Effects of small scale fluid motion on bacterial growth and respiration MARIA S. BERGSTEDT*, MIKI M. HONDZO* AND JAMES B. COTNER † *University of Minnesota, St Anthony Falls Laboratory, Minneapolis, MN, U.S.A. †Department of Ecology, Evolution and Behavior, University of Minnesota, St Paul,MN, U.S.A. SUMMARY 1. Laboratory experiments were conducted to investigate the effect of small-scale turbulen motion on the growth and respiration of bacteria in an oscillating grid apparatus. The experiments were performed under a range of energy dissipation levels similar to those occurring in freshwater systems. 2. The results showed that small-scale turbulent motion does have an effect on bacterial growth and respiration. A higher gradient in the dissolved oxygen time series, higher 5-day biochemical oxygen demand values, increased bacterial abundance, increased bacterial specific respiration, higher bacterial growth rate and increased nutrient uptake were all observed when the energy dissipation rate in the water column was increased. 3. This has implications for traditional laboratory procedures that are used to characterise bacterial metabolic rates under stagnant fluid-flow conditions, such as biochemical oxygen demand (BOD),which would be influenced by the effects of the small-scale fluid motion inherent in aquatic environments. According to our results, BOD values in natural systems experiencing fluid motion would be higher than traditional bottle-derived rates. Keywords: bacteria, biochemical oxygen demand, energy dissipation, respiration, turbulence Introduction Most aquatic ecosystems are in a periodic or continu- ous state of mixing, whereas many laboratory experi- ments and procedures are carried out under stagnant conditions,for example biochemical oxygen demand (BOD) measurements (Hyun et al., 1993;Hansen & Christoffersen,1995),bottle incubations for respira- tion rates (Biddanda, Opsahl & Benner, 1994; Biddanda,Ogdahl & Cotner, 2001)and/or primary productivity (Wetzel & Likens, 1991).This difference may have a large impact on the interpretation of laboratory results. In order to understand better the interaction between physical and biological processes in aquatic ecosystems, much recent work has explored the effects of turbulence on biological activity.These effects are also important in the wastewater treatment industry, which relies on biologicaltreatmentpro- cesses in reactors which are even more highly mixed than naturalenvironments. Research has focused on isolating the effectsof turbulence by investigating mixed communities or individual organisms of various sizes from fish and zooplankton down to organisms as smallas bacteria and individualcells (Moeseneder& Herndl, 1995;Estrada & Berdalet, 1997;Hondzo & Lyn, 1999).Current research shows that there is a strong influence of turbulence on most organisms,but in the case of bacteria the evidence is mixed (Peters & Marrase, 2000). There is actually little to no evidence that turbulence hasany effect on growth or nutrient uptake in systems composed solely of dispersed bacteria. This paper presents evidence that turbulent mixing does have an effect on bacterial growth and respiration. Correspondence: Maria S.Bergstedt, University of Minnesota, St Anthony Falls Laboratory, Mississippi River at 3rd Ave. SE, Minneapolis, MN 55414-2196, U.S.A. E-mail: berg0411@umn.edu Freshwater Biology (2004) 49, 28–40 28 2004 Blackwell Publishing Ltd