Effects of tidal flushing regimes on mangrove roots receiving wastewater contaminated with PAHs and PBDEs

The effects of tidal flushing regimes on roots of wastewater-stressed mangrove plants were unknown. The present study showed that patterns of radial oxygen loss (ROL) in two mangrove species, Excoecaria agallocha and Kandelia obovata, receiving wastewater containing polycyclic aromatic hydrocarbons (PAHs) and polybrominated diphenyl ethers (PBDEs), were not affected by tidal flushing regimes. However, more ROL was induced under less frequent tidal flushing regimes, and produced along a root of K.obovata than that of E.agallocha. The iron (Fe) plaque formed on roots were negatively correlated to ROL, suggesting that Fe plaque functioned as a ‘barrier’ to prevent ROL, with stronger effects under less frequent tidal flushing regimes and in K.obovata with weaker outer layers. These results revealed that PAHs- and PBDEs-stressed K.obovata was more sensitive to tidal flushing regimes than E.agallocha, and long-term exposure would lead to root damage and even plant death.