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Abstract

Planktic foraminifers are ubiquitous marine mesozooplankton, and contribute to the biological carbon pump by generating organic and inorganic carbon flux. We have analyzed the morphometry and quantified the protein-biomass of the test (i.e. shell) of 1441 living planktic foraminifers from 19 stations sampled during three consecutive years (2012-2014) in the Southwest Indian Ocean (30 degrees S-60 degrees S, 50 degrees E-80 degrees E). Transects were sampled across three main hydrographic fronts, the Subtropical Front (STF), Sub-Antarctic Front (SAF), and Polar Front (PF), including water bodies of different primary productivity and interannual variability. Differences in size-normalized planktic foraminifer protein-biomass between the years, species, and water bodies indicate that environmental parameters affect the production of planktic foraminifer organic (cytoplasm) carbon to varying degrees. Among the environmental parameters, trophic conditions (i.e., chlorophyll concentration) are best related to, and assumed most important for, the ontogenetic development of species and their individual biomass. We conclude that the contribution of planktic foraminifers to the biological carbon pump depends on ecological conditions and biological prerequisites at the species level, whereas the qualitative effect of prey (i.e., diatom species) remains to be determined. The applicability of planktic foraminifers as a proxy of the past biological carbon pump in mid to high latitudes would hence critically depend on the effect exerted by changing ecological conditions, and the presence of different species of both prey and planktic foraminifers on the regional scale. (C) 2016 Elsevier B.V. All rights reserved.