Large amounts of dislodged macroalgae and seagrasses accumulate on beaches, fulfilling numerous ecological functions [1
]. Among others, these depositions, known as wrack, protect coasts from erosion, act as refuge for numerous intertidal invertebrates and are sources of food and nutrients for marine ecosystems [3
]. However, nowadays, macroalgae and seagrasses are not the only items that end up accumulated on sandy beaches. In recent decades, high amounts of marine litter are deposited on the sand, becoming entangled in the natural wrack and altering its services to nearby ecosystems [5
]. However, up to now, just one study has assessed both artificial and natural accumulations on sandy beaches [5
Marine litter can be indirectly introduced on beaches by rivers, storms, winds, or sewages systems, but it can also be directly deposited by tourism, fishing, or recreational activities [7
]. Most of the litter that arrives to the shorelines is plastic and is mainly deposited on the beaches close to population centres, but it also occurs in more remote areas [8
]. Besides plastics, other materials such as wood, metals, glass, rubber, clothing or paper also frequently accumulated on the coasts. Marine litter can be deposited upon the seabed or can be incorporated and trapped under the sand, which complicates its quantification and removal [12
]. This litter can also become entangled in fauna and flora or can be ingested by marine organisms [14
]. Marine litter is also transported by ocean currents for substantial periods of time therefore increasing the potential for invasions of alien species, which is one of the major threats of coastal ecosystems [16
In addition to environmental impacts, marine litter can negatively influence beach aesthetic and increases pollution, affecting humans at economic, social, and health levels [17
]. Such is the concern for this type of pollution that the United Nations urges to prevent and reduce marine litter by 2025 (Target 14.1) as part of the Decade of Ocean Science for Sustainable Development (2021–2030). In this framework, the monitoring and quantification of marine litter is essential to determine the baseline data in the environment, to adopt effective management and mitigation measures along the coasts and to evaluate the main sources and activities which are causing this problem [19
]. This information allows us to detect future changes in the quantity of marine litter, determining a ‘standing stock’ that can help political and management interventions [20
The proximity of the source of marine litter, local hydrodynamics, social and economic factors (i.e., policy, education, tourism or other anthropogenic activities), can influence the accumulation of marine litter on beaches [21
]. Since environmental conditions and anthropogenic activities, such as fishing and tourism, often occur seasonally, the accumulation of marine litter can also vary over time. Similarly, natural wrack deposits on beaches are extremely variable around the world (i.e., [3
]). Natural wrack accumulations can be influenced by the physical environment and oceanographic conditions that promote wrack arrival near coasts and upon beaches [28
], but also by the life cycles, composition, and buoyancy of the species that compose it [29
]. Therefore, a sampling design that simultaneously includes the quantification of marine litter and wrack and their spatio-temporal variability could provide us useful information on the ecological status of the beaches and the potential relationship between both artificial and natural accumulations.
The objective of this study is to determine the composition and abundance of natural wrack and marine litter of five sandy beaches in the north of Portugal in two different dates to know their spatio-temporal variability and to explore a potential correlation between both artificial and natural accumulations. To do this, accumulations of natural wrack and marine litter were collected, identified, counted, and weighted across two different seasons to identify their spatio-temporal variability.
This study demonstrated that the quantity of marine litter that reaches the northern sandy beaches of Portugal is much greater than the amount of natural wrack that accumulates on the shoreline. Both natural wrack and marine litter accumulations had high variation that depended on the particular beach sampled. Although natural wrack depositions were greater in summer, the accumulation of marine litter was more intensive in winter. However, some beaches showed the inverse trend. These results indicate that, as well as other factors such as meteorological conditions and natural life cycle of the wrack species, the geomorphodynamics of the beaches could play an important role in the storage of marine litter and natural wrack on sandy beaches.
