# Linking zooplankton time series to the fossil record

Lukas Jonkers, Julie Meilland, Marina Rillo, Thibault de Garidel-Thoron, John Kitchener, Michal Kucera

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Lukas Jonkers, Julie Meilland, Marina Rillo, Thibault de Garidel-Thoron, John Kitchener, et al. Linking zooplankton time series to the fossil record. ICES Journal of Marine Science, 2021, 10.1093/icesjms/fsab123. hal-03514728

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ICES Journal of Marine Science (2022), 79(3), 917–924. https://doi.org/10.1093/icesjms/fsab123

Food for Thought

Linking zooplankton time series to the fossil record

Lukas Jonkers ID <fnref n="1" />,*, Julie Meilland<fnref n="1" />, Marina C. Rillo ID <fnref n="1" />,<fnref n="2" />, Thibault de Garidel-Thoron<fnref n="3" />, John A. Kitchener<fnref n="4" />, and Michal Kucera<fnref n="1" />

Jonkers, L., Meilland, J., Rillo, M. C., de Garidel-Thoron, T., Kitchener, J. A., and Kucera, M. Linking zooplankton time series to the fossil record. – *ICES Journal of Marine Science*, 79: 917–924.

Received 12 March 2021; revised 10 June 2021; accepted 14 June 2021; advance access publication 7 July 2021.

Marine zooplankton time series are crucial to understand the dynamics of pelagic ecosystems. However, most observational time series are only a few decades long, which limits our understanding of long-term zooplankton dynamics, renders attribution of observed trends to global change ambiguous, and hampers prediction of future response to environmental change. Planktonic foraminifera are calcifying marine zooplankton that have the unique potential to substantially extend our view on plankton dynamics because their skeletal remains are preserved for millions of years in deep-sea sediments. Thus, linking sedimentary and modern time series offers great potential to study zooplankton dynamics across time scales not accessible by direct observations. However, this link is rarely made and the potential of planktonic foraminifera for advancing our understanding of zooplankton dynamics remains underexploited. This underutilization of this potential to bridge time scales is mainly because of the lack of collaboration between biologists, who have mostly focused on other (zoo)plankton, and micropalaeontologists, who have focussed too narrowly on fossil foraminifera. With this food for thought article, we aim to highlight the unique potential of planktonic foraminifera to bridge the gap between biology and geology. We strongly believe that such collaboration has large benefits to both scientific communities.

**Keywords:** fossil record, global change, long-term zooplankton dynamics, planktonic foraminifera

**Introduction**

Marine zooplankton are sensitive indicators of the state of the pelagic ecosystem and play important roles in the marine food web and global biogeochemical cycles (Hays et al., 2005; Richardson, 2008). Therefore, observational zooplankton time series have great potential to determine the nature and functional consequences of the response of the marine pelagic ecosystem to ongoing anthropogenic pressure, including global change. However, most observational zooplankton time series are short and do not extend to before the 20th century. This limited time span makes it difficult to constrain pre-industrial zooplankton dynamics, which is a prerequisite for attributing observed biotic changes to human influence (Rosenzweig et al., 2008; Chiba et al., 2018; Jonkers et al., 2019).

Long time series are also needed to assess nonlinear dynamics and the existence of tipping points in ecosystems (Heinze et al., 2021). In addition, models used to predict how zooplankton will respond to and feed back on future climate change cannot be properly tested because we lack observations of zooplankton dynamics outside the current climate state. Longer time series are therefore highly pertinent and urgently needed (Brierley and Kingsford, 2009). Continuing the current zooplankton monitoring into the future will eventually provide sufficiently long time series to address community dynamics on longer time scales. However, this information will likely come when environmental change has progressed too far, rendering efforts to predict and anticipate ecosystem changes moot. In the absence of suitable collections, extending zooplankton time series into the past and leveraging the information from past zooplankton

<footnote n="1">¹MARUM Center for Marine Environmental Sciences, University of Bremen, Leobener Straße 8, 28359 Bremen, Germany</footnote>

