Abstract. Temperature appears to be the best predictor of species composition of planktonic foraminifera communities, making it possible to use their fossil assemblages to reconstruct sea surface temperature (SST) variation in the past. However, the role of other environmental factors potentially modulating the spatial and vertical distribution of planktonic foraminifera species is poorly understood. This is especially relevant for environmental factors affecting the subsurface habitat. If such factors play a role, changes in the abundance of subsurface-dwelling species may not solely reflect SST variation. In order to constrain the effect of subsurface parameters on species composition, we here characterize the vertical distribution of living planktonic foraminifera community across an east–west transect through the subtropical South Atlantic Ocean, where SST variability was small, but the subsurface water mass structure changed dramatically. Four planktonic foraminifera communities could be identified across the top 700 m of the transect. Gyre and Agulhas Leakage surface faunas were predominantly composed of Globigerinoides ruber, Globigerinoides tenellus, Trilobatus sacculifer, Globoturborotalita rubescens, Globigerinella calida, Tenuitella iota, and Globigerinita glutinata, and these only differed in terms of relative abundances (community composition). Upwelling fauna was dominated by Neogloboquadrina pachyderma, Neogloboquadrina incompta, Globorotalia crassaformis, and Globorotalia inflata. Thermocline fauna was dominated by Tenuitella fleisheri, Globorotalia truncatulinoides, and Globorotalia scitula in the west and by G. scitula only in the east. The largest part of the standing stock was consistently found in the surface layer, but SST was not the main predictor of species composition either for the depth-integrated fauna across the stations or at individual depth layers. Instead, we identified a pattern of vertical stacking of different parameters controlling species composition, reflecting different aspects of the pelagic habitat. Whereas productivity appears to dominate in the mixed layer (0–60 m), physical properties (temperature, salinity) become important at intermediate depths and in the subsurface, a complex combination of factors including oxygen concentration is required to explain the assemblage composition. These results indicate that the seemingly straightforward relationship between assemblage composition and SST in sedimentary assemblages reflects vertically and seasonally integrated processes that are only indirectly linked to SST. It also implies that fossil assemblages of planktonic foraminifera should also contain a signature of subsurface processes, which could be used for paleoceanographic reconstructions.

<p><strong>Abstract.</strong> Temperature appears to be the best predictor of species composition of planktonic foraminifera communities, making it possible to use their fossil assemblages to reconstruct sea surface temperature (SST) variation in the past. However, the role of other environmental factors potentially modulating the spatial and vertical distribution of planktonic foraminifera species is poorly understood. This is especially relevant for environmental factors affecting the subsurface habitat. If such factors play a role, changes in the abundance of deeper dwelling species may not solely reflect SST variation. In order to constrain the effect of subsurface parameters on species composition, we here characterize the vertical distribution of living planktonic foraminifera community across the subtropical South Atlantic Ocean, where SST variability is small but the subsurface water mass structure changes dramatically. Four planktonic foraminifera communities could be identified across the top 700 m of the E–W transect. Gyre and Agulhas Leakage faunas were predominantly composed of <i>Globigerinoides ruber</i>, <i>Globigerinoides tenellus</i>, <i>Trilobatus sacculifer</i>, <i>Globoturborotalita rubescens</i>, <i>Globigerinella calida</i>, <i>Tenuitella iota</i> and <i>Globigerinita glutinata</i>, and only differed in terms of relative abundances (community composition). Upwelling fauna was dominated by <i>Neogloboquadrina pachyderma</i>, <i>Neogloboquadrina incompta</i>, <i>Globorotalia crassaformis</i> and <i>Globorotalia inflata</i>. Thermocline fauna was dominated by <i>Tenuitella fleisheri</i>, <i>Globorotalia truncatulinoides</i> and <i>Globorotalia scitula</i> in the western side, and by <i>G. scitula</i> in the eastern side of the basin. The largest part of the standing stock was consistently found in the surface layer, but SST was not the main predictor of species composition, neither for the total fauna at each station nor in analyses considering each depth layer separately. Instead, we identified a consistent vertical pattern in parameters controlling species composition at different depths, in which the parameters appear to reflect different aspects of the pelagic habitat. Whereas productivity appears to dominate in the mixed layer (0–60 m), physical-chemical parameters are important at depth immediately below (60–100 m), followed by parameters related to the degradation of organic matter (100–300 m), and parameters describing the dissolved oxygen availability (> 300 m). These results indicate that the seemingly straightforward relationship between assemblage composition and SST in sedimentary assemblages reflects vertically and seasonally integrated processes that are only indirectly linked to SST. This also implies that fossil assemblages of planktonic foraminifera should also contain a signature of subsurface processes, which could be used for paleoceanographic reconstructions.</p>

