Environmental Controls of Size Distribution of Modern Planktonic Foraminifera in the Tropical Indian Ocean

Paleoceanographic studies often rely on abundance changes in microfossil species, with little consideration for characteristics such as organism size, which may also be related to environmental changes. Using a tropical Indian Ocean (TIO) core-top data set, we test the Optimum size-hypothesis (OSH), investigating whether relative abundance or environmental variables are better descriptors of planktonic foraminifera species' optimum conditions. We also investigate the environmental drivers of whole-assemblage planktonic foraminiferal test size variation in the TIO. We use an automated imaging and sorting system (MiSo) to identify planktonic foraminiferal species, analyze their morphology, and quantify fragmentation rate using machine learning techniques. Machine model accuracy is confirmed by comparison with human classifiers (97% accuracy). Data for 33 environmental parameters were extracted from modern databases and, through exploratory factor analysis and regression models, we explore relationships between planktonic foraminiferal size and oceanographic parameters in the TIO. Results show that the size frequency distribution of most planktonic foraminifera species is unimodal, with some larger species showing multimodal distributions. Assemblage size95/5 (95th percentile size) increases with increasing species diversity, and this is attributed to vertical niche separation induced by thermal stratification. Our test for the OSH reveals that relative abundance is not a good predictor of species' optima and within-species size95/5 response to environmental parameters is species-specific, with parameters related to carbonate ion concentration, temperature, and salinity being primary drivers. At the species and assemblage levels, our analyses indicate that carbonate ion concentration and temperature play important roles in determining size trends in TIO planktonic foraminifera. Key Points Optimum size-hypothesis holds true in planktonic foraminifera if one considers the main parameters driving each species' size distribution Size variations in planktonic foraminifera are linked to species' niches and diversity does not increase with productivity Within-species size is driven by CO32− concentration, temperature, and salinity; assemblage size by CO32− concentration and temperature Plain Language Summary In core-top samples from the tropical Indian Ocean (TIO), we investigate the optimum size-hypothesis, testing whether species' relative abundance or environmental parameter(s) are better descriptors of planktonic foraminifera species' optimum conditions. Further, we investigate the main environmental drivers of size variations in planktonic foraminifera at the assemblage-level, given that temperature has been reported to primarily drive assemblage size trends. We use a state-of-the-art machine (MiSo) to automatically identify planktonic foraminiferal species, analyze their size, and quantify fragmentation using machine learning techniques. When compared to identification carried out by human experts across 21 species, the machine classified the species accurately 97% of the time. The MiSo-generated size data was similar to that by other researchers. The frequency distributions of the species' size spectra show that most species have distributions that form bell-shaped curves. As species diversity increased, so did the assemblage size (95th percentile size); we attribute this observation to the effect of temperature-dependent niche separation. We find that, in the TIO, environmental parameters are better descriptors of optimum conditions in planktonic foraminifera than relative abundance. Our results also reveal that size variation at the species and assemblage levels is mostly driven by ambient carbonate chemistry and temperature.