The two groups of stations discriminated by cluster analysis (Fig. 4, a) have different sediment grain size characteristics. While the subtract of the stations EB-1 to EB-5 comprises sandy sediment, stations DK-1 and DK-2 have silty sediments, with fine fraction contents between ≈ 92–94%. The difference in sediment grain size found in the two regions allows us to suppose that there are different bottom hydrodynamic regimes. Bottom currents in the stations EB-1 to EB-5 area should be more energetic than in the DK-1 and DK-2 regions. However, it can also be hypothesized that the first region (with stations EB-1 to EB-5) may receive materials from the iceberg's melting, explaining the presence of coarse particle size fractions (medium sand to very coarse sand and granules) in the sediment composition. Such a record is not evident at stations DK-1 and DK-2. It should also be noted that DK-1 and DK-2 are much deeper (3700 m and 3850 m) than stations EB-1 to EB-5 (472–492 m).
The specimens analyzed live under the influence of the Deep Circumpolar Water, with low salinities and temperatures (between 34.60-34.73 and − 1.0 ºC and 0.53 ºC, respectively; Gordon and Nowlin, 1978; Garcia et al., 2002). In fact, A. glomeratum (an agglutinated foraminifera species) is frequently found in cold and/or deep-sea regions (Gooday, 1988; Gooday and Turley, 1990; Harloff and Mackensen, 1997; Kurbjeweit et al., 2000; Fontanier et al., 2002; Ernst and Zwaan, 2004; Bella et al., 2016) where the abundance of carbonate foraminifera is rare, due to the difficulty of forming and maintaining their shells due to the presence of corrosive waters. corrosive waters. In stations EB-1 to EB-5, the general average size of the longer axis of this species is ~ 130 µm, while in stations DK-1 and DK-2, that average is ~ 165 µm.
The results show that despite the highest number of observed measurements in the analyzed stations being between 90 and 180 µm (Figs. 2, 3), the biggest dimensions of A. glomeratum tests (between 200 and 320 µm) were recorded only in the deepest stations (DK-1 and DK-2), where the highest standard deviation values were found too. The statistical results (Fig. 5) show that station DK-2 is significantly different from the others but similar to station DK-1. The cluster analysis results (Fig. 4B) also suggest that the highest mean and maximum values of A. glomeratum sizes are associated with finer-grained sediments found in the deeper stations.
Adercotrima glomeratum is the most abundant species in the study area, mainly in the composition of the living assemblages from the upper 2 cm of surface sediments, where it is frequently the dominant species (Passos, 2019). This agrees with the fact that it is an opportunistic species with a great capacity to colonize environments and with wide tolerance to environmental variability, having a cosmopolitan character and generalist behavior (Fontanier et al., 2002; Ernst and Zwaan, 2004; Bella et al., 2016).
Many studies show that the difference in the size of tests of foraminifera species is related to depth (Theyer, 1969; Corliss, 1979; Loubere et al., 1988; Majewski and Pawlowski, 2010; Gooday et al., 2017). For instance, Globocassidulina sp. presented tests with reduced size in deeper regions of the Pacific Ocean, Southeast Indian Ocean, and North Atlantic (Theyer, 1969; Corliss, 1979; Loubere et al., 1988). Majewski and Pawlowski (2010) observed that, in the Antarctic Peninsula, some Globocassidulina spp. showed reduced sizes in deeper regions. These studies have shown a tendency for many species of foraminifera (the vast majority with carbonated-tests species) to decrease in size as depth increases.
The tests of A. glomeratum analyzed in this study, on the other hand, present an opposite pattern than that reported by the mentioned studies since they tend to show the biggest tests in deeper regions. When we analyze only the studies that have used foraminifera species with agglutinating tests, we can identify that this relationship is inversely proportional to that of calcareous test species. Even though research investigating differences in the size of tests from agglutinating species is rarer, it does show that individuals tend to increase in size according to depth. For example, Theyer (1969) found that Cyclammina cancellata had bigger individuals with more robust walls at a depth of 2000 m, while the individuals of this species found at 500 m were smaller and more fragile. Theyer (1969) deduced that the temperature can be the main factor that controls this pattern. The group of Xenophyophores (agglutinating, monothalamids, giant, and sensitive foraminifera) are found only in deep regions and indicate the ability to adapt to the abyssal areas by increasing the tests (Gooday et al., 2017). These examples suggest that the agglutinating species present systems of adaptation to the deep regions different from that of calcareous tests.
Most studies reported that A. glomeratum prefers muddy sediments in bottoms that receive phytodetritus from the sea surface (Gooday, 1988; Gooday and Turley, 1990; Rodrigues et al., 2015). Based on these remarks, we can say that the deepest regions of DP provide a more stable environment for this species' development since muddy substrates are generally related to calm hydrodynamic conditions. This calm and stable environment may allow the individuals to complete their life cycle and reach larger sizes. In addition, the deepest stations show the greatest variation in size, indicating the presence of organisms at different stages of development.
On the contrary, this work also suggests that, in environments disturbed by strong hydrodynamic conditions or by the deposition of sediments resulting, for example, from iceberg melting, this species has short life cycles and forms populations of smaller individuals. When the environmental conditions are stable, the life cycles of this species may be longer, and the populations may include larger organisms. Thus, the results of this study show that the size of A. glomeratum in the deep ocean can be an indicator of environmental disturbance or stability.