Pelagic carbonate production is an important element of the global carbon cycle. Through biomineralization of either calcite or aragonite, marine plankton binds large amounts of dissolved inorganic carbon in their shells, which are then exported from the productive zone (Milliman, 1993). A large part of the aragonite flux, which consists exclusively of pteropods (Buitenhuis et al., 2019;Fabry, 1989;Singh & Conan, 2008), is dissolved before being buried in the sediment (Berner & Honjo, 1981). However, calcite (especially low-magnesium calcite in planktonic foraminifera shells) is less prone to dissolution (Keir, 1980), making the burial of biogenic calcite the main mechanism to transfer carbon from the rapidly cycling ocean-atmosphere-biosphere to the slow geological reservoir (Archer, 1996;Catubig et al., 1998). In addition, biogenic carbonate production has an opposing fast effect on the carbon cycle, as well as the marine biological carbon pump. By consuming alkalinity, it induces degassing of carbon dioxide and thus acts as a "counter pump" in the process of biological carbon sequestration in the ocean (Frankignoulle & Canon, 1994). Planktonic foraminifera shells are a key component of marine biogenic calcite production and flux. Upon death of these organisms, their shells begin to settle through the water column, and the resulting export flux is estimated to constitute up to half of the global calcite flux (Schiebel, 2002;Schiebel et al., 2007). The rest of the pelagic calcite flux is dominated by coccoliths (Baumann et al., 2004;Milliman, 1993). However, in order to quantify the pelagic calcite budget, planktonic foraminifera cannot be ignored. The planktonic foraminifera shell flux is a variable in both space and time (Jonkers & Kucera, 2015;Žarić et al., 2005), yet the factors controlling the shell flux variability are not completely constrained and understood. Furthermore, the subject concerning how shell flux variability relates to mass flux variability has not been explicitly studied before. In theory, variability in the planktonic foraminifera calcite flux could arise from changes in the shell flux and thus reflect population growth and/or changes in the shell mass, which consequently reflect individual

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<p>Planktonic foraminifera precipitate calcareous shells, which after the death of the organisms are exported from the sea surface to the sea floor, where they are preserved on geologically relevant timescales. The export flux of planktonic foraminifera shells constitutes globally up to a half, and in the studied region off Cap Blanc (Atlantic Ocean) about one third, of the marine pelagic calcite flux. Given their importance for the marine calcite budget and for the pelagic carbonate counter pump, which counteracts the biological pump in terms of oceanic capacity for intake of CO<sub>2</sub>, it is crucial to gain an understanding of factors modulating the export flux of planktonic foraminifera calcite. In principle, variability in the export flux of planktonic foraminifera calcite could depend within one species on i) shell flux, ii) shell size and iii) calcification intensity, and where shell size and calcification intensity differ among species also on the species composition of the deposited assemblage. To assess the importance of these aspects in modulating the export flux of planktonic foraminifera calcite, we investigated two annual time series (from 1990-1991 and 2007-2008) from sediment traps moored in the Cap Blanc upwelling area. We assessed the predictability of foraminifera calcite flux variability on seasonal and interannual time scales, by determining the variability in species-specific shell fluxes, shell sizes and weights with bi-weekly resolution. We find a remarkable discrepancy in the contribution of the controlling factors between seasonal and interannual scales. On the seasonal time scale, 80% of the variability of the calcite flux is explained by shell flux. On the inter-annual time scale, on the other hand, variations in shell size and calcification intensity are key to explain the calcite flux, since the time series from 2007-2008 yielded 58% larger and 11% heavier specimens. These results imply that for the global estimate of planktonic foraminifera calcite flux, shell flux is likely the most relevant predictor. However, a prediction of the temporal evolution of the calcite flux will likely require estimates of changes in shell size and calcification intensity of the involved foraminifera species.</p>