Decomposition of Sargassum detritus varies with exposure to different plastic types.

Plastic pollution and ocean warming threaten crucial ecosystem processes, including detrital decomposition. We carried out a manipulative experiment using 20 outdoor raceways to test hypotheses about the influence of macroplastics (polyvinyl chloride (PVC), polyethylene terephthalate (PET), high-density polyethylene (HDPE), low-density polyethylene (LDPE), and biodegradable (BIO)) and ocean warming (as 3 °C above ambient sea surface temperatures) on the decomposition of Sargassum vestitum. All types of plastic significantly decreased rates of S. vestitum decomposition compared to controls. LDPE was associated with the greatest decrease in detrital decomposition (41%), followed closely by BIO (28%), whilst HDPE had the least influence (12%) during our 40-day experiment. Treatments with LDPE and PET retained more carbon (%) in S. vestitum than the control treatment. However, plastics neither affected nitrogen (%), nor C/N ratio of the decomposing detritus. Ocean warming significantly increased the decomposition of S. vestitum, but did not affect relative carbon or nitrogen, nor C/N of the remaining detritus, nor did temperature interact with plastic treatments. As detrital decomposition significantly contributes to marine biogeochemical cycling, food-web connectivity, and secondary production, our multiple stressor experiment demonstrates the value of management strategies that simultaneously address the impacts of ocean warming and plastic pollution in nearshore environments.