<footnote n="2">²ICBM Institute for Chemistry and Biology of the Marine Environment, Carl-von-Ossietzky University Oldenburg, Schleusenstraße 1, 26382 Wilhelmshaven, Germany</footnote>

<footnote n="3">³CEREGE, Aix Marseille University, CNRS, IRD, INRAE, Collège de, 13545 Aix-en-Provence, France</footnote>

<footnote n="4">⁴Australian Antarctic Division, Department of Agriculture, Water and the Environment, Kingston, 7050 Tasmania, Australia</footnote>

<footnote>*Corresponding author: tel: +49 421 218 65973; e-mail: ljonkers@marum.de</footnote>
response to perturbations of marine ecosystems sounds impossi-
ble. Yet some groups of zooplankton provide this potential because
of their fossil record. Apart from pteropods and radiolaria, plank-
tonic foraminifera, in particular, can provide a long-term perspec-
tive on global zooplankton dynamics because their skeletal remains
are well preserved across large parts of the world's ocean floor (Ya-
suhara et al., 2020). Up to now, planktonic foraminifera have been
mostly studied by micropalaeontologists, yet their potential to pro-
vide a long-term perspective on marine ecosystems can be ampli-
fied by stronger collaboration between zooplankton ecologists and
micropaleontologists. The lack of collaboration between ecologists
and micropalaeontologists is the result of focus on different tempo-
ral domains, perceived lack of importance of foraminifera as well as
collection practicalities. However, it is time to bring the biological
and geological communities together. With this food for thought ar-
ticle, we aim to foster collaboration between micropalaeontologists
and (zoo)plankton ecologists, because we believe that by connect-
ing these fields more strongly, we can better constrain zooplankton
dynamics on time scales from seasons to millennia.

Planktonic foraminifera: zooplankton with potential

Planktonic foraminifera are small, typically 100–1000 µm, unicel-
lular zooplankton that occur globally in the upper hundreds of me-
tres of the open ocean (Figure 1). The group is relatively species
poor, with about 50 species differentiated by shell morphology
(Schiebel and Hemleben, 2017; Morard et al., 2018) all displaying a
wide geographical distribution (Bé and Tolderlund, 1971). Plank-
tonic foraminifera are heterotrophic, but many species bear au-
totrophic endosymbionts (Takagi et al., 2019). Typical abundances
of planktonic foraminifera in the surface mixed layer are in the or-
der of tens to thousands of individuals per cubic metre, but abun-
dances of individual species show considerable spatial and tempo-
ral variation (Jonkers and Kučera, 2015; Schiebel and Hemleben,
2017) and they may in some areas represent the most abundant
(zoo)plankton group (Hunt and Hosie, 2006; Hosie et al., 2014).
Because of their low standing stocks, planktonic foraminifera are
often assumed to play a minor role in the marine food web, but re-
cent work has suggested that they could be an important prey in
oligotrophic regions, where they are relatively abundant (Stoecker
and Pierson, 2019).

Planktonic foraminifera build calcite shells that sink rapidly (hundreds of metres per day) to the ocean floor when the organisms die (Takahashi and Bé, 1984). Because of this export of calcite shells and associated organic matter from the surface ocean, planktonic foraminifera play an important role in the marine carbon cycle (Schiebel, 2002; Schiebel and Movellan, 2012). In regions above the calcite lysocline, shells exported from the near surface layer are preserved on the ocean floor (Berger, 1968). This process has been pervasive over long time scales, such that in some regions, the seafloor consists almost exclusively of foraminifera shells (Figure 1). It is precisely this sedimentary record that makes planktonic foraminifera unique among zooplankton (Figure 2). Their character-rich fossil shells preserve information not only on past species composition and trait distributions, but through their chemical composition also about their past habitat. The sedimentary record of planktonic foraminifera therefore allows studying zooplankton ecology and response to environmental change in multiple dimensions prior to human influence and, importantly, on temporal scales much larger

than is ever possible using ecological time series (Schmidt *et al.*, 2004; Ezard *et al.*, 2011; Yasuhara *et al.*, 2017; Rillo *et al.*, 2019; Lewandowska *et al.*, 2020). Such a long-term perspective is needed to better understand natural ecosystem variability related to, for instance, (multi)decadal climate oscillations; and to distinguish natural variability from human-induced changes.

