior previously described for Chasmagnathus granulata, its counterpart in salt marshes of South America (Iribarne et al., 2000). Expected consequences of the presence of plastics in the mangrove forest may be entanglement, plastic ingestion and even the complete obstruction of the crab’s hole. Buried debris will probably decay and fragment into smaller pieces (e.g., Barnes et al., 2009; Costa et al., 2010), remaining in the forest or being permanently buried (Costa et al., 2011). On the borders of the island (Fig. 1b and c), plastic items were more directly affected by tidal currents and carried from the forest (Ramos et al., 2011). In these habitats, independent of size categories, shape or polymer, exportation of plastic debris probably

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Fig. 2. (a) Number of the recovered tagged items every 24 h after the initial contamination in March (early rainy season) and September (early dry season), when the three habitats (river, high ground and tidal creek) are considered. To test if the behavior of the tagged items was significantly different between March and September, a Student t-test was carried out, but significant differences were not reported. (b) Number of the recovered tagged items every 24 h after the initial contamination, considering the three habitats separately. Factorial analysis of variance (ANOVA) detected significant differences (p < 0.05) among the higher grounds and the other monitored habitats.

prevails over decay and fragmentation. Once in the water, consequences of plastic debris are certainly present (e.g. Possatto et al., 2011; Dantas et al., 2012; Ivar do Sul and Costa, 2013). The number of the tagged items inside their own initial area decreased along the experiment, while the opposite occurred with items counted far from their original area in the three monitored habitats (Fig. 3a). In addition, during the late rainy season, seven tagged items (two polystyrene squares, four PET bottles and a plastic bag) released six months earlier were still on the higher grounds habitat. These evidences suggest that plastic debris can be retained in mangrove forests for at least six months (March to September), and probably for much longer periods, resisting even extreme tidal events and seasonal riverine flushes. Outcomes of these findings are observed in mangroves all over the world (e.g., UNEP, 2009) where plastic debris are often reported but seldom studied (Debrot et al., 2013). On the other hand, two tagged items were recovered from adjacent estuarine beaches during each experiment. Two other, a capclosed PET bottle (March) and a plastic bag (September), were found in the Goiana river. In September, one cap-closed PET bottle initially released on the higher grounds was found on the tidal

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creek, illustrating at this time the potential of the forest to transport plastic debris to the adjacent environments. Eighty percent of the tagged items completely disappeared from the surveying area, 133 during the late dry season and 137 during the late rainy season. Considering that experiments were conducted during equinoctial tides, the scenario represented the maximum exportation potential of the habitats after contamination events. So, during lower tidal amplitudes, higher quantities of plastic debris might be retained in the mangrove forests and accumulated there, mainly on the higher grounds. PET bottles were observed during both experiments mostly far from their original areas (Fig. 3b). Significant differences among the behavior of the PET bottles (cap-closed, open, open and squashed) were not detected (ANOVA; p > 0.05). On the other hand, the margarine tub was less observed, either inside, near or far from their original areas. This suggests that it is easily carried from the forest by tidal currents. Plastic bags were mostly retained by roots and branches inside the original areas, followed by plastic cups (Fig. 3b). All the plastic materials released in this experiment were made of buoyant polymers (PE, PP, PS and PET) (Table 2, Barnes et al., 2009). If items are not trapped by vegetation, it is expected that they will be carried away, floating until they become waterlogged or overloaded by epibionts (Barnes et al., 2009). Exportation occurs differently since plastics float differently according to its polymer density and shape (Table 2). Studies that associate hydrodynamic and plastic debris behaviors are rare in coastal and estuarine environments. The tagged items tested here did not present distinguishable behaviors in the mangrove forest (Fig. 3b) possibly because of the limited number of samples and length of the experiments. It is important to highlight, however, that a generalization of patterns of movements based on few tagged items may be misleading. The establishment of movement patterns will demand a very large sample size and different types of plastic items (Wilson and Randall, 2005). Mangrove forests occur in both rural and urban landscapes of the tropical and sub-tropical belt and are under similar threats (Debrot et al., 2013). Each forest habitat approached here plays important ecological roles within the estuarine ecosystem (Barletta and Costa, 2009; Ramos et al., 2011), and plastic debris can interfere directly with these services. Plastic debris is only another myriad of challenges faced by these ecosystems for their conservation. Studies like this highlight the need for works on the ecological consequences of anthropogenic interference in different mangrove habitats. Since the South-East Asia tsunami in 2004, appreciation of the value of mangroves to society has grown considerably. Their role in coastal protection has made mangroves importance soaring high. However, mangroves conservation is also dependent on solving the plastic pollution issue. Its prevention and environmental decontamination should be priorities of local and national concern. While our study shows trajectories of plastic debris on mangrove forests in an estuarine ecosystem, these results need to be added to others to establish quantities, size categories, and environmental behavior of plastic items in estuarine environments and their different mangrove habitats; patterns and models of accumulation; and finally exportation of plastics to the adjacent environments including the ocean. We believe our findings are applicable to other mangrove settings on tropical and subtropical latitudes, highlighting the importance of basic information on plastic debris contamination (e.g., sources, types of items), and movement (retention and exportation) on mangrove forests and estuarine environments. Therefore, considering the difficulties of remediation of this kind of pollution in mangrove forests, and the ecosystem importance for society, we suggest that sources control must be a priority target.

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Fig. 3. (a) Number of the recovered tagged items observed inside, near (10 m) their initial areas every 24 h after the initial contamination, considering the three habitats (river, high grounds and tidal creek) separately. ANOVA was carried out to detected significant patterns among monitored days, habitats and the remaining tagged items in relation to their initial area (inside, near, far). Significant differences were not reported at this time. (b) Studied tagged items and their behaviors (inside, near, far) in relation to their original areas.

Acknowledgments This project had the financial support of Fundação O Boticário de Proteção à Natureza (Proc. No. 0720-2006.2). Authors would like

to thank Calamares Mergulho Científico, for the technical support during fieldwork, CAPES (www.capes.gov.br) and CNPq (www.cnpq.br) for scholarships to members of LEGECE. MFC is a CNPq Fellow.

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Plastic debris retention and exportation by a mangrove forest patch.

An experiment observed the behavior of selected tagged plastic items deliberately released in different habitats of a tropical mangrove forest in NE B...
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