Spatial distribution of the calcareous dinoflagellate Thoracosphaera heimii in the upper water column of the tropical and equatorial Atlantic

The horizontal and vertical distribution of the phototrophic, calcareous di noflagellate Thoracosphaera heimii in the upper part of the water column (1 0-200 m) of the equatorial and tropical Atlantic Ocean has been studied. Th is first survey has been made to determine which part of the water column i s inhabited by T. heimii and is subsequently represented as a signal in the sediment record by this species. The study concentrates on the fossilisabl e vegetative coccoid life stage of the species and differentiates between i r. heimii shells with cell content and empty ones. The highest quantities o f T. heimii shells with cell content have been observed in water depths bet ween 50 and 100 m and coincide with relatively lower temperatures and relat ively higher salinities than respective surface conditions. We propose that in the tropical equatorial Atlantic the major part of the environmental si gnal given by T. heimii can be assumed to represent the deeper levels of th e upper water column.