Population dynamics and reproduction strategies of planktonic foraminifera in the open ocean

Abstract

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 favor 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 days. 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 in 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.

Daily timescale dynamics of planktonic foraminifera shell-size distributions

“…Jonkers et al (2015) demonstrated that for some PF species in the Gulf of Mexico, lunar synchronized periodicity in the shell flux was detectable mostly for the larger size fractions. Meilland et al (2021) revealed that more than half of the PF population (of the four studied species) in the tropical Atlantic do not take part in synchronized reproduction. Lin et al (2022) showed that the shell-fluxes of four PF species (>212 µm) were characterized by some degree of lunar periodicity but longer cycles (34-48 days) during three months of peak shell-fluxes in the Northern South China Sea.…”

“…The first, is that despite previous suggestions, PF life cycles are not paced by lunar phases, or that the lunar phasing modulation is very minor compared to the impacts of seasonal variations in SST, water column mixing and primary production. Such a case study was demonstrated by Meilland et al (2021) who reported that up to 70% of G. glutinata and G. ruber in the tropical Atlantic do not appear to take part in synchronized reproduction. The second, is that lunar phasing modulation is significantly weaker today relative to several decades ago due to a change in environmental conditions.…”

“…Jonkers et al (2010) documented the annual shell fluxes of the species Turborotalita quinqueloba coupled with oxygen isotopic measurements at the NW Atlantic, and suggested that dormant noncalcifying PF individuals may survive for several months under unfavorable conditions and will reproduce only upon improving conditions. Previous studies from the Gulf of Aqaba (GOA) by Bijma et al (1990) and Almogi-Labin (1984) suggested synchronized reproductive strategy for T. sacculifer, though more recently, Chernihovsky et al (2020) showed that a daily timescale -resolved record of PF shell sedimentation fluxes in the GOA, yields species flux patterns with no lunar (or semi-lunar) periodicity, challenging this long-standing paradigm (see also Meilland et al, 2021).…”

Orbital CO2 reconstruction using boron isotopes during the late Pleistocene, an assessment of accuracy

“…Future studies are needed to constrain the d 11 B spatial distribution in foraminiferal shells caused by potential variations in d 11 B from dissolution, ontogeny (e.g. Meilland et al, 2021) and/or vital effects (e.g. change in photosymbiotic activity throughout the life cycle, Lombard et al, 2009, Henehan et al, 2013, Takagi et al, 2019.…”