89 J. Phycol. 34, 89–93 (1998) A METHOD TO IMPROVE THE SPATIAL RESOLUTION OF PHOTOSYNTHETIC RATES OBTAINED BY OXYGEN MICROSENSORS 1 Carsten Lassen, 2 Ronnie N. Glud, Niels B. Ramsing, and Niels Peter Revsbech 3 Institute of Biological Sciences, University of Aarhus, Ny Munkegade, DK-8000 Aarhus C, Denmark ABSTRACT To determine gross photosynthesis in benthic microalgal communities, oxygen microelectrodes were used to measure the rate of decrease within the first 4 s after extinction of light. Photosynthetic rates calculated from third-order poly- nomial fits to the curve of decreasing O 2 concentration were compared to the rates obtained by the traditional method, where rates were estimated from linear regression. When photosynthesis was calculated for the fitted initial rates of O 2 decrease, maximum rates in microbial mats were up to 32% higher, and the depth-integrated gross photosynthesis was 5%–10% higher than the rates determined by the traditional method. The determinations from fitted initial rates also resulted in a more detailed profile of photosyn- thetic rate than that normally obtained. Computer simu- lation based on diffusion models, where the estimated ini- tial rates of O 2 decrease were assumed to represent actual photosynthesis rates, verified the validity of the curve-fitting procedure for obtaining high-resolution photosynthesis pro- files. Key index words: cyanobacteria; diatoms; high resolu- tion; microalgae; microelectrode; microphytobenthos; micro- sensor; modeling; photosynthesis; primary production; sed- iment Benthic microalgal photosynthesis has been stud- ied intensively during the past 15 years using the light–dark shift technique introduced by Revsbech and Jørgensen (1983). The high spatial resolution of the technique makes it a powerful tool for the investigation of the depth distribution of photosyn- thesis in sediments where the active zone ranges from a few tenths of a millimeter in dense biofilms (e.g. Glud et al. 1992) to 10 mm or more in gelati- nous cyanobacterial mats ( Jørgensen et al. 1983). The depth distribution of oxygenic photosynthe- sis in microbial mats and sediments has been stud- ied with the light–dark shift technique in relation to the following parameters: distribution of sulfate re- duction (Canfield and Des Marais 1991), nitrogen fixation (Dodds 1989, Villbrandt et al. 1990), deni- trification (Christensen et al. 1990, Nielsen et al. 1990), and spectral light distribution ( Jørgensen et al. 1987, Lassen et al. 1992, Ploug et al. 1993). The light–dark shift technique has also been applied on other microbial communities where high spatial res- olution is necessary, for example, epiphytic com- 1 Received 31 December 1996. Accepted 9 September 1997. 2 Present address: COWIconsult, Flegborg 6, 7100 Vejle, Den- mark. 3 Author for reprint requests. munities on submerged macrophytes (Sand-Jensen and Revsbech 1987), marine snow (Alldredge and Cohen 1987), and scums of pelagic Microcystis sp. (Ibelings and Mur 1992, Ibelings 1996). In ecological studies, the gross production of ben- thic microalgae is often calculated by subtracting bulk O 2 fluxes measured in the dark from fluxes measured during the light period. This may lead to significant underestimates of gross photosynthesis because benthic respiration in the light often ex- ceeds dark respiration by a factor of two (Linde- boom et al. 1985). With the light–dark shift tech- nique, gross photosynthesis can be measured di- rectly, making the technique suitable for use in sys- tem ecological studies (Pinckney and Zingmark 1991, 1993). The principle of the light–dark shift method is based on the fact that after a period of illumination, the O 2 profile reaches a steady state where O 2 pro- duction at each specific depth counterbalances the biological and chemical consumption of O 2 within this layer and counterbalances the diffusional im- port or export of O 2 between the layer and its sur- roundings. When photosynthesis is terminated in- stantaneously by switching off the light source, both consumption and diffusional transport are assumed to continue at initially unchanged rates. Measure- ment of the initial rate of decrease in O 2 concentra- tion can thus be used to quantify the photosynthetic rate. Total consumption of O 2 has been shown to continue at unchanged rates during the first 4 s af- ter switching off the light source (Glud et al. 1992), but the diffusional flux of O 2 changes rapidly as a result of changes in the steep O 2 gradients between the photosynthetically active layer and its surround- ings. This diffusional exchange with the neighbor- ing layers limits the spatial resolution of the tech- nique (Glud et al. 1992). Traditionally, the photosynthetic rates have been calculated from the decrease in O 2 1 s after switch- ing off the light source (Revsbech and Jørgensen 1983), but intervals of up to 4 s have been used in some studies (Ibelings and Mur 1992). In order to produce a photosynthesis profile with improved spa- tial resolution, computer modeling of experimen- tally obtained values has been performed using the following equation:  2 (x - y) -0.5 P (x) = P (y)(4D t) exp - dy t  0 e   4D t e - (1)