Molecular diffusive fluxes of oxygen in sediments of Port Phillip Bay in south-eastern Australia

Port Phillip Bay is a large, shallow, semi-enclosed bay in south-eastern Australia. Micro- electrodes were used to measure profiles of oxygen concentration in sediment cores taken from nine sites in the bay in January and February 1994. The effects of sediment surface topography, flow rate of the overlying water and irradiance on the distribution of oxygen in the sediments, and on the molecular diffusive fluxes of oxygen, were determined. Oxygen typically penetrated ≤3 mm into the sediment. Deeper penetration occurred when oxygen was photosynthetically produced in situ. In most cores the sediments consumed oxygen. Molecular diffusive fluxes of oxygen, determined from the gradient of oxygen in the DBL, were compared with fluxes modelled from the sediment gradient of oxygen. The modelled diffusive fluxes are considered to give better estimates of the diffusive fluxes in situ. Modelled fluxes ranged from 1.5 to 28.5 mmol O2 m–2 day–1, which was 43% (s.d. = 36%, n = 16) of the total flux at each site. Cores from two sites demonstrated efflux of oxygen, up to 71 mmol O2 m–2 day–1, as a result of photosynthesis in situ. The high degree of variability in oxygen fluxes within cores demonstrates the dynamism of oxygen cycling in these sediments.