The sedimentary record: a window to long-term
zooplankton dynamics

Sedimentary records of planktonic foraminifera assemblages have clearly shown that the composition of their communities has changed in response to global warming (Field et al., 2006; Spielhaugen et al., 2011). Such sedimentary time series not only demonstrate the usefulness of planktonic foraminifera as indicators of global change, but also provide the opportunity to assess the species composition before the onset of the industrial era, which is essential to attribute ongoing biodiversity changes to man-made climate change (Rosenzweig et al., 2008). Following this lead, Jonkers et al. (2019) have shown that the sedimentary record of planktonic foraminifera can be used to establish a global pre-industrial baseline of community composition. They compared this pre-industrial baseline to late-twentieth century planktonic foraminifera communities from moored sediment-trap time series, and showed that the sediment trap communities departed systematically from their pre-industrial counterparts recovered from the underlying sediment. As such, this study demonstrated the importance of linking the observational with the sedimentary record of planktonic foraminifera to assess the response of (zoo)plankton biodiversity to global warming.

Importantly, the sedimentary record not only preserves informa-
tion on community composition from before marked human in-
fluence, but also on traits of individuals species. These traits can
be used to assess the effects of other aspects of global change that
do not, or have not yet, affected biodiversity. For instance, shells of
modern planktonic foraminifera appear to be thinner than those
of their pre-industrial counterparts, possibly as a result of anthro-
pogenic ocean acidification (de Moel et al., 2009; Moy et al., 2009).
Because of the architecture of foraminiferal shells, there is substan-
tial potential to utilize other traits, such as shell size, growth rate, or
growth regularity to investigate how environmental change impacts
zooplankton.

In addition, planktonic foraminifera carry within their shells a chemical signal of the seawater they calcified in. For instance, stable oxygen isotope and magnesium/calcium ratios of foraminifera calcite can provide information about past seawater temperature (Emiliani, 1955; Nuernberg, 1995) and carbon and boron isotope ratios carry a signal of changes in seawater carbonate system (Spero et al., 1991; Foster and Rae, 2016). Palaeoceanographers therefore use the chemical composition of planktonic foraminifera to reconstruct changes in climate and biogeochemistry of the ocean. Stable oxygen isotope ratios of planktonic foraminifera were, for instance, used to quantify for the first time the temperature change between ice ages and warm climates (Emiliani, 1955). Importantly, the co-registration of environmental signals and assemblage changes in the sedimentary archive of planktonic foraminifera offers the potential to study their response to past environmental change in order to better understand modern changes in ecology and biodiversity of planktonic foraminifera.

However, the use of planktonic foraminifera as past thermome-
ters or pH-meters requires accurate calibration and a thorough
<figure><img src="image_2.png" /><figcaption>**Figure 1.** Living and fossil planktonic foraminifera. Panel (a) shows living specimens of planktonic foraminifera collected with a plankton net in subsurface water (40–60 m depth) of the North East subtropical Atlantic Ocean in August 2017 during cruise M140 (Kucera *et al.*, 2019). Panel (b) shows sedimentary specimens of planktonic foraminifera collected in 1996 during cruise M34/4 (Fischer *et al.*, 1996) using multi-corers from approximately 2000 m water depth near Barbados. The scale bar in panel (a) is valid for both pictures.</figcaption></figure>

understanding of their ecology. Time series play a crucial role in this regard. Sediment-trap time series of planktonic foraminifera have been used extensively for calibration purposes (Anand *et al.*, 2003; Wejnert *et al.*, 2013) and to investigate how variability in foraminifera seasonal abundance interferes with palaeoceanographic reconstructions (Jonkers and Kučera 2015, 2017). Next to the seasonal habitats, the vertical habitats of many species of planktonic foraminifera have been constrained to some degree using stratified plankton nets (e.g. Fairbanks and Wiebe, 1980; Rebotim *et al.*, 2017). However, given the scarcity of plankton haul time series resolving planktonic foraminifera, the temporal variability (life cycle, seasonal) in their habitat depth is virtually unconstrained.

