A research team from the University of Cádiz has combined satellite imagery and mathematical models to identify the origin and causes of the formation of floating debris strands in the northwestern Mediterranean basin. This technology allows for the reconstruction of a detailed timeline of the process and shows how extreme weather events, primarily torrential rains, can inject large quantities of trash into the marine environment. During the three months analyzed, the experts calculated the entry of 50 tons of waste into the sea, the vast majority concentrated in a single influx event lasting just three days.

The University of Cádiz research group ‘Structure and Dynamics of Aquatic Ecosystems’ has designed a system that establishes a timeline to detect and monitor debris in the Mediterranean. To do this, they applied a technique based on satellite imagery and mathematical models. The methodology analyzes how and when extreme weather events, mainly torrential rains, introduce large quantities of trash into the sea, which then accumulate in strands known as debris strands.

Specifically, the study shows how the entry of plastic into the sea is strongly regulated by heavy rainfall events and rising river levels. In figures, experts detected the entry of 50 tons of trash over 90 days, thus confirming that the Mediterranean is a major global accumulation zone for plastic waste, as they had already demonstrated in a previous study where they created the first global map of floating plastic debris accumulation.

The improvement of this predictive tool, currently in development testing, will allow for anticipating the arrival of waste on beaches several days after a storm is detected, facilitating cleanup efforts and enabling the implementation of environmental containment measures. According to the results of this study, titled ‘Storyline of an event of marine litter input and windrow formation in the Northwestern Mediterranean Sea’ and published in the journal Marine Pollution Bulletin, marine litter trails are filaments several meters wide that can reach more than 20 kilometers in length, accumulating plastics, plant debris, pollen, and even marine mucilage, a type of persistent foam formed in nutrient-saturated waters as a result of warm weather and water pollution.

All this marine litter enters the sea in pulses, that is, intermittently. Furthermore, these pulses that inject trash into the sea are recorded after extreme weather events, such as heavy rainfall.

Specifically, the study shows that most of the litter enters the sea during the so-called first flash—the initial surge following intense rainfall—in just a few hours or days. “Short rivers and urban areas respond almost immediately, while pulses of trash discharge from large rivers like the Rhône are delayed by two to three days relative to the peak rainfall,” explains Andrés Cózar, a researcher at the University of Cádiz and author of the study, to the Descubre Foundation.

Oceanographic ‘Storyline’

To reach these conclusions, the researchers analyzed different variables each day for three consecutive months during a specific event that occurred in the northwestern Mediterranean.
To do this, they applied, for the first time in oceanography, the methodology known as storyline, used in climatology as a key tool for making extreme weather events more tangible and understandable. This approach, applied to oceanography and, in this specific case, to the monitoring of marine litter accumulations, facilitates the reconstruction of the complete storyline of an extreme event, from the formation of storms to the final dispersal of the trash in the open sea.

During the study, the experts characterized the climatological conditions of the area from which the storm entered. They also observed the effects of the rain, how the rivers responded to the currents, and continued to monitor the spread of the trash. “The use of satellites offers a view of the ocean that is not possible from a ship. This allowed us to verify that the waste dispersed and reached up to 300 kilometers from the coast, and that days later, some of it washed back onto the beaches,” explains the UCA researcher.

Specifically, during the 90-day study, the experts estimated that 50 tons of trash entered the sea from land. “Of this amount, 26 tons, 52% of the total, was floating material. Of that, approximately 32% would eventually sink, while 68% would reach the beaches. The remaining 24 tons were deposited directly on the seabed near the coast,” Cózar quantifies.

The objective of this new approach is to verify the correlation between climatic, hydrological, and oceanographic phenomena in order to prevent irreversible environmental consequences. “Garbage patches are structures formed on the ocean surface that can accumulate plastics, plant debris, pieces of reeds, sludge deposits, pollen, and even marine mucilage, also called marine snot, a substance that often forms large amounts of foam and is a consequence of massive algal blooms, increased water temperature, and pollution,” Cózar explains.

Every two years

Regarding the frequency of these extreme pulses of land-to-sea debris flows, experts have confirmed that events of this magnitude occur, on average, every two to three years in the Mediterranean. “However, a single high-intensity episode can significantly alter the observed accumulation patterns.”

In this sense, the experts have reaffirmed the classification of the Mediterranean as the sixth major plastic accumulation zone, already identified in previous research conducted by this research group.

Along with these findings, the researchers have also verified that the rivulets can extend for tens of kilometers at sea. “Although their width is usually around 10 meters, their length can be extraordinary: the documented record in the Mediterranean exceeds 20 kilometers, which is equivalent to the length of the city of Málaga,” Cózar points out.

Future steps: towards a predictive system

According to the authors in their publication, this work is funded by the Regional Ministry of Universities, Research and Innovation of the Andalusian Regional Government and the European Space Agency and is currently in the experimental field phase.

The next step the experts are working on focuses on developing a system that globally automates the downloading and analysis of images, as well as the ability to generate early warnings and provide real-time information to government agencies. “We intend for this methodology to be able to anticipate, several days after a storm is detected, the arrival of trash on specific beaches, facilitating cleanup and mitigation efforts,” Cózar concludes.

In parallel, this research group is working on recovering and obtaining new hydrological and ecological data from the now-dried La Janda lagoon, which was located between the municipalities of Vejer de la Frontera, Benalup-Casas Viejas, and Tarifa.