A study recently published in the Marine Pollution journal has underscored a previously-overlooked aspect of seagrass meadows – i.e. the role they play in the trapping and distribution of litter within coastal waters and in the exchange of the same litter between land and sea areas. 

 

Six different coastal sites featuring Posidonia oceanica seagrass meadows in Spain and Malta were sampled during summer and autumn 2019, prior and just after heavy rainfall episodes, in order to assess the impact of coastal runoff in shuttling litter to the same meadows. The study focused on non-floating litter, i.e. litter items which sink to the seabed due to their density and which are then buried within the sediment, including microplastics (particles which are smaller than 5mm), cigarette butts, plastic caps, nylon and other litter items.

 

Results demonstrate the ability of seagrass meadows to trap land-sourced litter, with the landside edge just outside the same meadows being the main area of macro-litter accumulation and to block most litter originating in deeper waters from being washed back up on beaches. Areas away from the edge, both inside and outside the meadow, recorded lower litter-concentration values than the meadow edge. Storm waters trigger the transport of large loads of mismanaged litter from land to the sea, part of which has been shown to be retained by subtidal seagrass beds.

The outer margin of the meadows predominantly trapped easily-transportable litter, mainly plastic items, including bags and packaging, sanitary wipes or fragments of these items, whilst less mobile macro-litter (i.e. large fragments of plastic) as well as microplastics accumulated mainly along the inner margin. On average, macro-litter concentrations increased 3-fold after heavy rainfall. The trapping and retention of non-floating litter by coastal seagrass meadows facilitates the regular landward-seaward transport of the same litter and its fragmentation and breakdown before it is buried or transferred to deeper areas.

The study featured resident academic Prof. Alan Deidun, from the Oceanography Research Group within the Department of Geosciences of the Faculty of Science and was made possible through a collaboration between the University of Malta and the University of Cadiz through the SEA-EU (University of the Sea) network.