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Impacts of discarded plastic bags on marine assemblages and ecosystem functioning Dannielle Senga Green, Bas Boots, David James Blockley, Carlos Rocha, and Richard C. Thompson Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.5b00277 • Publication Date (Web): 30 Mar 2015 Downloaded from http://pubs.acs.org on April 5, 2015
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Impacts of discarded plastic bags on marine assemblages and ecosystem functioning
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Dannielle Senga Green1,2*, Bas Boots3, David James Blockley4, Carlos Rocha1 and Richard
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Thompson5
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Trinity College Dublin, Dublin, Ireland.
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Kingdom.
Biogeochemistry Research Group, Geography Department, School of Natural Sciences,
Queens University Belfast Marine Laboratory, Portaferry, Northern Ireland, United
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Ireland.
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South Atlantic Environmental Research Institute, Stanley Cottage, Falkland Islands
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School of Marine Science and Engineering, Plymouth University, Plymouth, Devon, United
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Kingdom PL4 8AA.
UCD School of Biosystems Engineering, University College Dublin, Belfield, Dublin,
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Keywords: contamination, pollution, polyethylene, biogeochemistry, nutrient cycling,
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aquatic, biodegradable
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Abstract
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The accumulation of plastic debris is a global environmental problem due to its durability,
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persistence and abundance. Although effects of plastic debris on individual marine organisms,
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particularly mammals and birds have been extensively documented (e.g. entanglement,
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choking), very little is known about effects on assemblages, and consequences for ecosystem
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functioning. In Europe, around 40% of plastic items produced are utilised as single-use
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packaging, which rapidly accumulate in waste management facilities and as litter in the
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environment. A range of biodegradable plastics have been developed with the aspiration of
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reducing the persistence of litter, however their impacts on marine assemblages or ecosystem
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functioning have never been evaluated. A field experiment was conducted to assess the
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impact of conventional and biodegradable plastic carrier bags as litter on benthic macro- and
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meio-faunal assemblages and biogeochemical processes (primary productivity, redox
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condition, organic matter content and pore-water nutrients) on an intertidal shore near Dublin,
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Ireland. After nine weeks, the presence of either type of bag created anoxic conditions within
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the sediment along with reduced primary productivity and organic matter and significantly
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lower abundances of infaunal invertebrates. This indicates that both conventional and
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biodegradable bags can rapidly alter marine assemblages and the ecosystem services they
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provide.
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1. Introduction
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Plastic items have become an integral part of daily life in many societies, and use is
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increasing, with an estimated annual global production of 299 million tonnes in 2013.1 Of
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this, single-use packaging items account for the majority, almost 40%, of total production.1,2 It
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is estimated that almost 5% of the plastic produced is transported via wastewater flows, inland
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waterways, wind or tides and ends up in the marine environment as litter3-7. Indeed, plastic 2 ACS Paragon Plus Environment
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waste accounts for up to 80% of all litter found in marine habtiats.4 Of this litter, plastic bags
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are one of the most common items8 especially on intertidal9,10 and subtidal11 benthos.
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Biodegradable plastics have been proposed as an alternative to conventional plastics such as
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polyethylene. Biodegradable plastic bags are intended to break down more rapidly than
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conventional plastic bags and are, therefore, believed to be less persistent as litter. Yet there
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have been few studies evaluating their degradation in natural habitats and the extent to which
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any enhanced degradation might reduce marine litter is not clear.12,13 Indeed some degradable
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polyethylene formulations have been shown to persist in the environment for years after their
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disposal.14 Given their relative recalcitrance to decomposition under natural conditions,
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conventional and biodegradable plastic bags pose a potential threat to organisms in coastal
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ecosystems when present as litter.
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Contamination of marine habitats by plastic litter can be aesthetically detrimental, leading to
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negative socio-economic consequences.15 There is also considerable evidence relating to
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consequences for wildlife. Over 660 species are known to encounter marine debris and
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negative consequences including physical damage from entanglement and choking, and
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mortality are reported for individuals from a wide range of species,4 including birds16
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mammals4 and invertebrates.17 While information on effects at the individual level is of
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considerable value, evidence of effects at higher levels of biological organisation, i.e. species
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assemblages, communities and populations, is often of critical importance to decision makers
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since it can be used to inform policy measures. Providing this information is necessary, for
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example, to inform decisions about legislation to reduce the quantity of single use, disposable
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items including plastic bags and / or about the efficacy of alternative materials with enhanced
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degradability.
