Coastal lagoons are transitional environments between inland and marine aquatic systems. They are of great ecological and economic importance, and provide valuable ecosystem services, such as generating food for humans and other animals. However, they are fragile environments and are subject to pressure from several different human activities, including pollution.
In a recent review, published in the journal Environmental Pollution, experts from Spain, Colombia and Chile identified and summarized the advances in research on microplastic (MP) pollution in coastal lagoons across the world. They searched peer−reviewed publications on this topic, published in English and Spanish, between the years of 2000 and 2022. In total, 57 relevant publications were found, with data on MP abundances and their characteristics in 50 coastal lagoons from 20 countries on different continents, including Europe (32 percent), Asia (20 percent), Latin America and the Caribbean (18 percent), Africa (12 percent), North America (10 percent), and Oceania (8 percent).
Although 58 percent of the lagoons studied have some form of environmental protection status (e.g., Ramsar Convention on wetlands of international importance, biosphere reserves, areas of importance for wildlife conservation), none of them was immune to contamination from microplastics. Fiber/filament and fragment shapes, and polyethylene, polyester, and polypropylene polymers were the predominant microplastic pollutants present.
“In these natural habitats, peak levels of microplastic pollution result from a combination of several factors. For example, the residence time and water renewal rate of lagoons, the nearby location of large urban and industrial developments with insufficient waste management, river and outfall discharges, seasonal climate fluctuations, natural phenomena (hurricanes, typhoons, etc.) and the typology of microplastics,” says study first author Ostin Garcés-Ordóñez, a member of the Consolidated Research Group in Marine Geosciences.
Coastal lagoons with slow, or very slow, water turnover rates are the most susceptible to high microplastic pollution. In the case of the Mosquito Lagoon in Florida (USA), it takes 200-300 days to renew 50 percent of the water, and therefore microplastics entering this lagoon can remain in this ecosystem for considerable periods of time.
“During the rainy season, the concentration of microplastics also increases in surface water compared to the dry season, a phenomenon that has been observed in the lagoon of Rio Lagarto (Mexico) and in the Ciénaga Grande de Santa Marta (Colombia), with maximum levels in areas where the most intense human activities are recorded,” says Garcés-Ordóñez.
Globally, the coastal lagoons most affected by MP pollution are found in Lagos (Nigeria), Sakumo (Ghana) and Bizerte (Tunisia), in close proximity to large urban centers that have no waste or sewage treatment systems. However, the study also found that the highest concentrations of microplastics have been detected in Barnes Sound and other small lagoons in a protected area in the north of Florida Bay (USA); in this case, they were transported there by hurricanes, from polluted areas further afield.
The study also reviewed the impact of microplastics on 96 different species of fish and found a maximum impact on fish that inhabit the coastal lagoons of Bizerte and Ghar El Melh in Tunisia. Fish are of great commercial interest and are also one of the best-studied groups of organisms when it comes to microplastic impacts. The study found that the species Liza aurata (golden grey mullet) and Sarpa salpa (dreamfish or salema) had intakes of up to 65 microplastics per individual. Mollusks, are also seriously affected, with levels of up to 17 microplastics per individual. The mollusks with the highest MP loads were Eastern oysters (Crassostrea virginica) found in the Mosquito Lagoon (USA).
Regarding the Iberian Peninsula, coastal lagoons are relatively few in number and small in comparison with other coastal lagoon systems in the world. Most of them are associated with the lower courses of rivers and their former mouths, as well as with the coastal drift of sediments, as is the case of the marshes of Empordà, the Ebro delta, the Valencia lagoon or the lagoon system of the Portuguese Algarve.
“The environmental state of these lagoons is variable and changes over time. Some suffer the effects of pollution from urban, industrial and agricultural sources, such as the Valencia lagoon and the Mar Menor in Murcia, where there have recently been massive deaths of fish due to episodes of anoxia,” notes Professor Miquel Canals, director of the UB Department of Earth and Ocean Dynamics and a leading member of the research team.
“The Mar Menor, which is the largest coastal lagoon in Spain, has unique characteristics among the Iberian coastal lagoons, given the high salinity of its waters as a result of its permanent communication with the open sea and the presence of several volcanic islets. Episodes of drought can have notable effects on some of these lake systems, as is already happening in the marshes of the Empordà or in Doñana.’
The study reveals the presence of microplastics in the water column, sediments and fish in the coastal lagoons of the Mar Menor (Murcia), the Pletera (Catalonia) and in Ria Formosa and Aveiro (Portugal). “In Spanish lagoons, an average of 20.1 ±2.9 microplastics per kilogram of the digestive tract has been found in sea bream (Sparus aurata) in the Mar Menor, and an abundance of up to 2.5 microplastics per individual in the eastern mosquitofish (Gambusia holbrooki) in the restored Pletera lagoon in Girona”, says Canals.
The review highlighted the fact that microplastics can end up being accumulated in certain areas of coastal lagoons, or they can be exported out to the sea, depending on the influence of seasonal weather, hydrodynamics, anthropogenic pressures, and the types of MPs involved.
“We can certainly say that the different environmental matrices of coastal lagoons –i.e., waters, sediments and organisms living in these habitats – are receptors of microplastics and are affected by the pollution they cause. However, we do not yet know to what extent coastal lagoons are ultimate sinks for microplastics,” says Canals.
“This is related to the characteristics and dynamics of each lagoon, as well as to the quantities and properties of incoming microplastics. Thus, some lagoons may be true sinks while others may function as temporary sinks in the microplastic cycle, easing, for example, their transfer from river systems to beaches and the marine environment. Such criteria could be used to classify coastal lagoons according to the balance between inward and, eventually, outward fluxes of microplastics.
The scientists found it challenging to combine data from all the different published studies on coastal lagoons all over the world. This was due to the fact that the research reported in these studies made use of a variety of different methods to assess the presence and impacts of microplastics.
“In many parts of the world, research is limited by the limited availability of financial, human and infrastructure resources (e.g. Raman or Fourier-Transform infrared spectrometers to characterize the composition of plastic polymers),” says Garcés-Ordóñez. “Extensively applying both simple and more advanced methodologies will answer many open questions about the dynamics of microplastics in these environments or the role of lagoons as temporary or permanent microplastic sinks,” he adds.
Ideally, the scientists would like to see preventative measures that stop microplastics from entering the lagoon systems in the first place. As Miquel Canals notes, “the best way to tackle and minimize the problem of microplastic pollution in general, and in coastal lagoons specifically, is to stop it from entering natural systems from the roots. It is necessary to act on the sources and the causes that favor their arrival in coastal lagoons and the rest of the ecosystems.”
Achieving this goal would require a series of combined actions at global, regional, national and local levels. Initially there would have to be an adequate regulatory framework, and then this would have to be implemented effectively. Actions would need to be taken to minimize plastic waste generation at all levels, for example by proper management of waste and wastewater discharged into aquatic ecosystems, and by instituting strategies to eliminate the accumulation of large waste in the environment.
“It is obvious that these examples are only some of the existing options, which should also be accompanied by environmental education and monitoring programs in order to be able to assess their effectiveness objectively. Other options, such as the recovery of microplastics from the natural environment, are not, for the moment, viable,” concludes Miquel Canals.
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