Introduction
Marine litter is found all around the world in all marine habitats (Pham et al.
2014), causing damage to wildlife (UNEP
2015), leading to economic losses and safety risks to people’s life (HELCOM
2015). The majority of marine litter consists of plastic (Reisser et al.
2013), making it one of the significant environmental issues of our planet (Fallati et al.
2019) at our time (Urban-Malinga et al.
2020). The number of species negatively affected by plastic has increased to more than 500 among all wildlife groups (Kühn et al.
2015). Plastic occurs in the deep sea (Van Cauwenberghe et al.
2013), in the Antarctic (Lacerda et al.
2019), in the open ocean (Eriksen et al.
2014), while the pollution on coastlines such as salt marshes, estuaries, mangroves, and beaches (UNEP
2016) is one of the most obvious signs of it (JRC
2011).
The Marine Strategy Framework Directive (MSFD) was adopted to protect the marine environment in 2008. MSFD is aiming to reach Good Environmental Status (GES) across the European Union (EU) by 2020 through the use of 11 descriptors (MSDF
2008/56/EC); with Descriptor 10 aiming for: “Properties and quantities of marine litter do not cause harm to the coastal and marine environment” (MSFD 2008/56/EC). On that basis and in addition to the “Regional Action Plan for Marine Litter in the Baltic Sea” (HELCOM
2015), there are legal obligations to record and reduce the marine litter pollution of the various marine habitats in the Baltic Sea (LUNG-MV
2015). A joined and harmonized monitoring strategy (JRC,
2011) was adapted from the OSPAR Guideline (OSPAR,
2010) and further developed, ensuring that data is comparable among monitoring surveys. This bare-eye method primarily focuses on stretches of sand or gravel beaches at least 1 km long, with surveys of 100 meters, and targets macro-litter (>25 mm). Long-term surveys such as the MARLIN Project “Baltic Marine Litter” (MARLIN
2011) of the marine litter of Estonian, Latvian, Swedish, and Finnish coasts have been conducted to meet these requirements. Beach surveys in Germany and Lithuania following OSPAR Guideline (OSPAR
2010) took place (Schernewski et al.
2018; Haseler et al.
2019,
2020). To gather more knowledge about meso- (5–25 mm) and large micro-litter (2–5 mm), different sieving (i.e., Rake method) and bare-eye methods were used at Baltic beaches (Haseler et al.
2018,
2019). However, these approaches focused on the open coastal beaches only, not considering the shores of the inner-coastal waters such as lagoons and estuaries.
The total Baltic Sea catchment area is four times larger than the surface area of the Baltic Sea, and it comprises nearly 1.8 million km
2 (Räike et al.
2021). More than 85 million people live in the Baltic Sea catchment area. The catchment area of those rivers is covered by agricultural fields, which substantially increases the pollution load (Schernewski et al.
2021). Supporting Source-to-Sea Framework for Marine Litter Prevention by Granit et al. (
2017), we believe that lagoons and estuaries as transitional zone could be an essential provider of information about river basin-related litter leakage. During the ice melting, its motion in the spring (Idzelytė et al.
2019), and accumulation on the coastline of coastal lagoons and estuaries, bigger litter pieces might be fragmented into smaller ones. However, it is unclear whether lagoons and coastal estuaries play a role as a sink, transition zone, or micro-litter source. Furthermore, the knowledge of pollution of lagoons is scarce worldwide.
To investigate plastic litter at coastal marine beaches, methods such as OSPAR 100 m transect are most often used (Schulz et al.
2015; Simeonova et al.
2017; Schernewski et al.
2018; Falk-Andersson et al.
2019; Haseler et al.
2020). However, the widely used OSPAR method for pollution of lagoons and estuaries assessment cannot be applied because beaches of lagoons and estuaries are mostly not long enough to survey a 100 m transect. Furthermore, this method does have a weakness in tackling meso- (5 – 25 mm) and large micro-litter (2 – 5 mm) (European Commission
2013). So far, few studies on coastal lagoon pollution (Oztekin et al.
2020; Velez et al.
