The seasonal succession of modern planktonic foraminifera: Sediment traps observations from southwest Taiwan waters

The seasonal succession and stable isotope compositions of living planktonic foraminifera collected in sediment traps from the continental shelf/slope off southwest Taiwan were investigated. Sediment trap moorings were deployed at water depths of ~816 m and ~233/250 m during October–November 2009, March–April 2010 and July–August 2010. The sampling duration was 3 days for each collecting cup, yielding 36 days/12 cups per individual mooring over the three sampling periods. All planktonic foraminiferal tests greater than 150 µm were picked, identified and counted for the faunal census. The variations in shell abundance (#/g; number of specimens per gram of original bulk sample) and size frequency show a pattern which seems to be related to the lunar cycle: shell abundances increase from low concentration at the new moon and reach their maxima before the full moon. Occurrences of larger foraminiferal shells also gradually increase as the collection approaching to full moon. The faunal assemblages were dominated by Globigerinella aequilateralis, Globigerinoides sacculifer, Globorotalia menardii, Globigerinoides ruber, Neogloboquadrina dutertrei, and Pulleniatina obliquiloculata; these six species constituting 30–80% of all foraminiferal tests found in the sediment traps. The seasonal contrast within the faunal assemblage between these three deployment periods, however, is not very evident except for the relative abundance of G. sacculifer and G. ruber. G. ruber is more abundant in summer than in spring whereas G. sacculifer shows an inverse pattern. Seasonal differences in the δ18O of P. obliquiloculata, G. sacculifer and G. ruber can be well explained by temperature profiles in the water column, while variations of δ13C are not related to the chlorophyll concentration hence to productivity. In addition, the calcifying depths reflected by three species are estimated to be ~40 m for G. ruber; ~60 m for G. sacculifer; and >80 m for P. obliquiloculata.