## The role of planktonic foraminifera in the carbon cycle

Besides their unique fossil record, planktonic foraminifera also are an important component of the marine carbon cycle (Schiebel, 2002; Salter *et al.*, 2014). When building their shell in the upper ocean, they contribute to the carbonate counter-pump which releases CO₂ into the atmosphere (Frankignoulle *et al.*, 1994). This process is counteracted by fixation of organic carbon in their cytoplasm and shell-bound membranes (Schiebel and Movellan, 2012; Meilland, 2015) and by the long-term removal of dissolved inorganic carbon through the burial of their calcite shells in deep sea sediments (Zeebe, 2012). Planktonic foraminifera may also affect the marine carbon cycle in ways that remain understudied. For example, whilst most of the organic carbon in foraminifera cytoplasm respires in the water column, the organic linings sandwiched within the foraminifera shell remain intact and are also buried in the sediment (Hemleben *et al.*, 1977; Bé *et al.*, 1979; Sigman *et al.*, 2001). This burial of organic carbon within the calcite shells is a potentially large, but still unquantified, long-term organic carbon sink. In addition, planktonic foraminifera possibly interact with marine snow (Hemleben *et al.*, 1989; Nöthig and von Bodungen, 1989; Itou *et al.*, 2001) and their shells may act as ballast, which could affect the efficiency of the biological pump at large. Finally, together with coccolithophores, planktonic foraminifera are the dominant pelagic calcifiers, with planktonic foraminifera estimated to biomineralise between 20 and 50% of all pelagic calcite (Schiebel, 2002). However,

if and how the contribution of planktonic foraminifera to the calcite budget varies in space and time and what controls such variability remains poorly known. Thus, time series of planktonic foraminifera calcification will help understand the response of their biomineralization to future climatic and environmental change.

## Why zooplankton time series should resolve planktonic foraminifera

The first observations of foraminifera in the plankton stem the start of ocean discovery expeditions (e.g. Murray, 1897). Most observations from the water column, however, are based on single plankton hauls, which provide only snapshots in space and time. To the best of our knowledge, the number of time series from plankton hauls resolving planktonic foraminifera is extremely low and the existing time series are short and/or have only a coarse temporal resolution (Schiebel *et al.*, 1997; Field, 2004). The patchy nature of plankton haul observations renders it extremely difficult to separate spatial from temporal variability in planktonic foraminifera distribution and traits. Therefore, important aspects of their ecology, phenology, and population dynamics remain poorly constrained (Jonkers *et al.*, 2015; Rebotim *et al.*, 2017; Chernihovsky *et al.*, 2020). This lack of knowledge is unfortunate because a deeper understanding of these aspects of planktonic foraminifera ecology would aid the interpretation of the geological record of long-term zooplankton dynamics.

Because of the virtual absence of plankton haul time series of living planktonic foraminifera, most of what we know about the short-term temporal dynamics of planktonic foraminifera populations comes from moored sediment traps (e.g. King and Howard, 2001). However, sediment-trap observations do not resolve the vertical habitat of species, nor the vertical stratification of communities and most time series are short (the longest time series spans 12 years, but most are only 1–2 years long; Jonkers and Kučera, 2015). Thus, our knowledge on interannual variability, phenology, and possible adaptations to ongoing climate change remains poor. Moreover, even though sediment trap studies have advanced planktonic foraminifera ecology (Jonkers and Kučera, 2015; Rillo *et al.*, 2019), such studies are by definition indirect since sediment traps provide a signal of export flux of empty, dead shells, providing an incomplete understanding of their biology. This indirect nature of
<figure><img src="image_3.png" /><figcaption>**Figure 2.** Schematic representation of the potential of planktonic foraminifera to provide insights in zooplankton dynamics across time scales ranging from seasons to millennia. In the upper ocean, living planktonic foraminifera (brown) dynamics are governed by interaction with phytoplankton and other zooplankton as well as with the physical environment (here represented by temperature and salinity). Thin arrows represent these dynamics, which have been constrained directly using plankton tows and CPR, and indirectly using sediment traps. However, gaps in our knowledge (indicated by question marks) on short-term dynamics remain. Deeper in the water column, dead shells of planktonic foraminifera (white) sink to the ocean floor sediments where they form a long-term archive. This fossil record of long-term zooplankton, carbon, and climate dynamics can be accessed using sediment cores.</figcaption></figure>