2020) have been conducted, and most of them used methods applicable to long, sandy beaches. Therefore, a new method for all sizes of litter investigation in the inner-coastal waters needs to be established. This method needs to meet specific requirements, such as (i) it needs to be low-cost and straightforward to enable monitoring of many locations; (ii) it should be cost-efficient - meet the demand of authorities to keep costs low; (iii) it should be suitable to involve trained laymen (citizen science) and with a possible benefit to serve for environmental awareness rising; (iv) litter monitoring method and the gained data should be suitable to be combined or compared with the data of the OSPAR 100 m beach monitoring method (OSPAR
2010) and/or “Joint List of Litter Categories for Marine Macrolitter Monitoring” (Fleet et al.
2021) to enable comprehensive pictures of the state of pollution of coastal and marine waters; (v) it should allow an analysis of litter sources to support and enable mitigation measures; (vi) it needs to be generally applicable around the Baltic and in Europe to meet the requirements of Descriptor 10 of the MSFD.
The objectives of this study are: (a) to develop a methodology suitable for large micro-, meso-, and macro-litter monitoring at sandy coastal strips of lagoons and estuaries; (b) to test this method in a wide range of Baltic lagoons and estuaries to get an overview of present pollution state; (c) to analyze the composition, type, and abundance of beach litter (d) to assess significant pollution sources and address the extent to which lagoons serve as a sink for river-borne and land-based litter; (e) to discuss this methodology’s applicability and suitability concerning expanding and complementing existing beach litter monitoring methods towards transitional waters and inner-coastal waters.
Discussion
This study investigates the two largest lagoons in the Baltic Sea and one of the largest bays located in the northeastern part of the sea for all-sizes litter pollution. The entire Szczecin Lagoon, the Lithuanian side of Curonian Lagoon, and parts of Pärnu Bay have been declared as Birds Directive Sites, Habitats Directive sites, or both Birds and Habitats directive sites. Thus, studied lagoons and bays are essential to ensure the long-term survival of Europe’s most valuable and threatened species and habitats (Kotta et al.
2008; Radziejewska and Schernewski
2008; Povilanskas et al.
2014). Furthermore, they have tremendous economic value. Tourism, fisheries, and port activities are the main socio-economic activities in the considered areas (UNEP
2016; Inácio et al.
2018). Lagoons of the Baltic Sea face similar problems, with eutrophication being the main (Nehring
1992; Raateoja and Setälä
2016); however, knowledge about litter pollution in such water bodies is scarce. Many large Baltic rivers do not enter the sea directly but pass lagoons and estuaries (bays). Therefore, lagoons and bays can be regarded as pollution accumulation zones.
This study’s average macro litter density varied from 0.06 pieces/m
2 in the Pärnu Bay to 0.53 pieces/m
2 in the Curonian Lagoon, comparable to the other lagoons in Europe: mean density in Sarıkum Lagoon (the Black Sea) was 1.51 ± 0. 57 pieces/m
2 (Oztekin et al.
2020); Ria Formosa Lagoon (the Atlantic Ocean) from 0.12 ± 0.01 pieces/m
2 to 9.10 ± 2.05 pieces/m
2 (Velez et al.
2020).
Comparing litter densities per 1 m
2 found on the coastal beaches and lagoon sites, we see that, i.e., in Lithuania, the density of litter in 1 m
2 is the same on the lagoon (0.93 pieces/m
2) as on the coastal beaches of the Baltic Sea (0.93 pieces/m
2) (Haseler et al.
2020). However, litter densities in Szczecin Lagoon were lower than those reported for the coast of the German Baltic Sea (Haseler et al.
2018). There are several possible explanations for these results. Firstly, lagoon beaches are less visited than coastal ones (Haseler et al.
2018; Kataržytė et al.
2019); therefore, they are less polluted. Secondly, the fate of litter items in the lagoon depends on water exchange between the lagoon and the sea: if the river outflow is more intensive, litter is washed out on the coastal beaches.
