Ingestion and effects of microplastics on marine planktonic food webs

Plastic is one of the most persistent pollutant in the environment. The current high production of plastic and poor management plastic waste facilitate its entry in the marine environment. The degradation of plastic involves the breakdown of big debris into small pieces that eventually can reach sizes smaller than 5 mm. Those plastic particles are defined as microplastics (MPs). MPs can also be directly manufactured for different purposes in sizes smaller than 5 mm. Independently of their origin, MPs enter the marine environment by different routes, e.g. atmospheric deposition, sewage systems and run-off. MPs have become ubiquitous in marine environments arising global concern about their impacts on the ecosystems.

The size of MPs overlaps with the natural prey of a wide range of marine organisms and consequently pose a potential risk of MP ingestion. Size similarity of small MPs < 100 μm and phytoplankton, makes zooplankton a particularly vulnerable group to MP ingestion. Despite the increasing knowledge about the ingestion and impact of MPs on zooplankton, there is still lack of environmental realistic conditions in the laboratory exposure experiments. The mechanisms behind the interactions between MPs and marine organisms is also understudied. Copepods are the most abundant metazoan in the oceans. They are a key group in the marine food webs as grazers on primary producers and as prey for many economical and environmentally relevant organisms, such as fish larvae. Copepods are also essential part of the biological carbon pump with the transportation of organic matter through fecal pellet production. With this thesis, I provide an overview of the ingestion and impact of MPs on copepods. Furthermore, I elucidate some mechanism behind the copepods-MPs interactions considering realistic MP characteristics, applying small-scale video observations and a trait base approach. In addition, with a literature review, I evaluate the trends in MP research in marine food webs.

Quantification of small size fraction of MPs in the marine environment is challenging due to lack of standardized methodologies and difficulties in comparing data. Besides, some areas are understudied, such as Arctic, Antarctic and Indian Ocean. Therefore, there is still a long way to have a complete overview of the MP concentration and distribution in the oceans. This thesis provides data on the MP concentration and composition for arctic surface waters, with an average of 142 MPs m^-3, when sampling with a free-plastic pump system. Given the presence of MPs in the arctic ecosystems, I evaluated the impact of virgin MPs on the most abundant copepods in the Arctic. Our studies show ingestion of MPs (20 μm) in all the studied species, but not impact, at environmental realistic concentrations. However, when co-exposed to crude oil, the MPs triggered feeding suppression in Calanus hyperboreus.

Microplastics in the environment present a high variety of characteristics that can affect their bioavailability and their potential ingestion by planktonic copepods. Using traditional laboratory incubations and small-scale video observations I demonstrated the ability of copepods to select their prey and reject 80% of the encountered MPs, independently of the MP characteristics. Copepods have different feeding behaviors, which allow them to target the prey that provide them with higher energy uptake. This Ph. D. thesis is the first, to our knowledge, evaluating the response of copepods to MPs based on their different feeding strategies. The worldwide distribution of copepods and the high ingestion of MPs by these organisms in previous laboratory studies, led us to consider copepods a potential main entry of MPs in marine food webs. However, in this project, a minimal ingestion of MPs was observed independently of the copepod´s feeding behavior and microplastic characteristics. In consequence, I did not identified globally, areas with high risk of entry of MPs into the marine food webs via copepods.

Microplastics are pollutants of high concern in the marine environment. But, based on the findings from this thesis, planktonic copepods have a low risk of MP ingestion even at concentrations above those found in the environment. Hence, planktonic copepods do not play an important role for the entry of MPs in marine food webs.