the signal is particularly problematic because we still lack precise knowledge on planktonic foraminifera lifespan. Hence, it is difficult to investigate biotic and abiotic interactions that influence planktonic foraminifera populations and species using sediment trap data alone.

The scarcity of planktonic foraminifera time-series observations from plankton nets is certainly not due to the lack of suitable collections or planktonic monitoring efforts (Figure 3). Instead, it seems that marine ecologists and micropalaeontologists have rarely worked together and planktonic foraminifera remain overlooked in zooplankton time series. This unfortunate tradition of leaving the study of modern planktonic foraminifera to micropaleontologists means that planktonic foraminifera biology is not only understudied, but also studied in isolation from other plankton groups. Thus, ecological interactions between planktonic foraminifera and other species, e.g. feeding, competition, parasitism, remain largely unexplored (Grigoratou et al., 2019). This lack of knowledge ham-

pers progress in understanding their ecology from modern observations and renders interpretations based on the fossil record overly reliant on environmental aspects alone. Studying time series of living planktonic foraminifera together with other plankton groups would improve our understanding of how they interact. Dedicated studies can, for instance, help to better characterize the position of planktonic foraminifera in marine food webs. In turn, if we better understand how planktonic foraminifera population dynamics are coupled to other plankton groups, we can potentially use their fossil record to obtain a more holistic picture of past dynamics of marine ecosystems. Thus, there are obvious mutual benefits to collaboration between biologists and micropalaeontologists: by including planktonic foraminifera in zooplankton time series, we can obtain a more complete picture of marine ecosystem functioning and modern observations can be put into a long term perspective that will ultimately aid in the prediction and anticipation of future (anthropogenic) changes that marine ecosystems face.
<figure><img src="image_4.png" /><figcaption>**Figure 3.** Existing zooplankton time series present opportunities for collaboration between marine ecologists and micropalaeontologists. Distribution of plankton time series included in the NOAA Copepod Time Series Metabase (https://www.st.nmfs.noaa.gov/copepod/metabase/). The map only shows time series in water depths of at least 100 m where planktonic foraminifera occur and could be included in the monitoring. The southern ocean CPR is not indicated because collaboration already exists within the framework of this survey. The map is certainly not complete with other suitable plankton time series available elsewhere; but it serves to highlight the potential for collaboration between micropalaeontologists and marine biologists working with oceanic time series.</figcaption></figure>

## An example of successful collaboration: continuous plankton recorder surveys

The case of the continuous plankton recorder (CPR) clearly demonstrates the benefits of integrating planktonic foraminifera in (zoo)plankton time series. The continuous plankton recorder (CPR) has been designed to quickly and efficiently record mesoscale biogeographical distribution of zooplankton in the surface ocean (Reid et al., 2003; Beaugrand, 2009). In the northern hemisphere, the CPR survey started in 1931 under the umbrella of the Sir Alister Hardy Foundation for Ocean Sciences and it has been known as the Continuous Plankton Recorder Survey since 2018. Planktonic foraminifera have been counted in this survey, without distinguishing species, since 1993. This rich dataset showed that, together with other plankton groups, foraminifera are migrating toward in response to increasing temperatures (Beaugrand et al., 2012). This study therefore demonstrated the potential of planktonic foraminifera to provide insights that are representative for other plankton groups. The CPR observations could be taken even further by resolving the data at species level and considering trait distributions. Such data would reveal not only how plankton communities transform during climate-induced range shifts, but also how their growth and biomineralization are affected.

