Population dynamics and reproduction strategies of planktonic foraminifera in the open ocean
Authors: Meilland Julie , Siccha Michael , Kaffenberger Maike , Bijma Jelle , Kucera Michal 
Published: October 2021

Abstract. It has long been assumed that the population dynamics of planktonic
foraminifera is characterised by synchronous reproduction associated with
ontogenetic vertical migration. However, due to contradictory observations,
this concept became controversial, and subsequent studies provided evidence
both in favour and against these phenomena. Here we present new observations
from replicated vertically resolved profiles of abundance and shell size
variation in four species of planktonic foraminifera from the tropical
Atlantic to test for the presence, pattern, and extent of synchronised
reproduction and ontogenetic vertical migration in this oceanic region.
Specimens of Globigerinita glutinata, Globigerinoides ruber ruber, Globorotalia menardii and Orbulina universa were collected over the first 700 m resolved at nine
depth intervals at nine stations over a period of 14 d. Dead specimens
were systematically observed irrespective of the depth interval, sampling
day and size. Conversely, specimens in the smaller size fractions dominated
the sampled populations at all times and were recorded at all depths,
indicating that reproduction might have occurred continuously and throughout
the occupied part of the water column. However, a closer look at the
vertical and temporal size distribution of specimens within each species
revealed an overrepresentation of large specimens in depths at the beginning
of the sampling (shortly after the full moon) and an overrepresentation of
small individuals at the surface and subsurface by the end of the sampling
(around new moon). These observations imply that a disproportionately large
portion of the population followed for each species a canonical reproductive
trajectory, which involved synchronised reproduction and ontogenetic
vertical migration with the descent of progressively maturing individuals.
This concept is consistent with the initial observations from the Red Sea,
on which the reproductive dynamics of planktonic foraminifera has been
modelled. Our data extend this model to non-spinose and microperforate
symbiont-bearing species, but contrary to the extension of the initial
observations on other species of foraminifera, we cannot provide evidence
for ontogenetic vertical migration with ascent during maturation. We also
show that more than half of the population does not follow the canonical
trajectory, which helps to reconcile the existing contrasting observations.
Our results imply that the flux of empty shells of planktonic foraminifera
in the open ocean should be pulsed, with disproportionately large amounts of
disproportionately large specimens being delivered in pulses caused by
synchronised reproduction. The presence of a large population reproducing
outside of the canonical trajectory implies that individual foraminifera in
a fossil sample will record in the calcite of their shells a range of
habitat trajectories, with the canonical trajectory emerging statistically
from a substantial background range.