Response of sediment-dwelling bivalves to microplastics and its potential implications for benthic processes

Sediments are considered an ultimate sink for microplastic (MPs) but knowledge on MPs impacts on the benthic organisms is still limited. A microcosm experiment was performed to assess the effect of MPs on two sediment-dwelling bivalve species that differ in terms of their life¬styles and feeding behaviors (the cockle Cerastoderma glaucum and the Baltic clam Limecola balthica). Fitness of both bivalves, their vertical distribution in the sediment and activity at the sediment-water interface were studied following addition of PE microspheres of three different size fractions (63-75, 150-180 and 250-300 μm) in two concentrations (0.1 and 0.5% sediment dwt) to the sediment surface. Bivalve survival was generally high and did not vary significantly among treatments. Both bivalve species captured MPs but no significant effect on their body conditions and energy reserves was observed. However, some behavioural alterations were found. The near-surface-dwelling C. glaucum emerged from the sediment less often and in lower numbers suggesting lower activity in sediments amended with large and medium MPs served in higher concentration. Also vertical distribution of deeper-dwelling L. balthica differed among treatments. The clam penetrated deeper sediment layers in microcosms treated with MPs than in the control (on average 62% of the total number of bivalves found in the sediment layer 2-4 cm vs. 65% in the upper 2 cm) which may suggest an avoidance behaviour or escape reaction in response to MPs addition to the topmost sediment. Total sediment community oxygen consumption was significantly higher in MPs-amended sediments than in the control, with the highest rates in treatment with the largest microspheres. However, oxygen uptake increase seemed to be unrelated to addition of MPs per se, suggesting rather an interaction between MPs and bivalves. The need for future studies on MPs effects on benthic community structure and functioning is highlighted.