1 Introduction Plastic, the lightweight and long-lived material, has become a serious environmental hazard (Thompson et al., 2009). The annual global production of the organic polymer has rapidly increased from 1.7 to 280 million tonnes within the last 60 years (Plastics Europe, 2012) resulting in the accumulation of plastic litter in virtually all habitats (Browne et al., 2011). Marine systems are sinks for pre- and post-consumer plastic and the multifaceted negative impacts of plastic pollution on wildlife (reviewed in Cole et al., 2011; Derraik, 2002; Oehlmann et al., 2009) as well as several aspects of debris composition, distribution and abundance have been described here (reviewed in Ryan et al., 2009). Although accumulation of plastic in the ocean is prevalent, there is scarce data on plastic inputs in the oceans (Law et al., 2010). Marine plastics originate from ship or land-based sources (Coe and Rogers, 1997) with the latter to be of greater relevance (Andrady, 2011). A significant portion of the terrestrial plastic is transported to the seas by rivers. Nevertheless, quantifications of plastic loads in rivers found in primary literature are minimal (Moore et al., 2011). Realistic estimations of the plastic flow from rivers to oceans are very important in helping to raise the awareness of the sources of plastic debris and ultimately to drive measures to reduce it. In this article, we present results from a two-year (2010, 2012) survey on plastic litter transport in Europe's second largest river, the Danube. The main aim of the study was to categorize and to quantify drifting plastic items. In a second step we compare plastic abundance and plastic mass in the river with those of ichthyoplankton (drifting fish larvae and juveniles). Adverse health effects may arise when small fish confuse plastic particles with food items (zooplankton, fish eggs) and ingest them (Carpenter et al., 1972). Finally we give a rough estimate of the input of plastic litter via the River Danube into the Black Sea. To our knowledge, this is the first report on plastic transport in a large river. The whole study was embedded in a scientific project that highlights larval dispersal and the conservation of riverine fish populations. All sacrificed individuals were handled according to applicable regulations and used for comprehensive analysis (Lechner et al., 2013b). 2 Methods 2.1 Study site The study was conducted in a free flowing stretch of the Austrian Danube between Vienna and Bratislava. All sampling sites were situated within the “Danube Alluvial Zone National Park” which preserves the last remaining major wetlands environment in central Europe (http://www.donauauen.at). Here, the average river width is 350 m and the discharge at mean flow is 1930 m3 s−1. Featuring the world's most international river basin (19 countries, 800.000 km2, 81 million people), the Danube is a special case study regarding conservation and management issues (Sommerwerk et al., 2009). As the main tributary (input of 6444 m3 s−1 at mean flow) and major nutrient pathway, the Danube directly affects the Black Sea (BSC, 2009). Beside eutrophication, the vulnerable ecosystems of this continental water face an increasing threat of plastic litter pollution (Topcu et al., 2013). Inputs from land-based sources have gained less attention but are supposed to be high, especially via the Danube River System (Lebreton et al., 2012). 2.2 Sampling The sampling procedure has been accurately described elsewhere (Lechner et al., 2013b). Briefly, we utilized stationary conical driftnets (0.5 m diameter, 1.5 m long, 500 μm mesh) that were fixed to iron rods driven into the riverbed and sampled the top 0.5 m of the water column. Nets covered 60% of the water column in more than 75% of all cases. The mesh size we used is in the range of other studies that quantified suspended plastics (reviewed in Hidalgo-Ruz et al., 2013). A flowmeter (2030R, General Oceanics®, Miami) was attached to the lower third of each net entrance to measure the volume of filtered water. In this volume-reducing approach, the filtered sample (containing plastics, fish larvae, organic debris and other items) is collected in a jar attached to the net-end and can be taken to laboratory for further processing. Duplicates (2010) and triplicates (2012) of driftnets were simultaneously exposed at three (2010) to four (2012) sampling stations along both river margins with maximum distances of 1 km between the single stations and 25 m between the shoreline and driftnets. In 2010, we sampled circadian (24 h) periods with hourly intervals between single sample events. In 2012, sampling started 2 h before sunset (according to ephemeris) and was continued in hourly intervals until midnight. Collecting day and night samples was essential in consideration of realistic comparisons between ichthyoplankton and plastics abundance: larval fish drift is known to exhibit a distinct diurnal rhythm with nocturnal peaks in individual numbers (Pavlov et al., 2008). Therefore, exclusive daytime sampling would have underestimated fish densities by far. The sampling period (Apr–Jul) was chosen to comprise the entire drift season (Lechner et al., 2013a). Before preservation in 96% alcohol, all fish were overdosed (500 mg/l) with the anesthetic tricaine methanesulfonate. 2.3 Sampling processing In the laboratory, plastic items and fish larvae were separated from the samples in a two-step process. Each sample was suspended in a water bath and a density separation (buoyant plastic particles and larvae with intact swim bladders were removed), was followed by a careful visual sorting of the remaining material by the naked eye. 2.4 Characterization and quantification of plastics All plastic pieces and larvae were counted. A subsample (n = 500) of fish larvae was taken and all individuals were weighed to the closest 0.01 g (moist mass). Each plastic particle was allocated to one of the categories shown in Fig. 1. Pellets, spherules and flakes characterize different types of industrial raw material that serve as precursors for plastics production. The category “others” encapsulates all other pieces and fragments of plastic consumer products. A subsample (n = 500) of each category was taken and all containing items were weighed to the closest 0.01 g and measured to the closest 0.01 mm (Zeiss® Axio Imager M1 with Axio Vision 4.8.2 software for image analysis). Referring to the size-ranges of the defined groups, the collected plastic may be termed mesodebris (2–20 mm; pellets, flakes, big spherules, others) or microdebris ( 5 cm), which did not enter driftnets through the small gap between net-frame and water surface. But especially large material contributes to the plastic mass in oceans (Lattin et al., 2004). 3) Compared to Germany and Austria, all other neighbouring countries of the Danube feature lower standards in their wastewater and sewerage treatment (http://www.icpdr.org). Their potentially higher contributions to the Danube's plastic load should considerably cumulate and increase the average input at the mouth. Plastic is the dominant debris in the Black Sea with a high percentage of items (47%) sourcing in neighbouring countries (among them several of the Danube basin), potentially introduced by river currents (Topcu et al., 2013). There is rare information about land based litter sources and the “Development and improvement of the existing monitoring system to provide comparable data sets for pollutant loads (from direct discharges and river inputs)” is a high priority task of the “Black Sea Strategic Action Plan” (BSC, 2009). Giving first answers on abundance and composition of plastic litter in the river Danube we hope to serve the cause and help to strengthen the enforcement of national and international regulations on land-based pollution sources (i.e. Operation Clean Sweep®, http://www.opcleansweep.org) Furthermore, our results shall give impetus to continuative studies on freshwater plastic pollution. All harmful consequences of plastic contamination described in marine systems (ranging from ingestion of plastic particles by a wide range of organisms to introduction of alien species which raft plastic litter) may operate in rivers and lakes and deserve closer attention.
