Seasonality of marine plastic abundance in central Red Sea pelagic waters

The Red Sea holds one of the lowest concentrations of floating plastic worldwide and no evident congregation zones were identified so far, despite peculiar oceanographic features that candidate the basin as an accumulation area for floating debris. However, the Red Sea exhibits a complex pattern of surface currents, which changes according to the monsoon season, possibly affecting the abundance of plastic throughout the year. To explore the effect of seasonality on plastic concentration in surface waters, we conducted a fortnightly time series sampling, using a neuston net, for 21 months at a pelagic station in the central Red Sea, where the major seasonal overturn of the Red Sea surface circulation occurs. The estimated average abundance (±SE) was 58,563 ± 19,272 items Km−2 (73.5 ± 40.75 g Km−2), highly variable according to season, lower during the summer monsoon. Indeed, the winter monsoon pushes oceanic surface waters inside the Red Sea, transporting alongside floating plastic items, whilst surface currents exit the basin during the summer monsoon, depleting central Red Sea waters from floating plastic. Composition of plastic items also changes through time. Particularly, the higher proportion of films and foams during summer months suggests that the main source of plastic at the sampling station from June to September is a short-range transport, while during winter months, the higher contribution of small fragments indicates that, from October to May, plastic is also transported to the central Red Sea through surface currents for long distances, possibly coming all the way from the Indian Ocean.