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DIURNAL MOVEMENTS OF METALIMNETIC PHYTOPLANKTON 1

A metalimnetic population of the colonial chysophyte Chrysosphaerella longispina Lauterborn was observed using in vivo fluorometry to undergo a 1.5 m diurnal migration through a 5° C temperature gradient over four summers in a mesotrophic lake on the edge of the Canadian Shield. The population was highest in the water column at midday and early afternoon. The minimum depth reached during the day by the population was best correlated with the mean irradiance of the day of the observation plus that for the previous day. Rarely did the population cross the thermocline. Estimates of the migrational velocity determined by in vivo fluorometry were higher (maximum rate of 10.8 m·d−1) than estimates obtained with in situ settling chambers (maximum rate of 0.43 m·d−1); however the pattern of migration obtained by both methods was similar. The migratory ability of this alga appeared to provide little advantage in nutrient uptake. Oscillatoria agardhii Gomont, which comprised only 5% of the total algal biomass in the metalimnion, did not undergo pronounced diurnal migrations in the chambers and was positively buoyant over 24 h.