Does warming change the effect of microplastic on marine benthic invertebrates

Coastal areas are subject to multiple stressors generated by human activities on land and at sea. Combined effects of multiple stressors on marine biota may be very complex and cannot easily be predicted from individual stressor effects. Among the anthropogenic threats to the coastal environment, accumulation of plastic waste in the ocean has recently caught public attention. Small plastic particles(< 5 mm), so-called microplastics, can adversely affect the physiology of marine inve1iebrates. Filter-feeding organisms in particular may be impaired because they ingest microplastics while feeding. Ocean warming due to ongoing greenhouse gas emissions constitutes another known stressor for marine ecosystems. As warming has a strong influence on metabolic rates in ectothermic invetiebrates, we hypothesized that the effect size of a response induced by the microplastic patiicles changes with increasing temperature. We exposed the Azorean barnacle Megabalanus azoricus from Madeira Island to an 01ihogonal combination of three temperature levels (22°C, 25°C, 28°C) and four microplastic concentrations (0 mg 1-1, 2 mg 1-1, 20 mg 1-1, 200 mg 1-1) for 69 days. Moulting frequency, cirral activity, respiration rates and m01iality served as response variables to assess individual and combined effects of temperature and microplastic. Our study demonstrated that M azoricus ingests as well as excretes microplastic particles. Warming significantly increased motiality and moulting frequency and induced changes in cirral beating behaviour of the barnacles. Microplastic alone did not have an effect on any of the measured response variables. Contrary to expectations, we found the highest survival rate in the group of the highest microplastic concentration, whereas the group without microplastic featured the lowest survival rate. Furthermore, we found a significant interaction of microplastic and warming with regard to extended cirral beating behaviour. Repeated measures of respiration rate and total cirral beating rate indicated a strong effect of time kept in the laboratory on the barnacles. Detailed analysis of our results revealed that the dominant stressor driving the magnitude of all observed effects was presumably food availability. Insufficient food supply over the course of the experiment may have led to energy depletion, which induced a considerable decrease of respiration rates and total cirral beating rates. Elevated water temperature, the second most influential stressor in the experiment, increased metabolic rates and by association moulting frequency of the barnacles. High metabolic rates may have forced the animals to consume their energy reserves even faster, which presumably caused increased mortality at higher temperature. Since we did not find individual effects of microplastic on M azor;cus, we were unfo1tunately not able to test our core hypothesis. Nevertheless, we found evidence that high concentrations of microplastic in combination with elevated temperature may elicit changes in metabolic activity of M azo,•;cus either caused by pseudo-satiation or in the wake of a protective mechanism. Our results would seem to suggest that ocean warming poses a larger risk to M azoricus than microplastic particles. Moreover, as adult M azoricus appeared to be rather unaffected by microplastic, the present study also highlights the need to investigate whether effects of microplastics on organisms differ from effects of naturally occurring non-nutritious particles in seston.