high concentrations of sulphide in the sediment can harm seagrasses since sulphide is a plant toxin inhibiting respiration sulphide is present in sediment rich in organic matter and poor in iron to be toxic, sulphide has to enter the plants, which is not possible under normal conditions when oxygen is present in the below ground tissue and in micro zones around the roots sulphide is oxidized in the root zone to the harmless compound sulphate before reaching the root surface during oxygen deficiency in the water column