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It has been estimated that up to 70% of all plastic debris settles onto the benthos18 with
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considerable accumulations in intertidal habitats worldwide19; yet to date, there have been few 3 ACS Paragon Plus Environment
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studies assessing the effect of plastic debris on these habitats and no work on the ecosystem
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services they provide, such as primary productivity and nutrient cycling. Coastal ecosystems
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are extremely diverse and productive, supplying vital ecosystem services such as the
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production of food, stabilisation of shorelines, removal of pollutants, and nutrient turnover.20
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Within these, sedimentary habitats are of particular importance, recycling nutrients that
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support economically important benthic and pelagic food-webs through a myriad of
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biogeochemical processes.21 It is possible that plastic debris might decrease the biomass of
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microphytobenthos (important primary producers) in sedimentary habitats by blocking light.
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Consequently this could alter biogeochemical processes such as decomposition of organic
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matter, which is essential for the release of inorganic nutrients (such as nitrogen as
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ammonium). This, in turn, supports primary production and therefore, food-webs. To date,
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two studies have examined the effects of plastic debris at ecologically relevant scales; one
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mensurative study on infauna22 and one manipulative subtidal experiment on epibiota and
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infauna.23,24 It has been suggested that when on the surface, plastic debris might physically
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asphyxiate the underlying sediment, potentially impeding nutrient exchange processes at the
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sediment-water interface, leading to anoxia and decreasing the abundance of infaunal
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organisms,6 but neither this nor the effects of plastic debris on infaunal assemblages in
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intertidal habitats have ever been experimentally evaluated.
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To assess the impact of plastic bags as litter within a coastal habitat, a manipulative field
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experiment was done to test the following hypotheses: the addition of plastic carrier bags
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locally alters (i) invertebrate assemblage structure and composition and (ii) biogeochemical
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processes within the sediment and iii) carrier bags made of different types of material
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(biodegradable or conventional non-biodegradable plastic) have different effects on biological
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assemblages and biogeochemical processes within the sediment.
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2. Materials and Methods
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2.1. Site description and experimental design.
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Booterstown Marsh (Booterstown, Co. Dublin, Ireland, 53°18.65' N, 6°11.9' W) is a nature
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reserve, sheltered from the open sea, managed by An Taisce, the National Trust for Ireland
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(An Taisce 2013) and has been proposed to be a Natural Heritage Area. Permission to execute
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the experiment at this nature reserve was granted by An Taisce. It is a designated bird
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sanctuary with hydrologically low energy depositional areas consisting of very fine clay and
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silt forming mud-flats overlain with brackish water. As the marsh receives freshwater input
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from a stream, salinity fluctuates with the tidal cycle. At the time of sampling, the salinity of
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the surface water was 20 ppt during falling to low tide and during rising to high tide it was 35
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ppt. It is in close vicinity to a highly urbanised area and is prone to accumulate waste litter,
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including plastic items.
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White plastic carrier bags made of either conventional high density polyethylene (HDPE) or
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biodegradable plastic manufactured from corn starch were used. Both bags were of a single-
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use disposable nature rather than of a more substantial ‘bags for life’ thickness. The
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biodegradable bags were labelled as “biodegradable” and “compostable” and claimed to
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completely disintegrate into carbon dioxide, water and biomass within 10-12 weeks in
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standard composting conditions and are certified "OK COMPOST" by EU standard EN 13432
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& International standard ASTM D6400-99. The dimensions of both types of bag were
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approximately 38 x 46 cm and 65 µm thick. On 13 March 2014, conventional and
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biodegradable plastic bags and control plots without bags (n = 10 for each treatment) were
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randomly interspersed on the surface in an area (10 m by 25 m) of pristine mud-flat, i.e. free
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of observable plastic debris on the surface. Each plot was separated by at least 2 m to avoid
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potential edge effects crossing over between treatments and was assumed to be similar at the
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beginning of the experiment. Each bag was securely positioned flat using metal pins inserted 5 ACS Paragon Plus Environment
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into each corner to simulate the manner in which plastic bags had been observed to become
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trapped at the surface of the sediment elsewhere in the marsh (Personal Observation). Control
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plots (n = 10) were also marked out with metal pegs at the corners to identify dedicated plots
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of the same size as the bags. Prior to deployment of the bags, each plot including the controls,
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received a custom made, in situ profiler to allow for pore water measurements (see section
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2.2.1 for details). Compared to the typical persistence of plastic debris in the environment our
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experiment was short term and was destructively sampled to assess impacts on infaunal
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assemblages (section 2.2) and ecosystem functioning (section 2.3) after 9 weeks (i.e. after
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75% of the time stated for complete disintegration of biodegradable plastic bags under
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composting conditions). Weather conditions (daily temperature and hours of sunshine) during
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the experimental period are displayed in Supplementary Figure S1.
