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Modeling seasonal and vertical habitats of planktonic foraminifera on a global scale

Abstract. Species of planktonic foraminifera exhibit specific seasonal production
patterns and different preferred vertical habitats. The seasonal and vertical
habitats are not constant throughout the range of the species and changes
therein must be considered when interpreting paleoceanographic
reconstructions based on fossil foraminifera. However, detecting the effect
of changing vertical and seasonal habitat on foraminifera proxies requires
independent evidence for either habitat or climate change. In practice, this
renders accounting for habitat tracking from fossil evidence almost
impossible. An alternative method that could reduce the bias in
paleoceanographic reconstructions is to predict species-specific habitat
shifts under climate change using an ecosystem modeling approach. To this
end, we present a new version of a planktonic foraminifera model, PLAFOM2.0,
embedded into the ocean component of the Community Earth System Model
version 1.2.2. This model predicts monthly global concentrations of the
planktonic foraminiferal species Neogloboquadrina pachyderma,
N. incompta, Globigerina bulloides, Globigerinoides ruber (white), and Trilobatus sacculifer throughout the world
ocean, resolved in 24 vertical layers to 250 m of depth. The resolution
along the vertical dimension has been implemented by applying the previously
used spatial parameterization of carbon biomass as a function of temperature,
light, nutrition, and competition on depth-resolved parameter fields. This
approach alone results in the emergence of species-specific vertical
habitats, which are spatially and temporally variable. Although an explicit
parameterization of the vertical dimension has not been carried out, the
seasonal and vertical distribution patterns predicted by the model are in
good agreement with sediment trap data and plankton tow observations. In the
simulation, the colder-water species N. pachyderma, N. incompta, and G. bulloides show a pronounced seasonal cycle in
their depth habitat in the polar and subpolar regions, which appears to be
controlled by food availability. During the warm season, these species
preferably occur in the subsurface (below 50 m of water depth), while
towards the cold season they ascend through the water column and are found
closer to the sea surface. The warm-water species G. ruber (white)
and T. sacculifer exhibit a less variable shallow depth habitat with
highest carbon biomass concentrations within the top 40 m of the water
column. Nevertheless, even these species show vertical habitat variability
and their seasonal occurrence outside the tropics is limited to the warm
surface layer that develops at the end of the warm season. The emergence in
PLAFOM2.0 of species-specific vertical habitats, which are consistent with
observations, indicates that the population dynamics of planktonic
foraminifera species may be driven by the same factors in time, space, and
with depth, in which case the model can provide a reliable and robust tool to
aid the interpretation of proxy records.
