The role of gas vacuoles and carbohydrate content in the buoyancy and vertical distribution of Anabaena minutissima in Lake Rotongaio, New Zealand

Murray-Darling Freshwater Research Centre

MDFRC item.

The late summer phytoplankton of Lake Rotongaio contained an almost pure population of Anabaena minutissima var. attenuata, a gas vacuolate species producing thin solitary filaments. The population was uniformly distributed through the top meter of the water column and then declined with depth to near zero at 6 m. The majority of the Anabaena filaments were buoyant with flotation velocities of c. 0.1 m/d. When samples of lakewater were artificially held at the surface the proportion of buoyant filaments dropped to 88.9% after 9 h of daylight, and to 44.9% at the end of the second day. Turgor pressure varied from 0.33 to 0.43 MPa and could therefore be neglected as a cause of buoyancy loss. Most of the cellular ballast could be accounted for by protein (P) and carbohydrate (C), and increases or decreases in carbohydrate ballast appeared to be the main cause of buoyancy change. A transect along the east-west axis of the lake showed an increase in C/P with distance down-wind at 0 m, and a decrease with distance upwind at 3 m. Hydrodynamic measurements on this date indicated the existence of a wind-driven circulation cell with upwelling at the western (upwind) end of the lake. A computer simulation of this population in Lake Rotongaio indicated that under completely stable conditions the Anabaena would form a maximum at 0.88 m with oscillation of +/-0.01 m over the day/night cycle. In the real lake this depth lies within the mixed layer and stratification would therefore never occur. The time scale of vertical mixing in the Lake Rotongaio was extremely short relative to that required for buoyancy control, and was considered to select in favor of positively buoyant but slowly moving filaments with an ability to exploit a low average irradiance.