One of the investigated transboundary lagoons in this study – the Szczecin Lagoon, indicates that the amount of litter found on the beaches on Germany’s side of the lagoon was precisely the same as on the Poland side of the lagoon – 0.16 pieces/m2. Litter circulates and distributes within the lagoon if outflow from the river is less intensive.
Investigating the top 10 litter items found in this study’s lagoons and bays, we recognized several items to be very different from the top 10 litter from the coastal beaches around the Baltic Sea (Haseler et al.
2020). The main difference in the top litter items was the paraffin (micro- and meso-size) which is washed out of the ships in the harbors and often ends up in the open sea (outside the 12-mile zone). Therefore, it is found in high densities on the coastal beaches, including the beaches of Pärnu Bay investigated in this study. In the meantime, paraffin was not present among the top 10 litter items on the beaches of two lagoons investigated in this study. On the other hand, two out of the top 10 litter items found in this study, “Construction material” and “Plastic construction material” were allocated by the experts to the “Land based industry and trade”. It means that lagoons and bays could be potential sink of land-based litter through the connection of the major rivers. River runoff as the primary source of litter (22%) was also indicated in the study performed on the SE Black Sea beaches (Aytan et al.
2020). Sewer overflows and stormwater were of the highest importance for large micro- and meso-litter emissions in the south-eastern Baltic Sea estuary of Warnow (Schernewski et al.
2021). Further supporting our findings, the study case of Ria Formosa (Portugal) indicated that construction material (heavier materials like ceramics, glass, and metal) derived from land-based sources was dominant in the lagoon compared to the open sea coastal sites (Velez et al.
2020).
Strengths and Weaknesses of Methodology for Litter Monitoring of Coastal Lagoons
Our results showed that the monitoring method of litter at the coastal lagoons and bays could determine micro, meso, and macro-litter. An evident strength of the methodology established in this study is relatively low-costs and time efficiency. Initial costs to implement this methodology are lower than a UAV methodology (Escobar-Sánchez et al.
2021), somewhat equal to the Sand-rake method costs (Haseler et al.
2020), and slightly higher than OSPAR. The annual running costs of the lagoon litter monitoring method are also much lower than those of the previously mentioned methods (OSPAR, UAV, and Sand-rake) due to the fewer hours needed for field activities. Surveying two polygons of 50 m
2 by the Sand-Rake way takes twice as much time (5 h) (Haseler et al.
2020), whereas investigating two polygons of 40 m
2 by the methodology established in this study—takes 2.5 h. In contrast to the Sand-Rake method, it is applicable when the sediment is wet.
In general, the lagoon litter monitoring methodology was established to be implementable for volunteer-based monitoring, which would significantly decrease annual running costs. This method is easy to explain and does not require fancy tools to apply. It would also promote community-engaged citizen science, at least in the field activities, and allow litter pollution investigation at a smaller scale of lagoons and bays. However, the perception of litter pollution differs among people and depends on various socio-demographic factors (i.e., age, income level, educational background, and gender) (Rayon-Viña et al.
2018). The difference in perception, especially for hardly visible or small objects, could be an additional challenge for litter data integrity. For all that, we recommend that every litter monitoring activity is complemented with the polymer-type analysis done using MicroPhazir hand-held device (or similar instrument/equipment). While items are listed according to the “OSPAR Marine Litter Monitoring Survey Form”. Furthermore, before implementing volunteer-based monitoring of litter pollution at coastal lagoons and bay beaches, training on methodology given by an expert should take place.
The main idea to establish a new method for coastal lagoon litter pollution was driven by the fact that the OSPAR 100 m monitoring methodology could not be applied due to relatively short lagoon beaches and the loss of litter below 25 mm in size. Moreover, the Sand-Rake method proposed by Haseler et al. (
2018) could not be used due to the granulometry of lagoon beaches and wrack accumulation, which would complicate the sieving of sediments for smaller-size litter. However, the data obtained by a new method must be suitable to combine with the data of a 100 m method by OSPAR. For example, the average density of all-sizes litter pieces in Pärnu Bay was determined as 0.28 pieces/m
2 using the Sand-Rake method (Haseler et al.