In the southern hemisphere, CPR surveys have been carried out in the Southern Ocean since 1991. Like in the north, planktonic foraminifera were initially counted without identifying species. The surveys showed that planktonic foraminifera are at least as abundant as other zooplankton, such as pteropods and copepods, and that they sometimes are the dominant zooplankton group (McLeod et al., 2010; Takahashi and Hosie, 2020). Thus, in 2013, a collaboration started between CPR and planktonic foraminifera researchers to improve the CPR observations by identifying the most abundant species of planktonic foraminifera. This collaboration in the

Southern Ocean Continuous Plankton Recorder (SO-CPR) framework has delivered a wealth of new data on unprecedented spatial and temporal scales. It resulted in specific studies dedicated to foraminifera that provided a unique, fine-scale biogeographical surface distribution of species across hydrological fronts and revealed unexpectedly-high species diversity of planktonic foraminifera in the Antarctic sector of the Southern Ocean (Meilland, 2015; Meilland et al., 2016). Since 2014, a specific protocol is followed to maximize the preservation of calcifiers in the samples and several CPR analysts have been trained in foraminifera taxonomy. Planktonic foraminifera are now a target group within the SO-CPR programme to monitor possible changes in abundance and species composition in response to acidification (Takahashi and Hosie, 2020) and, given their sensitivity to temperature (Morey et al., 2005; Beaugrand et al., 2012; Jonkers et al., 2019), there is a great potential to use them to monitor the effect of ongoing warming on zooplankton biogeography in the Southern Ocean.

## Recommendations

Currently, the potential of planktonic foraminifera to provide a long-term view on marine ecosystem dynamics is severely underexploited because they are rarely resolved in marine ecological time series. A search of the IGMETS (International Group for Marine Ecological Time Series) metabase (https://igmets.net) provided only a single mention of planktonic foraminifera. However, given the large global network of zooplankton time series, there is plenty of opportunity to incorporate planktonic foraminifera in ongoing monitoring work and we hope that the successful collaboration within the southern ocean CPR framework can be repeated in ongoing or historical monitoring efforts (Figure 3). To better constrain foraminifera population dynamics in the water column, planktonic foraminifera could, in principle, be included in any ongoing time series (Figure 3). The insights gained from these efforts could then be applied to the sedimentary record of foraminifera to bridge the gap between ecological monitoring and geological time scales. However, the time scales can also be linked directly at locations with long-term sediment trap time series and good preservation of carbonate on the seafloor, such as in the Southern Ocean off Australia (https://imos.org.au/facilities/deepwatermoorings/sots) or in the North Atlantic off Mauretania (Fischer et al., 2016). The latter monitoring design offers the advantage to also study the transfer of the signal from the water column into the sediment in detail (Loncaric et al., 2007).

Several key characteristics of planktonic foraminifera make their collection easier than other (zoo)plankton. First of all, planktonic foraminifera are abundant in the upper 100s of metres of the water column and can be found in virtually all pelagic settings where the water depth exceeds approximately 100 m. Second, in contrast to other zooplankton groups that are extremely fragile and easily damaged during sampling (e.g. salps), planktonic foraminifera shells are robust and rigid and can be manipulated easily. Moreover, because their shells carry the taxonomic traits used to identify species, they can be stored easily for future analyses without loss of information or the need for expensive conservatives. Finally, planktonic foraminifera are relatively species poor and most species can be identified with relatively little training. The fact that their diversity is limited has also facilitated the development of a comprehensive catalogue of genetic barcodes (Morard et al., 2015). This catalogue makes planktonic foraminifera ideally suited for analy-
ses by metabarcoding and other environmental DNA approaches which have recently proved their potential for large plankton sampling survey and long-term monitoring in data series (Deagle *et al.*, 2018; Morard *et al.* 2018, 2019; Suter *et al.*, 2020).

