The control of sub-surface maxima of diatoms in a stratified lake by physical, chemical and biological factors

The sedimentation patterns of Asterionella formosa Hass. wereobserved in two lake basins, in one of which a mid-water populationmaximum of the diatom occurred. Comparison of the alga's distributionwith vertical profiles of temperature and dissolved reactivesilicon suggested that the formation of a mid-water maximumwas associated with a sharp silicon gradient. Sinking velocitiesof the diatom were reduced at the maximum compared to thoseat lesser or greater depths. The suspension of cultures at differentdepths, in either nutrient-replete medium or exposed to theprevailing nutrient conditions, following thermal stratification,confirmed that an increase of sinking velocity occurred in silicon-or light-deficient populations. It is suggested that the mid-watermaximum of A.formosa was formed by differential colony sinkingvelocities with depth in response to variations in silicon availability.The formation of a strong silicon gradient was dependent uponthe growth of the population being restricted to the epilimnionby the onset of thermal stratification.