2020) and 0.06 pieces/m
2 using the methodology established in this study. Furthermore, nine out of ten top litter items found along the Estonian coast of the Baltic Sea (Haseler et al.
2020) were also obtained using the newly established methodology of this study, meaning that this study’s results are comparable with the results of the Sand-Rake method. In the meantime, the results of large micro- and meso-litter are also reliable, as the sieving is comparable to the Frame method (9 m
2) and should have similar recovery results (Haseler et al.
2018). Furthermore, we assume that the macro-litter results of 40 m
2 are more reliable than the 100 m OSPAR results as a much smaller area is investigated. Regarding monitoring sites, we recommend surveying one sampling site for 100 km
2 of a lagoon or bay area. Choosing sampling sites, the length of a beach (enough to sample two replicates of 40 m
2) should be preferred. Beaches that are recognized as official bathing sites and sites that are possibly affected by urbanization or port activities should be considered, too.
Although no statistically significant amounts of litter between seasons were found in several studies (Balčiūnas and Blažauskas
2014; Schernewski et al.
2018; Oztekin et al.
2020), the OSPAR Guidelines suggest evaluating the trend of litter abundance every three months. Based on this suggestion, calculations of litter monitoring costs were done considering the same frequency of litter monitoring (four times a year). In addition, event-based (i.e., storms, heavy rain) litter pollution monitoring could be implemented in the methodology.
Weaknesses of the method and ways to eliminate them are:
i.
When there is a large amount of reed thrown by waves and mixed with debris on the lagoon shore in wrack accumulation zones (wrack lines), it is challenging to make a representative analysis of the presence of micro- and meso-litter. Micro-litter could be stuck on the reed and accidentally discarded while removing the reed from the 1 m2 sampling areas. It is proposed to remove and wash the reed layer by layer in a separate bucket on the 1 m2 areas (some stems could be cut with a knife) and then drain this water with the resulting suspension through a 2 mm sieve.
ii.
Extrapolation of the results from the 2 × 1 m2 areas is problematic because it is mainly based on the accumulation zone, while smaller items are missed in the 40 m2 rectangular areas.
iii.
On the wide shores of lagoons, several wrack lines could be observed. The proposed method should primarily account for the wrack line closest to the water body. The more “distant” wrack lines could be sampled additionally by placing another 40 m2 polygon parallel to the first one.
iv.
If there are pebbles, gravel, or shells on the lagoon’s shore, it is recommended to use a cascade of 10 mm and 2 mm sieves to remove the increased load on the 2 mm sieve.
v.
It is difficult to identify small wet particles and films (black, white, transparent) and distinguish them from objects of biological origin in the field.
vi.
While the method implies sampling at quite a small rectangle of 4 × 10 m and thus requires a beach of a least 10 m length, there was a problem finding even such a small sandy coast in some areas. For example, Curonian Lagoon (especially on the eastern and southern coast) has shores covered with reed beds, muddy swamps, or a thick layer of shells of zebra mussels.
vii.
In this study, only accessible lagoon/bay beaches with parking spots have been sampled; therefore, rural beaches should be included in the monitoring to understand tourism’s impact better.
Conclusions
The litter monitoring method developed in this study aims to investigate all-size litter pollution on the beaches of coastal lagoons and bays, which are relatively short, with a larger sediment size fraction than the open-coast beaches. In total, 23 beaches from the inner-coastal waters of the Baltic Sea were investigated using the methodology developed in this study. In two major Baltic Sea lagoons and one bay, 817 litter pieces were found (0.22 pieces/m2). Substantial differences were observed in the total number and litter densities between the water bodies. Micro- and meso-litter size category and macro-litter resulted in somewhat equal parts, 339 and 478 pieces, respectively. The latter consisted of various types of construction material (plastic or glass/ceramic), indicating that lagoons could be a potential sink of land-based litter pollution. This study’s findings suggest that litter pieces were mainly introduced to the inner-coastal beaches from the tourism sector, wastewater treatment and stormwater drainage, and land-based industry and trade. We believe that results obtained using a newly established monitoring method are reliable, comparable, and fit the requirements of MSFD.
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