The effect of low-density polyethylene on the decomposition of seagrass and kelp detritus

Marine plastic pollution threatens some coastal ecosystems, as it can negatively impact ecosystem processes, such as the decomposition of macrophyte detritus. While mesocosm studies have shown how plastic pollution slows the decomposition of marine macrophytes, this has yet to be evaluated in a field situation. To address this, we conducted a litter bag experiment near Coffs Harbour (Australia) to investigate the impact of low-density polyethylene (LDPE) bags on the decomposition of detritus from a dominant seagrass (Zostera muelleri) and kelp (Ecklonia radiata) species. Exposure to 0.8 g of LDPE mixed with 10.0 g of detritus reduced E. radiata decomposition by 92.2 % (P < 0.05) but did not significantly affect the decomposition of Z. muelleri. Carbon release was increased by 14.9 % and 23.3 % in E. radiata and Z. muelleri when exposed to plastic pollution, respectively (P < 0.05). Similarly, LDPE significantly increased nitrogen liberation by 409.7 % and 57.3 % during the decomposition of E. radiata and Z. muelleri (P < 0.05), respectively. As a result of slowed decomposition, the change in the C/N ratio of E. radiata was significantly influenced by the addition of LDPE, but this was not the case for Z. muelleri detritus (P < 0.05). Given the prominent role of macrophyte decomposition in coastal and estuarine ecosystems, our results show how excessive plastic pollution can significantly decrease macrophyte detrital decomposition, which could have flow-on effects on associated biogeochemical cycling and detritus-based production.