Planktic foraminifera are widely used in palaeoceanographic and paleoclimatic studies. The accuracy of such reconstructions depends on our understanding of the organisms' ecology. Here we report on field observations of planktic foraminiferal abundances (>150 µm) from 5 depth intervals between 0-500 m water depth at 37 sites in the eastern tropical Indian Ocean. The total planktic foraminiferal assemblage comprises 28 morphospecies; with 11 morphospecies accounting for ~90% of the total assemblage. Both species composition and dominance in the net samples are broadly consistent with the published data from the corresponding surface sediments. The abundance and vertical distribution of planktic foraminifera are low offshore west Sumatra, and increase towards offshore south Java and the Lesser Sunda Islands (LSI). Average living depth of Trilobatus trilobus, Globigerinoides ruber, and Globigerina bulloides increases eastward, while that of Neogloboquadrina dutertrei, Pulleniatina obliquiloculata, and Globorotalia menardii remains constant. We interpret the overall zonal and vertical distribution patterns in planktic foraminiferal abundances as a response to the contrasting upper water column conditions during the southeast monsoon, i.e., oligotrophic and stratified offshore Sumatra (non-upwelling) vs. eutrophic and well-mixed offshore Java-LSI (upwelling). Overall, the inferred habitat depths of selected planktic foraminifera species show a good agreement with those from sediment trap samples and from surface sedimentss off Sumatra, but not with those from surface sediments off Java-LSI. The discrepancy might stem from the different temporal coverage of these sample types. Our findings highlight the need to consider how foraminiferal assemblages and ecology vary on shorter timescales, i.e., from “snapshots” of the water column captured by plankton net to seasonal and interannual variability as recorded in sediment traps and how these changes are transferred and preserved in deep-sea sediments.