We finish this article with some practical recommendations on how planktonic foraminifera can become an integral part of zooplankton monitoring time series. For this to happen, collaboration is needed to share expertise and to balance potentially contrasting collection requirements. We, hence, explicitly reach out to the zooplankton research community and encourage collaboration between micropalaeontologists and ecologists.

Given the scarcity of observations, any information on the presence, abundance, species composition, size, and weight of planktonic foraminifera is useful to increase our understanding of their ecology, especially when combined with information on other plankton groups. We hope that, whenever possible, researchers will, after having finished working on the plankton group of their interest, preserve their plankton samples and will reach out to the micropalaeontological community to raise awareness of the existence of valuable sample material and facilitate collaboration. If a specific treatment of zooplankton samples does not allow planktonic foraminifera preservation after analysis, it would be extremely useful to count or image the specimens present. Imaging has the advantage that foraminifera species can be identified and measured later, either manually or automatically, making use of encouraging developments in image recognition techniques adapted to foraminifera (Hsiang *et al.*, 2019; Marchant *et al.*, 2020).

Researchers keen on including foraminifera in their analyses are encouraged to isolate the shells on micropaleontological slides, where they can be safely dry-stored almost indefinitely. For optimal planktonic foraminifera sampling, we recommend the use of nets with fine mesh (100 µm), but we would like to emphasize that, if adjusting the mesh size is not feasible, information from larger size fractions is also useful. The size range of planktonic foraminifera between what is usually observed in phytoplankton and zooplankton surveys also has implications for emerging continuous automated *in situ* imaging systems (Wiebe and Benfield, 2003; Ohman, 2019), which already have shown their promise to track large foraminifera species (Hull *et al.*, 2011). The 100–1000 µm size range should be taken into account in the design and optics of such systems to cover planktonic foraminifera and other key microzooplankton.

To prevent dissolution of the calcite skeletons of planktonic foraminifera, we recommend buffering liquid samples to a pH of 8 (with the amount of buffering agent adjusted to the amount of organic matter in the sample). Other preservation options are freezing or drying, but care needs to be taken when mrganic matter concentrations are high as this could hamper recovery of foraminifera shells later on. We realize that a different sample handling protocol means additional work, but we think planktonic foraminifera are worth the extra effort. Moreover, preventing carbonate dissolution in zooplankton samples will also ensure the preservation of other calcifying zooplankton groups such as pteropods and ostracods and hence make zooplankton time series more complete.

It is likely that valuable zooplankton collections containing planktonic foraminifera exist in various laboratories, but are not (fully) used. Historical collections have proven particularly useful to elucidate long-term changes in planktonic foraminifera calcification in response to ocean acidification (Fox *et al.*, 2020). Linking different collections therefore has great potential and we explicitly encourage collaboration between biologists and micropalaeontologists to make sure that existing collections are fully investigated.

## Concluding remark

In conclusion, planktonic foraminifera are zooplankton with an exceptional fossil record that provides unique long-term information on marine biodiversity and environmental change, and plays an important role in the marine carbon cycle. Despite their potential to expand research on zooplankton dynamics across time scales that cannot be covered with modern observations, they remain understudied in modern zooplankton time series. We hope that we have inspired marine biologists to consider planktonic foraminifera in their research and that the recommendations offer a good starting point to do so. We explicitly reach out to zooplankton ecologists because by combining micropalaeontological and ecological expertise, we can unlock the great potential of planktonic foraminifera to understand marine zooplankton dynamics in a rapidly changing world.

## Data availability statement

No data were used or generated for this manuscript.

## Acknowledgements

We would like to thank Todd O'Brien (NOAA) for providing the data shown in the map in Figure 3. LJ, JM, MR, MK, and TG-T are part of the "FORCIS—Foraminifera response to climate stressors" project funded by the synthesis center CESAB of the French Foundation for Research on Biodiversity (FRB; www.fondationbiodiverse.fr). JM and MR are funded by the Cluster of Excellence "The Ocean Floor—Earth's Uncharted Interface" funded by the German Research Foundation (DFG).

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