The decomposition of leaf litter is a vital ecological process. The dominant mangrove species in Oman is Avicennia marina, which can thrive in a highly arid and saline environment. The decomposition of A. marina leaves drives the detrital ecosystem in mangroves. Any human activities that disrupt this process may have harmful impacts on the overall health of the mangrove system. Metallic nanoparticles are man-made materials that are being increasingly used in industrial, medical and consumer products. Some studies suggested that when ZnO NPs is discharged into ecosystems may cause toxicity through release of Zn2+ ions and production of reactive oxygen species (ROS). Mangroves are dynamic systems with widely fluctuating salinity. The aim of the present study was to investigate if salinity would impact the toxicity of ZnO nanoparticles. The present study investigated the effects of different ZnO-NP concentrations and salinity on decomposition of A. marina leaves by measuring leaf mass loss and bacterial and fungi density. Additionally, dissolved ionic zinc was measured and the behaviour of the nanoparticles characterized. Finally, the effects of the nanoparticles on leaf surface morphology was characterized by scanning and transmission electron microscopy. In the first experiment the effects of four different concentrations of ZnO nanoparticles,two different salinities (0, 1, 10 and 50 mg/L) and two different salinity concentration (50 and 100 %) were measured. In the second experiment the effects of four different seawater salinities (25, 50, 75 and 100%) and one concentration of ZnO NPs (10 mg/L) was measured. Finally, the effects of ZnO-NPs on the survival and body morphology of brine shrimp (Artemia salina) as an example of marine plankton were examined. For experiment 1, after 4 weeks the maximum mass loss was in the controls and 1mg/L and there was a significant difference between the 1 and 50 mg/L treatments. After six weeks of incubation, salinity had a more significant effect (p=0.0001) on leaf decomposition than nanoparticles (p= 0.101). There was no significant effect of ZnO-NPs or salinity on fungi biomass (p> 0.05). The TEM showed that there was aggregation of NPs. At 50nm magnification, ZnO-NPs tend to form hexagonal morphology compared to spherical shaped particles at 100nm and 200nm magnification. By using ICP-OES, dissolved ionic zinc did not exceed 1 µg/L except at 50mg/L after 2 weeks (2.016 µg/L in 50% seawater) and (2.461 µg/L in 100% seawater). The SEM suggests some direct damage to leave surface. The 1 mg/L leaf sample at 50% still appeared to be intact but all other treatments at 50% and 100% seawater showed significant decay. In the 100 x photos of the 50 mg/L, there appear to be numerous NPs on the surface of leaves. For experiment 2, there was a significant effect of salinity on the percentage mass loss (p< 0.05) but no significant effect of nanoparticles (p> 0.05). Moreover, the bacterial biomass was significantly affected by nanoparticles (p< 0.05).) For experiment 3, the SEM images showed no significant alteration on A. salina morphology caused by ZnO-NPs but there survival were affected significantly (p< 0.05) (after 4 and 6 weeks especially at 50mg/L) likely due to both ionic Zn and NPs. Overall, the results suggest that nanoparticles may interfere with microbial community and indirectly affect the rate of leaf decomposition, but that variation in salinity is a stronger factor affecting the rate.