PRODUCTION AND VERTICAL MIGRATION OF CERATIUM-HIRUNDINELLA IN RELATION TO PHOSPHORUS AVAILABILITY IN EAU-GALLE RESERVOIR, WISCONSIN

Seasonal production of Ceratium hirundinella and its diel migratory patterns were examined in relation to phosphorus (P) availability in eutrophic Eau Galle Reservoir, Wisconsin (USA). During mid-June, hypolimnetic P gradients (0.030-1.045 mg.L-1) developed as internal P loading was high (14.7-18.0 mg.m-2.d-1). Ceratium migrated as much as 4 m into the upper hypolimnion at night. Subsequent increases in Ceratium biomass, gross primary productivity, and chlorophyll a indicated retrieval of hypolimnetic P for production. During early July, anoxia restricted vertical migration of Ceratium into the hypolimnion. Surplus cellular P was low during this period, while alkaline phosphatase activity increased to a maximum, suggesting P limitation of Ceratium production. During late July and August, P-rich interflows from the Eau Galle River entered the reservoir at the base of the epilimnion. Ceratium migrated into these interflows at night, with corresponding increases and decreases in surplus cellular P and alkaline phosphatase activity, respectively. Ceratium production increased to a maximum in early September, following these periods of high external P input. These results directly support the hypothesis that Ceratium can access multiple P sources through vertical migration.