Diel buoyancy changes by the cyanobacterium Aphanizomenon ovalisporum froma shallow reservoir

In the summer of 1999, a bloom (11 100 filaments ml(-1)) of the gas vacuola te cyanobacterium Aphanizomenon ovalisporum developed in a shallow (1.7 m d eep) reservoir containing nutrient-enriched water from Lake Kinneret (Israe l). During 4 days, A. ovalisporum showed a marked diet periodicity in buoya ncy: the proportion of floating filaments fluctuated between 76-84% from mi dday to evening and 94-98% at the end of the night, in both surface and bot tom samples. Buoyant filaments were present throughout the water column, pr esumably due to wind-driven vertical mixing Aphanizomenon filaments collect ed from the reservoir were maintained under mean photon irradiances of 15 ( LL), 150 (ML) and I 100 (HL),mu mol m(-2) s(-1) in a computer-controlled se t-up, which simulated the diet light changes at different depths in the res ervoir. In the LL cultures, filament buoyancy showed no diel fluctuation pa tterns during the 4 days of incubation, but ML and HL cultures showed regul ar diet changes, with a higher proportion of filaments floating at the end of the night than during midday-evening. There was no evidence for either t urgor-driven collapse of gas vesicles or dilution of gas vesicles by cell g rowth by any of the treatments. Gas vesicles of A. ovalisporum had a relati vely low mean critical pressure (p(c) of 0.57 MPa), but the daytime rise in turgor pressure was too small to cause gas vesicle collapse. The observed diet buoyancy changes may be explained by accumulation of carbohydrate ball ast during the day and decrease during the night.