A deep maximum of green sulphur bacteria (‘Chlorochromatium aggregatum’) in a strongly stratified reservoir BRIDGETT R. K. CHAPIN,* ,† FRANK DENOYELLES JR,* DAVID W. GRAHAM † AND VAL H. SMITH* *Department of Ecology and Evolutionary Biology, University of Kansas, Lawrence, KS, U.S.A. † Department of Civil, Environmental and Architectural Engineering, University of Kansas, Lawrence, KS, U.S.A. SUMMARY 1. Fine-scale physical and chemical gradients and deep photosynthetic microbial popu- lations were assessed to provide an initial characterisation of a small, thermally stratified reservoir (Cross Reservoir, Kansas, U.S.A.) and its deep chlorophyll maxima (DCM). Factors were identified that may affect vertical positioning of subepilimnetic photosyn- thetic sulphur bacteria (PSB) in lakes. 2. Results indicate that Cross Reservoir is a mesotrophic, dimictic lake with large subepilimnetic chlorophyll maxima containing dense layers of PSB. Characteristics of the deep PSB community of Cross Reservoir strongly correlate with both light and nutrient gradients. 3. The deep bacterial community mostly contained single-celled and aggregating green sulphur bacteria, specifically free-living Chlorobium limicola and the conspicuous motile ectosymbiotic consortium known as ‘Chlorochromatium aggregatum’. The bacteria were within the anaerobic hypolimnion, beneath a metalimnetic plate of Cryptomonas spp. and within very low sulphide and light conditions [mean of 67 lgS L )1 and 0.05% photosynthetically active radiation (PAR)]. Pigment concentrations and fluorescence trends indicate that the bacteria made up a larger proportion of the DCM biomass than did phytoplankton in 1996. 4. Cross Reservoir shares characteristics with natural lakes world-wide that also include a deep PSB community containing dense layers of ‘C. aggregatum’. Correlation analyses indicate that PSB community positioning and density are related to light, sulphide supply, redox potentials and pH. A 2-factor principal components analysis (PCA) and other data trends supported these interpretations and indicated that PSB are sensitive to the thermal stability of the water column, are nitrogen limited and regulated more by sulphide or sulphide to light ratios than local levels of light. The sensitivity of these deep photosynthetic bacteria to environmental gradients, and their significance to some aquatic systems, demonstrate their potential as indicators of environmental disturbance. Keywords:‘Chlorochromatium aggregatum’, deep chlorophyll maxima, photosynthetic sulphur bacteria, reservoir, resource gradients Introduction Studies of aquatic phototrophs in stratified lakes have traditionally focused on epilimnetic phytoplankton. However, significant primary production in stratified lakes can occur much lower in the water column, Correspondence: B.R.K. Chapin, Kansas Biological Survey, 2101 Constant Ave. Lawrence, KS 66047-3759, U.S.A. E-mail: bridgett@ku.edu Freshwater Biology (2004) 49, 1337–1354 doi:10.1111/j.1365-2427.2004.01270.x Ó 2004 Blackwell Publishing Ltd 1337