Global marine litter research 2015-2020: Geographical and methodological trends.

A systematic review of research on marine macrolitter densities in the past five years (2015-2020) revealed considerable knowledge gaps in the field. Nearly half he reviewed studies were on stranded litter. Data are scarce from many of the regions estimated to mismanage the largest amounts of plastic waste. In regions where data are available these are typically from coastal areas with few data from the high and deep seas; 57% and 87% of studies on pelagic and seafloor litter, respectively, took place within 100 km from shore. Data on pelagic litter are generally constrained to the sea surface and only two of 30 pelagic studies have measured macrolitter deeper in the water column. Reported litter densities are generally highest for stranded litter, although seafloor litter densities by weight are high in some areas. Reported densities of floating litter are several orders of magnitude lower. However, a lack of standardisation of methods makes it difficult both to assess and to compare litter densities within and across the different environmental compartments in time and space. The review illustrates a great need for survey design development within the field of macroplastics and point to some long-established considerations from ecological research pertaining to independence of data points, spatial autocorrelation, sampling scale, and plot size and shape which are highly relevant also for marine litter research. These considerations are relevant both for global standardisation efforts and for independent studies. Furthermore, the knowledge gaps created by geographic and compartment biases in research needs to be addressed to identify further research needs, validate models and inform policy.

Spatial trends and drivers of marine debris accumulation on shorelines in South Eleuthera, The Bahamas using citizen science.

This study measured spatial distribution of marine debris stranded on beaches in South Eleuthera, The Bahamas. Citizen science, fetch modeling, relative exposure index and predictive mapping were used to determine marine debris source and abundance. Citizen scientists quantified debris type and abundance on 16 beaches within three coastal exposures (The Atlantic Ocean, Great Bahama Bank and The Exuma Sound) in South Eleuthera. Marine debris, (~2.5 cm or larger) on each beach was monitored twice between March-May and September-November 2013 at the same locations using GPS. Approximately, 93% of all debris items were plastic with plastic fragments (≤2.5 cm) being the most common. There were spatial differences (p ≤ 0.0001) in plastic debris abundance between coastal exposures. Atlantic Ocean beaches had larger quantities of plastic debris by weight and by meter (m) of shoreline. Stranded plastic may be associated with Atlantic Ocean currents associated with leakage from the North Atlantic sub-tropical gyre.

A novel GIS-based tool for predicting coastal litter accumulation and optimising coastal cleanup actions.

Effective site selection is a key component of maximising debris removal during coastal cleanup actions. We tested a GIS-based predictive model to identify marine litter hotspots in Lofoten, Norway based on shoreline gradient and shape. Litter density was recorded at 27 randomly selected locations with 5 transects sampled in each. Shoreline gradient was a limiting factor to litter accumulation when >35%. The curvature of the coastline correlated differently with litter density at different spatial scales. The greatest litter concentrations were in small coves located on larger headlands. A parsimonious model scoring sites on a scale of 1-5 based on shoreline slope and shape had the highest validation success. Sites unlikely to have high litter concentrations were successfully identified and could be avoided. The accuracy of hotspot identifications was more variable, and presumably more parameters influencing litter deposition, such as shoreline aspect relative to prevailing winds, should be incorporated.

Carbon dynamics within cyclonic eddies: insights from a biomarker study.

It is generally assumed that episodic nutrient pulses by cyclonic eddies into surface waters support a significant fraction of the primary production in subtropical low-nutrient environments in the northern hemisphere. However, contradictory results related to the influence of eddies on particulate organic carbon (POC) export have been reported. As a step toward understanding the complex mechanisms that control export of material within eddies, we present here results from a sediment trap mooring deployed within the path of cyclonic eddies generated near the Canary Islands over a 1.5-year period. We find that, during summer and autumn (when surface stratification is stronger, eddies are more intense, and a relative enrichment in CaCO3 forming organisms occurs), POC export to the deep ocean was 2-4 times higher than observed for the rest of the year. On the contrary, during winter and spring (when mixing is strongest and the seasonal phytoplankton bloom occurs), no significant enhancement of POC export associated with eddies was observed. Our biomarker results suggest that a large fraction of the material exported from surface waters during the late-winter bloom is either recycled in the mesopelagic zone or bypassed by migrant zooplankton to the deep scattering layer, where it would disaggregate to smaller particles or be excreted as dissolved organic carbon. Cyclonic eddies, however, would enhance carbon export below 1000 m depth during the summer stratification period, when eddies are more intense and frequent, highlighting the important role of eddies and their different biological communities on the regional carbon cycle.

Flushing submarine canyons.

The continental slope is a steep, narrow fringe separating the coastal zone from the deep ocean. During low sea-level stands, slides and dense, sediment-laden flows erode the outer continental shelf and the continental slope, leading to the formation of submarine canyons that funnel large volumes of sediment and organic matter from shallow regions to the deep ocean(1). During high sea-level stands, such as at present, these canyons still experience occasional sediment gravity flows(2-5), which are usually thought to be triggered by sediment failure or river flooding. Here we present observations from a submarine canyon on the Gulf of Lions margin, in the northwest Mediterranean Sea, that demonstrate that these flows can also be triggered by dense shelf water cascading (DSWC)-a type of current that is driven solely by seawater density contrast. Our results show that DSWC can transport large amounts of water and sediment, reshape submarine canyon floors and rapidly affect the deep-sea environment. This cascading is seasonal, resulting from the formation of dense water by cooling and/or evaporation, and occurs on both high- and low-latitude continental margins(6-8). DSWC may therefore transport large amounts of sediment and organic matter to the deep ocean. Furthermore, changes in the frequency and intensity of DSWC driven by future climate change may have a significant impact on the supply of organic matter to deep-sea ecosystems and on the amount of carbon stored on continental margins and in ocean basins.