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2.2. Assessing infaunal diversity.
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A 10 cm diameter corer was inserted down to 5 cm depth as much to the centre of each plot as
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possible. The core sample (~393 cm3) was transferred to sealable bags, stored at 4°C and upon
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arrival in the laboratory immediately sieved through a 125 µm mesh sieve pan to retain
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macrofauna and large meiofauna. All material recovered from the sieves was preserved
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separately for each sample in 70% methylated ethanol for enumeration and identification to
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the lowest discernible clade possible using a dissecting microscope.
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2.3. Assessing ecosystem functioning.
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2.3.1. Redox and pore-water nutrient concentrations.
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Prior to any other measurements being taken, five plots from each treatment were randomly
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selected to determine the redox potential of the surface sediment (~0.5 cm) using a redox
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the other sampling procedures. Pore-water samples were collected using purpose-built in situ
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profilers based on the design in Seeberg-Elverfeldt et al.2 and as described in Rocha et al.26
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and Green et al.27 Briefly, the profilers, made of polymethylmethacrylate sheets, had grooves
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cut at intervals to attach RhizonTM membranes (Rhizosphere Research Products B.V., The
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Netherlands) on a vertical sequence, allowing sampling at 0 (sediment-water interface) 2 and
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4 cm depths. The profilers were inserted vertically into the sediment, secured by metal pins,
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and a single biodegradable or conventional bag was placed centred on top. Control plots had a
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profiler inserted and secured with pins only. At the end of the experiment, pore-water was
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sampled by attaching a needle to each RhizonTM membrane and water was collected directly
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into sterile vacuum tubes (according to Ibanhez and Rocha.28 Within 24 h, ammonium (NH4+)
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and biogenic silicate (Si(OH)4) were measured from the water samples using a Lachat
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QuickChem 8000 flow injection autoanalyser (Lachat Instruments, USA) following Lachat
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methods 31-107-06-1-B (NH4+) and 31-114-27-1-A (Si(OH)4). All concentrations of pore-
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water were corrected for sediment porosity and were standardised to dry bulk density
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following Eleftheriou and McIntyre.29 NH4+ and Si(OH)4 inventories were calculated within
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the depth profile by integration of linear pore-water concentration gradients, corrected for
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porosity, down to 4 cm depth.
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2.3.2. Chlorophyll content and light measurements.
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Chlorophyll measurements were done colourimetrically. The oxic surface layer of sediment
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(approximately the top 0.5 cm) was sampled using clean spatulas, immediately wrapped in tin
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foil to protect from light and stored at 4°C. Within 24 h of collection, 10 ml of 90% acetone
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was added to ~1 g of field-moist, homogenised sediment, left to extract total chlorophyll for 1
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h at room temperature under constant shaking in the dark and centrifuged at 3000 g for 5 min
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to settle sediment.30 Chlorophyll-a, -b and -c concentrations were measured from the 7 ACS Paragon Plus Environment
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supernatant using a spectrophotometer (at λ = 430 & 664, 460 & 647, and 630 nm
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respectively) and concentrations of chlorophyll were calculated according to equations by
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Jeffrey and Humphrey31 and expressed as µg chlorophyll g-1 dry sediment.
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The plastic bags became colonised with algae during the experiment, therefore, chlorophyll-a,
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-b and -c were also measured from the surfaces of the bags. Bags were kept in the dark and
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stored at 4°C for 24 hours before being processed for analysis in order to preserve chlorophyll
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at levels similar to field conditions. Total chlorophyll contents of microalgae that had
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colonised the surfaces of the bags were estimated by cutting 4 cm2 squares from each corner
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(leaving 5 cm margins at all sides) and one from the centre, representing a surface area of 20
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cm2. From each bag the five squares were placed together into 10 ml of 90% acetone, left to
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extract for 1 h at room temperature in the dark under constant shaking with intermittent
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vortexing to thoroughly mix the plastic fragments. After centrifuging at 3000 g for 5 min to
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settle the plastic fragments, 1 ml of supernatant was used for measuring on a
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spectrophotometer in a similar fashion as was done with the sediment samples.
