Manon Harrewynn (MSc Thesis 2020)
Evaluation of the distribution and abundance of floating marine litter in the Gulf of Cádiz (SW Spain).
|Supervisor: Jesus Gago Piñeiro (IEO, Vigo)
|The Gulf of Cádiz presents such peculiar characteristics, concerning its location and hydrodynamics, that it became the perfect study area for researchers. In between the Atlantic Ocean and theMediterranean Sea, influenced by surface currents coming from the Azores and rivers plumes, the Gulfof Cádiz allows scientists to better understand the exchanges between two big water masses.ECOCADIZ campaign was launched in 2004 primarily, to map the neritic pelagic resources andoceanographic / biological conditions of the continental shelf of the Gulf of Cádiz. Since 2018, otherstudies such as the abundance assessment and distribution of microplastics and floating marine litterjoined the campaign to tackle the problem of these contaminant’s accumulation and acknowledge theirtrajectories. Microplastics and floating marine litters were therefore studied to follow the requirementsof the Marine Strategy Framework Directive, which is asking for an initial evaluation of littercharacteristics in coastal and oceanic waters, as well as monitoring its trend and accumulation pattern.However, this report represents the first attempt to estimate floating marine litter distribution andabundances, related to ocean dynamics in the southern Spanish and Portuguese coastal waters. Litterobservations were done in August 2018 and 2019, representing summer characteristics, all along thecontinental shelf of the Gulf. The calculated densities were not in the same order of magnitude than theones previously measured in the Mediterranean Sea, characterized as highly contaminated. However,important variations were observed at a local scale, leading to a hypothesis on the influence of thehydrodynamics of the Gulf on the spatial distribution of floating items. Divided into three sectors for adeeper comprehension of the results, the studied area showed significant differences of densitiesconcerning the type of litter (Anthropogenic Marine Debris had a density comprise from 0.16 items/km2to 1.09 items/km2 in the most polluted sector while Natural Marine Debris were from 0.03 items/km2 to0.07 items.km2 over the two years). Sector 2 was characteristic of an accumulation zone while Sector 3was the least polluted of the study area. Factors such as river input, geophysical characteristics,anthropogenic activities and distance from the coast were suggested to explain such spatial distributionand abundances. According to statistical analysis of the dataset, floating marine litters mainly originatedfrom the coast, with a majority of them being anthropogenic litter and, more specifically, plastics(61.9%). Hypothesis were made concerning the trajectory of these debris, suggesting they couldinfluence the high concentrations found inside the Mediterranean Sea. However, sampling effort andcomplementary analyses are still needed to explain floating marine litter dynamic in the Gulf of Cádiz.Beaches and estuaries are strategic (and low coast/effort) sampling sites to target in future anthropogenic and natural macro-contamination. Specific treatment and collection effort could therefore be proposedto efficiently manage the coastline suffering from litter accumulation, and eventually, the influence ofthe Gulf of Cádiz in the input of litter into the Mediterranean Sea could be verified.