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 pa tterns were examined in relation to phosphorus (P) availability in eut rophic Eau Galle Reservoir, Wisconsin (USA). During mid-June, hypolimn etic 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 biom ass, gross primary productivity, and chlorophyll a indicated retrieval of hypolimnetic P for production. During early July, anoxia restricte d vertical migration of Ceratium into the hypolimnion. Surplus cellula r P was low during this period, while alkaline phosphatase activity in creased to a maximum, suggesting P limitation of Ceratium production. During late July and August, P-rich interflows from the Eau Galle Rive r entered the reservoir at the base of the epilimnion. Ceratium migrat ed into these interflows at night, with corresponding increases and de creases in surplus cellular P and alkaline phosphatase activity, respe ctively. Ceratium production increased to a maximum in early September , following these periods of high external P input. These results dire ctly support the hypothesis that Ceratium can access multiple P source s through vertical migration.