The effect of interspecific and intraspecific diversity on microplastic ingestion in two co-occurring mussel species in South Africa.

Interspecific and intraspecific diversity are essential components of biodiversity with far-reaching implications for ecosystem function and service provision. Importantly, genotypic and phenotypic variation within a species can affect responses to anthropogenic pressures more than interspecific diversity. We investigated the effects of interspecific and intraspecific diversity on microplastic ingestion by two coexisting mussel species in South Africa, Mytilus galloprovincialis and Perna perna, the latter occurring as two genetic lineages. We found significantly higher microplastic abundance in M. galloprovincialis (0.54 ± 0.56 MP items g-1WW) than P. perna (0.16 ± 0.21 MP items g-1WW), but no difference between P. perna lineages. Microbeads were the predominant microplastic (76 % in P. perna, 99 % in M. galloprovincialis) and polyethylene the prevalent polymer. Interspecific differences in microplastic abundance varied across locations, suggesting diverse sources of contamination. We suggest that microplastic ingestion can be species-specific even in organisms that coexist and play similar functional roles within ecosystems.

The relative effects of interspecific and intraspecific diversity on microplastic trapping in coastal biogenic habitats.

Our understanding of how anthropogenic stressors such as climate change and plastic pollution interact with biodiversity is being widened to include diversity below the species level, i.e., intraspecific variation. The emerging appreciation of the key ecological importance of intraspecific diversity and its potential loss in the Anthropocene, further highlights the need to assess the relative importance of intraspecific versus interspecific diversity. One such issue is whether a species responds as a homogenous whole to plastic pollution. Using manipulative field transplant experiments and laboratory-controlled hydrodynamic simulations, we assessed the relative effects of intraspecific and interspecific diversity on microplastic trapping in coastal biogenic habitats dominated by two key bioengineers, the brown intertidal macroalgae Fucus vesiculosus and F. guiryi. At the individual level, northern morphotypes of F. guiryi trapped more microplastics than southern individuals, and F. vesiculosus trapped more microplastics than F. guiryi. Canopy density varied among species, however, leading to reversed patterns of microplastic accumulation, with F. guiryi canopies accumulating more microplastics than those of F. vesiculosus, while no differences were observed between the canopies of F. guiryi morphotypes. We emphasize the importance of assessing the effects of intraspecific variation which, along with other crucial factors such as canopy density, flow velocity and polymer composition, modulates the extent of microplastic accumulation in coastal biogenic habitats. Our findings indicate that a realistic estimation of plastic accumulation in biogenic habitats requires an understanding of within- and between-species traits at both the individual and population levels.

Species-specific plastic accumulation in the sediment and canopy of coastal vegetated habitats.

Plastic waste has become ubiquitous in ecosystems worldwide. Few, recent studies report evidence of coastal vegetated habitats acting as sink for plastics, yet assessments have been completed either for macro or microplastics and focussing on just one type of vegetated habitat. Here, we investigated the role of marine coastal vegetated habitats as sinks for macro (≥5 mm) and microplastics (<5 mm) through a comprehensive, multi-habitat approach. We assessed the occurrence, abundance and physical properties of macro and microplastics in the canopy and superficial sediment of two intertidal (seagrass Zostera noltei, saltmarsh Sporobolus maritimus) and two subtidal (mixed seagrass meadows of Cymodocea nodosa and Zostera marina, rhizophytic macroalga Caulerpa prolifera) habitats in the Ria Formosa lagoon (Portugal). Our results showed that coastal vegetated habitats trapped macro and microplastics in the sediment at variable degrees (1.3-17.3 macroplastics 100 m-2, and 18.2-35.2 microplastics kg-1). Macroplastics accumulated in all vegetated habitat but not in nearby unvegetated areas, yet only S. maritimus habitat presented a significant trapping effect. Microplastics occurred in the sediment of all vegetated and unvegetated areas with similar abundances and high variability. Microplastics, all of type fibre, were recorded on all canopies except for S. maritimus. Overall, the trapping capacity of microplastics in the sediment and on the canopy was higher for subtidal than for intertidal vegetated habitats. We conclude that generalizations in the trapping effect of coastal vegetated areas should be done with caution, since it may be highly variable and may depend on the plastic size, habitat and tidal position. Since these habitats support a high biodiversity, they should be included in assessments of plastic debris accumulation and impacts in coastal areas. Further research, including experimental studies, is needed to shed more light on the role of coastal vegetated habitats as plastic sinks.