Oxygen microgradients in the rhizosphere of the mangrove Avicennia marina

Oxygen concentration was measured in aerial roots (pneumatophores), horizontal cable roots and surrounding sediment of the tree Avicennia marina (Forsk.) Vierh. in a mangrove swamp at Ao Nam Bor, on the southeast coast of Phuket Island, Thailand. The O₂ measurements were made in the field with a polarographic oxygen minielectrode. The O₂ concentration inside pneumatophores was 63 to 88 % of air saturation, whereas the cable roots showed a lower concentration (62 to 73 %) indicating an O₂ gradient from the emerging parts to the subsurface roots. The O₂ concentration in the roots was highest in the outer part of the aerenchyma. The oxic zone of the sediment around roots was thin (ca 0.5 mm) which may indicate that very litte O₂ is released from the roots. Recording around a fiddler crab (Uca sp.) burrow revealed, for comparison, that the oxic layer of the burrow wall was ca 1 mm thick. The penetration depth for O₂ at the surface of this tropical mangrove sediment (1.5 to 3 mm) was similar to that measured in temperate coastal sediments.