Microplastics are abundant and widespread in the marine environment. They are a contaminant
of global environmental and economic concern. Due to their small size a wide range
of marine species, including zooplankton can ingest them. Research has shown that
microplastics are readily ingested by several zooplankton taxa, with associated negative
impacts on biological processes. Zooplankton is a crucial food source for many secondary
consumers, consequently this represents a route whereby microplastic could enter the
food web and transfer up the trophic levels. In this review we aim to: 1) evaluate
the current knowledge base regarding microplastic ingestion by zooplankton in both
the laboratory and the field; and 2) summarise the factors which contribute to the
bioavailability of microplastics to zooplankton. Current literature shows that microplastic
ingestion has been recorded in 39 zooplankton species from 28 taxonomic orders including
holo- and meroplanktonic species. The majority of studies occurred under laboratory
conditions and negative effects were reported in ten studies (45%) demonstrating effects
on feeding behaviour, growth, development, reproduction and lifespan. In contrast,
three studies (14%) reported no negative effects from microplastic ingestion. Several
physical and biological factors can influence the bioavailability of microplastics
to zooplankton, such as size, shape, age and abundance. We identified that microplastics
used in experiments are often different to those quantified in the marine environment,
particularly in terms of concentration, shape, type and age. We therefore suggest
that future research should include microplastics that are more representative of
those found in the marine environment at relevant concentrations. Additionally, investigating
the effects of microplastic ingestion on a broader range of zooplankton species and
life stages, will help to answer key knowledge gaps regarding the effect of microplastic
on recruitment, species populations and ultimately broader economic consequences such
as impacts on shell- and finfish stocks.