Microplastic pathways in marine pelagic systems

Marine litter and microplastics in particular have for some time now been postulated to be of environmental concern. Despite the ever-increasing amount of microplastic studies, conclusive evidence of the adverse effects of microplastics in the pelagic realm is currently deficient. Specifically, there is a need to look at contamination of key species in the environment and to conduct well-constructed cause and effect studies in order to make tangible conclusions about the potential environmental risks of microplastics. This type of study proves to be particularly challenging for the pelagic realm due to its diversity and dynamic nature. The work presented within this thesis aimed to assess how key ecological players interact with microplastics in the pelagic realm and what potential adverse effects these interactions present to the environment. Study organisms were chosen along two gradients, being organism size (macro to micro) and environmental nutrient saturation (eutrophic to oligotrophic). By doing so I was able to gain important insights into the pathways of microplastics in the pelagic realm. Microplastic contamination of seven species of mesopelagic fishes, sampled from a nutrient rich warm-core eddy in the Northwest Atlantic, was investigated. With 73% of all of 233 assessed fish containing microplastics these fish seem to be particularly prone to microplastic contamination. Internalisation of microplastics could have happened either through direct ingestion from the sea water or through transfer from their planktivorous prey. Similarity in microplastic colour, size and shape to those sampled from the seawater indicates direct ingestion. Nevertheless, through the application of small mesh sizes of microplastic-retaining filters, this is the first study reporting microplastics in the gut contents of these fishes which overlap in size with food targeted by their planktivorous prey. Such high rates of microplastic contamination may have knock-on effects on individual fish health and those of their predators. For the gelatinous filter-feeding salps, which were exposed to microplastics in the laboratory, an effect on an individual level is unlikely as microplastics were simply in- and subsequently egested through incorporation into their faecal pellets. The incorporation of microplastics into salp faecal pellets did, however, slow down the pellets’ sinking speed. High sinking velocities are a pivotal characteristic of this type of particulate organic matter as these allow it to bypass recycling in the upper water column and thus enable carbon sequestration to the ocean floor. Therefore, I show that microplastics may pose a threat to the distribution of organic matter in the water column and the cycling of carbon within the biological pump. Lastly, microplankton communities sampled in the oligotrophic waters of the subtropical South Pacific were exposed to nanoplastic particles and ingestion by microzooplankton was investigated. Through experimental observations and encounter rate calculations it was shown that ingestion by microzooplankton is unlikely. For waters containing higher amounts of organic matter, coagulation of nanoplastics with such matter was demonstrated. This re-packages nanoplastics in a way that allows for their dispersal in the water column and increases their bioavailability. In waters where such matter is not sufficiently present a likely pathway could be filter-feeding ingestion by mucous grazers such as larvaceans and salps