Contrasting Diel Patterns of Vertical Migration in the Dinoflagellate Ceratium hirundinella in Relation to Phosphorus Supply in a North Temperate Reservoir

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Abstract
Very different diel patterns of vertical migration in Ceratium hirundinella were observed during two midsummer periods of study in a north temperate reservoir. During the first study (29–30 July), Ceratium migrated within a 2-m-deep oxygenated zone, moving upward near to the surface during the day and downward to the oxic–anoxic interface at night. During the second study (7–8 August), Ceratium did not migrate towards the surface during the day. High photosynthetic oxygen production and mixing processes increased oxygen distribution from 2 to 3 m providing Ceratium with access to a significantly greater water volume, with a greater nutrient content. Riverine phosphorus loading during the week prior to the first study was about 10 times greater than during the week prior to the second study. Limiting cell phosphorus concentrations and no measurable soluble reactive phosphorus in the upper (0–3 m) water column suggested a relationship between phosphorus limitation and the cessation of positive phototaxis. Diel patterns of vertical migration in this species may be influenced by episodic variations in phosphorus and/or other nutrient loadings via riverine inputs as interflows to this reservoir.