FEMS Microbiology Ecology86 (1002) 195-203 © 1992 Federation of European Mtcroblologtcal Sooeltes 0168-6496/92/$05 00 Pubhshed by Elsevter 195 FEMSEC 00364 Microprofiles of photosynthesis and oxygen concentration in Microcystis sp. scums Bas W Ibehngs and Luuc R Mur Untt erslty of Amsterdam Laboratory for Mwrobtology Time Netherlands Received 25 March 1991 Revision reccwed 18 September 1991 Accepted 20 September 1991 Key words. Cyanobactena, Araflclal scum conditions; Oxygen mlcroelectrodes; Downwelhng photon trradlanee, Colony size SUMMARY Oxygen mlcroelectrodes were used to monitor oxygen concentration and rates of gross photo- synthetic activity in Mtcrocystts sp scums which were formed and incubated under laboratory conditions The depth of the phouc layer, rate of photosynthesis, oxygen concentration and the lo- cation of the transition to anoxla m the scum depended on trradtance levels and colony size Gross photosynthetic activity never extended be- low 2 5 mm depth m the scum At high trradtance levels oxygen concentration tn the upper 1 5 mm of the scum decreased and the oxygen concentra- tion peak shifted to greater depth Oxygen con- centrations in scums composed of small colonies (< 55 /.tm) were higher than concentrations tn large colomes scums (> 300 ~.m) but small colonies showed stronger indications of photoln- Correspondence to B W Ibehngs, University of Amsterdam, Laboratory for Microbiology,Nteuwe Achlergrachl 127, 1018 WS Amsterdam, The Netherlands hfbltton. In a nat,tral scum small colonies are presumably shielded from inhibitory intensities by larger colonies which will dominate the upper layers Accumulation of low-light adapted, smaller colonies in deeper layers likely yielded a second peak m photosynthetic activity. ?n order to sys- tematically d~scuss scums and scum formation a distinction is made in three different scum types. 2 INTRODUCTION The occurrence of cyanobactenal surface blooms has been documented under different en- vironmental conditions During periods of calm weather populations of buoyant, gas vacuolated cyanobacterta float to the surface of a lake to create nocturnal blooms [1,2]. During day time mechanisms which cause loss of buoyancy oper- ate through turgot-mediated collapse of gas vesi- cles [3,4] and ballast accumulation of photosyn- thetically fLxed carbohydrate [5] Dynamic ex- change of colomes at the lake surface as demon- strated in a computer model [6] may give the