Dynamic Aspects of Dinoflagellate Distribution Patterns in a Small Productive Lake

Abstract

(1) Vertical and temporal distribution patterns of the dinoflagellate Ceratium hirundinella 0. F. Muller, in relation to controlling factors, were studied for a small English lake (Esthwaite Water). General and grosser features of distribution were established from regular sampling at one station; examples are given from 1966-67 and 1975-77. Finer resolution and analysis were possible from more intensive study of selected episodes over the whole lake in 1976 and 1977. (2) The seasonal cycle of the euphotic population comprised a phase of nearexponential increase (May-July), a stationary phase of high numbers (JulySeptember), and a very rapid decline, often accompanied by cyst-formation (late September-October). The motile cells avoided the anoxic hypolimnion and, at times, the strongly illuminated surface layer. Depth-maxima often formed at the boundaries of these regions. (3) Under conditions of low wind stress during the rapid (exponential) growth phase, vertical distribution appeared to be mainly controlled by the underwater light climate and thermocline (pycnocline). Maximum density of Ceratium occurred at 3-4 m depth, at which depth about 10% of the surface-penetrating irradiance was measured. Surface-avoidance by cells was often found, but not appreciable vertical migration. During the stationary phase of the population cycle there was a change of behaviour, and Ceratium aggregated near the surface during daytime. (4) Conditions of light wind stress along the long axis of the lake (c. 3 m s -) appeared to cause laminar horizontal flow of the surface layers, which induced upwelling of subsurface maxima of Ceratium at the windward end of the lake, or downwind transport of the alga when concentrated in the surface waters. Moderate to strong wind conditions (> 4 m s'-) caused sufficient turbulent mixing to eliminate vertical stratification of Ceratium within the epilimnion. (5) Photo-inhibition of cellular fluorescence by Ceratium occurred in the upper layers during the day. Cells collected below the euphotic zone quickly (< 2 min) showed large decreases in fluorescence when exposed to strong sunlight.