During the summer, the beaches of the north of Portugal suffer sedimentation and the sand end up accumulating along the foreshore as a high berm (as shown in Figure S1
). Contrarily, in winter, erosion due to waves and adverse meteorological conditions flatten the berm of the sandy beaches. These morphodynamic changes could result in natural and anthropogenic materials that are deposited on the beaches remaining buried under the sand in summer, and in winter, they being uncovered due to erosion. While the natural wrack degrades over time [30
], marine litter can persist intact under the sand for months or years [9
] which could explain the highest amount of litter obtained in our study during the winter. This matches with the results obtained in previous studies from the North Atlantic, in which marine litter accumulations were also greater in the winter months [41
]. Moreover, the presence of macroalgal species in the natural wrack of the winter that are usually frequent during the summer (i.e., S. polychisdes
) reinforces our explanation that the beach morphodynamics could play an important role in the accumulation of both depositions. Nevertheless, the biomass of the natural wrack and the weight of marine litter were not statistically correlated in our study, which suggests different mechanisms of accumulation for these kinds of depositions on sandy beaches (e.g., temporal differences in the supply dynamics, different depositional behaviour).
Other factors, such as recruitment strategies of the taxa found within the wrack, could also influence the accumulation on sandy beaches. For instance, S. polyschides
was one of the most abundant species of the natural wrack in summer, together with the biennial Himanthalia elongata
(Linnaeus) S.F. Gray 1821,
the pseudoperennial Sargassum muticum
(Yendo) Fensholt 1955 or the opportunistic Ulva
spp. These macroalgal taxa are characterized by having sustained active growth in spring and summer, reaching maximal size and reproductive stage very quickly during these periods [27
]. This greater biological development during summer means that the reproductive or vegetative branches of these species may be detached by waves and currents, then accumulating in the natural wrack. Nevertheless, after the summer, these macroalgal species usually decay as it is an unfavourable period in terms of nutrition so they cannot cope with that level of extensive growth over the long-term [49
]. In contrast, perennial macroalgae are slow-growth species which develop large nutrient stored pools during the winter, allowing them to survive better in the winter months [47
]. This explains the dominance of Fucus
spp. A. nodosum
spp. in the natural wrack during the winter in our study.
In concordance with previous studies, our results showed a high temporal and spatial variability in the diversity and quantity of natural wrack on sandy beaches [4
]. No study was found in the literature that allowed us to compare the composition of natural wrack taxa in our study with other Portuguese beaches. However, our results coincide with other studies in Galician beaches, which obtained a similar number of taxa annually and also showed high abundance of the taxa S. polyschides
spp., S. muticum
]. Despite the fact that the quantity of natural wrack per beach in the North of Portugal was generally much lower than those found in Galicia, the taxa Fucus
spp., S. polyschides
and A. nodosum
were more abundant in the north of Portugal. In addition to recruitment strategies, morphological adaptations of the natural wrack species can also influence such spatio-temporal variability. For instance, the drifting of the invasive alga S. muticum
increases in spring and summer, when it is also more abundant in the natural wrack [50
]. This is due to this species self-thins to be dispersed to complete its life cycle [54
]. Moreover, species with air bladders in their structures, such as Fucus
spp., A. nodosum
spp., are more frequently found in the wrack after periods of high wave exposure [50
], such as that occurred during the winter in our study. As these species were especially abundant in Cabedelo’s beach in winter, we could speculate that the largest amount of wrack and marine litter on this beach could be intensified by adverse meteorological conditions prior to sampling. This could also explain the greater amount of marine litter found in winter in comparison with summer.
