Planktonic foraminifera preserved in marine sediments archive the physical
and chemical conditions under which they built their shells. To interpret the
paleoceanographic information contained in fossil foraminifera, the recorded
proxy signals have to be attributed to the habitat and life cycle
characteristics of individual species. Much of our knowledge on habitat depth
is based on indirect methods, which reconstruct the depth at which the
largest portion of the shell has been calcified. However, habitat depth can
be best studied by direct observations in stratified plankton nets. Here we
present a synthesis of living planktonic foraminifera abundance data in
vertically resolved plankton net hauls taken in the eastern North Atlantic
during 12 oceanographic campaigns between 1995 and 2012. Live
(cytoplasm-bearing) specimens were counted for each depth interval and the
vertical habitat at each station was expressed as average living depth (ALD).
This allows us to differentiate species showing an ALD consistently in the
upper 100 m (e.g., Globigerinoides ruber white and pink), indicating
a shallow habitat; species occurring from the surface to the subsurface
(e.g.,
Globigerina bulloides, Globorotalia inflata,
Globorotalia truncatulinoides); and species inhabiting the
subsurface (e.g., Globorotalia scitula and Globorotalia
hirsuta). For 17 species with variable ALD, we assessed whether their depth
habitat at a given station could be predicted by mixed layer (ML) depth,
temperature in the ML and chlorophyll a concentration in the ML. The
influence of seasonal and lunar cycle on the depth habitat was also tested
using periodic regression. In 11 out of the 17 tested species, ALD variation
appears to have a predictable component. All of the tested parameters were
significant in at least one case, with both seasonal and lunar cyclicity as
well as the environmental parameters explaining up to > 50 %
of the variance. Thus, G. truncatulinoides, G. hirsuta and
G. scitula appear to descend in the water column towards the summer,
whereas populations of Trilobatus sacculifer appear to descend in
the water column towards the new moon. In all other species, properties of
the mixed layer explained more of the observed variance than the periodic
models. Chlorophyll a concentration seems least important for ALD, whilst
shoaling of the habitat with deepening of the ML is observed most frequently.
We observe both shoaling and deepening of species habitat with increasing
temperature. Further, we observe that temperature and seawater density at the
depth of the ALD were not equally variable among the studied species, and
their variability showed no consistent relationship with depth habitat.
According to our results, depth habitat of individual species changes in
response to different environmental and ontogenetic factors and consequently
planktonic foraminifera exhibit not only species-specific mean habitat depths
but also species-specific changes in habitat depth
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