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In addition, the percentage of light blocked by the plastic bags (transparency) was measured
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using a LI-COR LI-250A light meter (LI-COR Inc., USA). On average, the conventional bags
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blocked 51.1 (± 0.6) and biodegradable bags blocked 50.3 (± 1.6) % of photosynthetically
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active radiation.
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2.3.3. Organic matter content of the sediment and grain size.
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Surface (top ~0.5 cm) sediment samples were collected by scraping using a clean spatula
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while the underlying sediment (approximately 4 cm depth) was sampled using a mini-corer
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adapted from a 60 ml syringe (26.7 mm diameter). Two samples were randomly taken from
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each plot to account for within-plot heterogeneity. Organic matter content was determined by
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loss on ignition (LoI) following Eleftheriou and McIntyre.29 Briefly, ~2.5 g of oven-dried 8 ACS Paragon Plus Environment
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(80°C, 12 h) sediment was placed in a muffle furnace at 450°C for 24 h. The difference in
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weight is expressed as a percentage of ash-free dry weight.
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Grain size distribution of the sediment was estimated by laser diffraction using a Mastersizer
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2000 (Malvern Instruments) from three plots randomly selected from each treatment. Briefly,
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1 g of dry sediment was digested in hydrogen peroxide to remove organics prior to analysis
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according to Gray et al.32
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2.4. Statistical analyses.
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All data were analysed using R v3.1.1.33 The data were checked for normality (normal
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quantile plots and Shapiro-Wilk tests) and homogeneity of variance (Levene’s tests) and no
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transformations were deemed necessary. All response variables from the sediment samples
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(total amount of individuals (N), number of species (S), Shannon-Wiener index (H’),
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chlorophyll contents in sediments, loss-on ignition (LoI), nutrient concentrations at each depth
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and nutrient pools and fluxes) were individually analysed using a one-way ANOVA with
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”Treatment” as a factor having three levels (Control, Conventional and Biodegradable).
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Redox potential was measured from only five randomly selected replicates from each
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treatment in the field due to time restrictions posed by the tide. The redox data were analysed
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using the same model as before. When the main test was significant, a post-hoc pair-wise
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comparison of the means was computed (Tukey HSD test, at α = 0.05) to ascertain the a
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priori hypotheses. Chlorophyll-a, -b and -c contents on the bags (two levels: Conventional
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and Biodegradable) were compared using separate two-tailed Welch’s t-tests.
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Multivariate data were analysed using the vegan package v2.2-0.34 Assemblage structures
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were computed using species count data (4th-root transformed abundance data to account for
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highly dominant observations), whereas assemblage compositions were computed based on
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and differences are shown using two-dimensional non-metric multidimensional scaling
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(nMDS) ordinations. The nMDS were calculated using 250 iterations or until the lowest 2D
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stress was reached using the metaMDS function in vegan employing the monoMDS engine.
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Before calculating permutational analyses of variance (PERMANOVA), multivariate
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homogeneity of variances were checked (betadisp function in the vegan package). Differences
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between assemblage structure and composition within the sediment were analysed with
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“Treatment” as a single factor with three levels (Control, Conventional and Biodegradable) by
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computing a one-way PERMANOVA using the adonis function in vegan, with probabilities
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calculated based on 9999 permutations of the raw data. To assess which identified groups
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contributed most to dissimilarities between treatments, similarity percentages were computed
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using the simper routine in the vegan package.
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3. Results
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3.1. Effects of plastic bags on infaunal diversity, assemblage composition and structure.
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All bags were still in situ when collected and no immediate signs of degradation nor
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fragmentation were observed. A total of 14 different groups of infaunal invertebrates were
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identified within the top 5 cm sediment layer. The total numbers of individuals (N) were
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significantly lower beneath plastic bags compared to in control plots (One-way ANOVA, F
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2,27
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and conventional bags (Table 1). Sediment beneath biodegradable and conventional bags had
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on average 6.2 and 6.4 times fewer individuals compared to that of control plots, but species
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richness (number of species - S) and Shannon-Wiener diversity (H’) was not significantly
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different from that found in control plots (Table 1). The most abundant taxa were Nematoda
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(potworms), Capitella capitata (gallery worm), Hydrobia ulvae (mudsnails) and
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Chironomidae (midge) larvae which, although numerically dominant in all treatments, were
= 48.97, P
