Limnol. Oceanogr.. 31(2), 1986, 293-304 0 1986, by the American Society of Limnology and Oceanography, Inc. Oxygen production and consumption in sediments determined at high spatial resolution by computer simulation of oxygen microelectrode data l Niels Peter Revsbech Institute of Ecology and Genetics, University of Aarhus, Ny Munkegade, DK-8000 Aarhus C, Denmark Bent Madsen Institute of Mathematics, University of Aarhus Bo Barker Jmgensen Institute of Ecology and Genetics, University of Aarhus Abstract Microprofiles of oxygen and oxygenic photosynthesis were measured in a photosynthetically active sediment by an oxygen microelectrode. The di&.tsion coefficient of oxygen in the uppermost 1 mm of the sediment was determined in poisoned sediment by microelectrode measurement of changes in the oxygen profile during nonsteady state conditions. The experimentally obtained data were inserted into computer models to calculate the vertical profile of the oxygen consumption rate. The calculations showed that the rate of oxygen consumption was highest at the oxic-anoxic boundary where sulfide was oxidized and in the lowest part of the photic zone. Computer models were also used to obtain more accurate profiles of oxygen production. The oxygen profiles calculated by the computer were very close to those obtained experimentally both during the steady state developed under light and during the transient conditions developed at the onset of darkness. The surface layers of many shallow water sediments are characterized by very high microbial activities. The activities are high- est when the input of organic material to the sediment is high. This input can be sup- plied by autotrophic processesnear the sed- iment surface, or it can be supplied as de- trital material from the overlying water. When light is available, very dense popu- lations of photosynthetic microorganisms can develop. A very tight coupling between microbial respiration and microbial pho- tosynthesis can be expected in these pho- tosynthetically active layers (Riemann 1983). The analysis of microbial activities in sol- id or semisolid substrates with high num- bers of microorganisms has until recently been hampered by the lack of suitable an- alytical methods. The development of mi- croelectrodes for the analysis of oxygen (Revsbech 1983; Revsbech and Ward 1983), 1This study was supported by grants 198 l/82 114/ IV and 1983/84 124/IV from the Carlsberg Founda- tion. dissolved sulfide, and pH (Revsbech et al. 1983) has made detailed analysis possible. These microelectrodes have been used not only to demonstrate the distribution of oxy- gen, sulfide, and pH in sediments (Revsbech et al. 1983) but also the vertical distribution of oxygenic photosynthetic activity (Revs- bech et al. 198 1) with a spatial resolution of about 0.1 mm (Revsbech and Jorgensen 1983). Intuitively, one would expect it to be as easy to determine a profile of respiratory activity as a profile of photosynthesis, but this is not so. The process of diffusion does not have to be considered for the calculation of photosynthetic rates from experimental data obtained by the oxygen microelec- trodes (Revsbech et al. 1983) but it must be for the calculation of respiratory rates. We will show here how computer models can be used to simulate the combined effects of oxygen consumption (respiration), photo- synthesis, and diffusion on the oxygen pro- files in a microbial mat. The vertical dis- tribution of oxygen consumption rates in the mat is estimated from the results of these simulations. 293