In the sandy beaches of northern Portugal, the total weight of marine litter was four times greater than the total weight of natural wrack, especially during winter. Coinciding with other studies, the most abundant material of the marine litter was plastic [41
]. Most of this plastic originated from fishing materials such as ropes, tangled nets, cords or strings, fishing lines or plastic stoppers for mussel farming, which could be harmful to marine organisms [58
]. Small plastic pieces were also very numerous, which probably came from the breakdown or degradation of larger plastic debris items because of the wave action or physical abrasion [9
]. Industrial packaging or plastic sheeting and other plastic instruments used for human sanitary purposes, such as cotton bud sticks, were especially abundant in our study during the winter. Such type of plastic materials have great buoyancy, so they can be easily washed away by runoffs from rivers to the sea after periods of heavy rains, which would explain their greatest proportion in winter. In winter, in Cabedelo the number of cotton bud sticks increased among other items. Similarly, in Âncora, the quantity of cotton bud sticks increased in winter, but in the rest of the beaches was similar in both periods. During the winter, the rivers of the north of Portugal have large volume discharges of water to coastlines because of heavy storms, which could also cause the inefficiency of sewage treatment plants and wastewater problems, promoting the direct discharge of sanitary items such as cotton bud sticks onto the beaches [22
]. Thus, such high densities of cotton bud sticks and other marine litter items found on the beaches of Cabedelo and Âncora during winter could be explained by exceptional adverse weather conditions prior to sampling coupled with inefficient sewage systems and human behaviour (disposing of these materials in sewage rather than garbage streams).
In 2017, more than 300 tons of plastic were produced globally [62
] and it is expected that this amount of plastic will double in the next few decades [63
]. A recent study has demonstrated that the presence of plastic can negatively influence the decomposition and nitrogen liberation from the natural wrack to the ecosystem [6
]. Moreover, many invertebrates feed on natural wrack, playing a crucial role in the recycling of nutrients in coastal habitats [64
]. On the other hand, some studies proved that wrack consumers can ingest marine plastic litter accumulated on wrack [66
]. Other animals can feed on the wrack consumers [3
], so the presence of plastics and toxic elements (i.e., cigarette butts) in this natural wrack could also promote cascading effects on the food chain. Thus, urgent management is required to end plastic pollution on the sandy beaches of Northern Portugal and to maintain the healthy functioning of the coastal ecosystems.
Our study also showed as in summer, cigarette butts and other pollutants (mainly formed by leftover food) were also very abundant in the marine litter, probably because of the increased frequency of bathers and the influence of tourism activities. In this area, people visit beaches much more frequently in summer than in winter. While leftover food is easily degradable, cigarette butts are very perdurable in the coastal ecosystems and can cause metal contamination [69
]. The north Portuguese coast is very vulnerable to marine litter, since the economy of this area is linked to fishing and tourism activities. The west of the Iberian Peninsula also experiences numerous recreational and maritime activities and is an important route for commercial vessels and cruise ships, thus adding increased sources of pollution to coastal ecosystems in the form of marine litter. The total amount of marine litter found on the beaches of northern Portugal was somewhat higher compared to other places of the west of the Iberian Peninsula, such as Cádiz [57
], Galicia [41
], Madeira [56
], and Açores [42
], but was much lower than that found in other highly polluted areas such as Papua (New Guinea [71
]), Rio de Janeiro (Brazil, [72
]) or Chile [73
]. The high marine litter pollution found on the sandy beaches in this study, which are located far away from urban centres where tourists and local bathers are rare, provides some evidence for the urgent need to manage and control this problem in the North of Portugal.
Until now, research of marine litter in Portugal have been focused on quantifying the items of small size such as microplastics [74
] and have done on the deepest areas of the sea [76
]. In addition, the quantification of natural wrack diversity has been never studied on the north Portuguese coast. Therefore, our study provides the first assessment of wrack and marine litter depositions on the sandy beaches in the North of Portugal and shows that marine litter depositions considerably exceed the amounts of natural wrack found upon beaches. This fact could have serious consequences by altering the ecological services of the natural wrack, for example, limiting the cycle of nutrients on the beaches [6
]. In addition, the northern Portugal sandy beaches showed a greater amount of marine litter than other north Atlantic beaches, which could have negative social and economic implications. This study suggests that beach morphodynamics and meteorological conditions can play a very important role in the spatio-temporal variability of the natural wrack and marine litter accumulations. Thus, future research should consider this spatio-temporal variability when quantifying marine litter and wrack on sandy beaches. These findings are essential to the future design of management plan strategies that effectively remove marine litter on beaches.