HOTMIC research questions and objectives

The open ocean represents a major, but yet unconstrained, sink for plastics that enter the environment no matter where on the planet. It is furthermore very likely that plastic-biota interactions are the main driver for the fragmentation, aggregation, and vertical transport of plastic litter from the ocean surface to sedimentary sinks. At the same time the material may also have a detrimental impact on biota health as a result of the interaction. However, the magnitude of the plastic flux as well as the flux rates from the coasts to the open ocean and to the deep sea are very poorly constrained, as is the impact of plastic-biota interactions on transport and ecological health. Therefore, HOTMIC integrates coastal field work, open ocean research cruises, and experimental investigations to build a mechanistic understanding of MP
transport and its biological impact reaching from coastal seas via the accumulation spots in the open ocean gyres to its eventual final sink at the seabed (Fig. 2).

Fig. 2. Schematic of HOTMIC project structure

The major questions that arise here are as follows:
1. What is the lateral flux of MP from Western European coastal waters to the North Atlantic open ocean gyre, and what mechanisms drive this transport?
2. How does ageing, weathering, and alteration of MP along lateral transport pathways affect the composition and character of MP entering the open ocean? JPI Oceans HOTMIC - ID: 87 17 of 44
3. What is the vertical flux of MP from the surface to the deep ocean, and what is the resultant accumulation of MP in deep sea sediments?
4. What is the oceanic distribution of MP in different size classes, especially the numerically dominant smallest size fractions?
5. What is the role of biota in formation, alteration, and transport of MP, through processes such as biofouling, bioshredding, ingestion, and bioturbation?
6. To what extent are marine biota between coastal oceans and the deep sea subject to and affected by ingestion of MP particles?

HOTMIC will characterize MP transport, fate, and impact along a source-to-sink gradient between Western European coastal sources and its eventual accumulation in oceanic deep waters and sediments in the range of the central North Atlantic gyre (Fig. 1). The open ocean
working area is located in a region that has been classified as part of the inner accumulation zone of the North Atlantic garbage patch (Cózar et al. 2014). This classification is based on surface water samplings and we therefore expect considerable amounts of plastic debris
at the sea surface. Particle sinking velocities are expected to be on the order of 100 m d-1 (Villa-Alfageme et al., 2016; Lemaitre et al., 2018), and abundant plastic particles are expected in the water column and seafloor within the proposed area.

We will sample the ocean surface during fieldwork in the putative coastal source sites in the Azores, Madeira, and Ireland, as well as during transits between the source regions and stations within the garbage patch. Trawls will be used to collect samples for MP distribution in both water and meso- and macrofauna. Multiple depths throughout the water column will be sampled during coastal fieldwork and during transits between stations in the garbage patch. Innovative technology for MP collection will be tested using the newly developed Flow2Vortex device by SDU. Particulate MP and dissolved plastic leachate compounds will be sampled throughout the water column within the garbage patch using in situ pumps. Seafloor sediments will be collected in the garbage patch and coastal source regions to evaluate benthic accumulation and uptake by epi- and infauna. Field and laboratory experiments will evaluate MP biofouling, weathering, and alteration at different depths in the water column, and how this may affect ingestion by biota.

Cutting edge techniques for detection and characterization of MP smaller than 10 µm will be refined and employed on field-collected samples, and compared with a suite of spectroscopic and chemical composition analyses to improve and validate methods for analysis
(sampling, sample preparation and detection) of MP in water, sediment, and biota.

WP1 will characterize the oceanic distribution of MP, and will be closely linked to WP2 for understanding timescales, fluxes, and physicochemical mechanisms of transport, weathering and fragmentation. WP3 will examine how marine biota affects and augments these
processes through biofouling, ingestion, and bio-shredding. The MP distribution and concentrations determined in WP1 will provide the environmental context for understanding ingestion and transfer of MP particles determined in WP3. WP4 will address analytical challenges of detecting, characterizing, and quantifying MP, especially particles and fibers < 10 µm in environmental samples provided by WP1 and WP 3. The (further) development, improvement and validation of methods for the analysis of MP will provide a comprehensive and systematic picture on the degree of contamination and distribution of MP particles in the ocean (Fig. 2). This includes sampling and sample preparation, and particle detection, identification and quantification.

Choice of study sites

HOTMIC will examine the plastic transport pathway between sources at Europe’s west coast into the North Atlantic gyre. Some 20% of the global inventory of floating plastic debris is accumulated in the North Atlantic (Cózar et al., 2014; Eriksen et al., 2014). Most of this material is concentrated in the inner accumulation zone of the North Atlantic subtropical gyre (the North Atlantic “garbage patch”), between the Azores and Bermuda (Fig. 1; Cózar et al., 2014). Plastic particles within the North Atlantic gyre comprise a wide range of compositions and sizes, including nanoplastics (ter Halle et al., 2017), and small size particles may comprise the major mass fraction in the region (Poulain et al., 2019). It is not well-known how MP are transported into the North Atlantic gyre, but eddies may be an important mechanism (Brach et
al., 2018).

HOTMIC will focus on the Azores, Madeira, and Ireland as source regions or intermediates for transport to the North Atlantic gyre. Coastal zones around and between the Azores archipelago and Madeira are extensively contaminated with plastic debris (Pieper et al., 2015; Pieper
et al., 2016; Chambault et al., 2018). Due to oceanographic circulation structures, the Azores and Madeira act as a retention zone for plastic debris from Europe (Chambault et al., 2018), and the interface between continental sources and the open ocean accumulation zone. Similar
plastic debris is present in Ireland coastal and continental shelf waters (Lusher et al., 2014; Martin et al., 2017). Ireland is positioned at the Gulf Stream eastern divergence, and provides the link between southward transport into the North Atlantic gyre. The northern branch
carries debris toward high latitudes, corresponding to study regions of linked consortia (see below).

The North Atlantic garbage patch will be targeted during research cruise opportunities aboard F/S Poseidon (scheduled August 2019) and F/S M.S. Merian (cruise not scheduled, but planned Winter/Spring 2021). Coastal sites will be sampled in conjunction with ongoing
monitoring work by project partners IPMA and MARE (Azores and Madeira) and associated collaborator Marine Institute (Ireland; see support letter).