Field and laboratory studies on the ontogeny and ecology of some globorotaliid species from the Sargasso Sea off Bermuda

Abstract

The life habits of Globorotalia truncatulinoides (d''Orbigny) and Globorotalia hirsuta d''Orbigny have been investigated. These nonspinose planktonic foraminifers occur in high abundance in early spring and reproduce in the subtropical waters around Bermuda. Size measurements were made in order to trace test growth, and deep tows were taken to establish descent to deeper water. The observations and measurements revealed reproduction and juvenile habitats in the upper 200 m of the water column. Comparison of test sizes of sediment trap specimens with those of surface tow specimens demonstrates an increase due to addition of chambers as these species descend downward through the water column. Additional calcite is secreted on top of the normal wall when the tests reach water temperatures less than 8-10.degree. C. The boundary of 8-10.degree. C was established by temperature-controlled experiments in the laboratory. Stable isotope investigations reveal a distinct shift from negative to positive .delta.18O values between juveniles and adults, and the adults still show a vital effect of 0.25-0.5 per mil. The carbon isotope data suggest a change in feeding behavior or food source. The .delta.18O values of adult specimens collected with plankton tows differ only by approximately 0.2 per mil from those specimens obtained by sediment trap. This can be explained as being due to the addition of the calcite crust and/or one or two chambers.