Size distribution of Holocene planktic foraminifer assemblages: biogeography, ecology and adaptation
Introduction
Size is an obvious morphological characteristic, readily preserved in fossils, easy to measure, conspicuous, ecologically important, comparable across taxa and extremely variable in time and space. Consequently, this parameter has been studied for several groups of organisms (Peters, 1983, Skelton, 1993, Futuyma, 1998). Much of this work has focused on terrestrial animals, showing both the role of ecology (Bergmann’s rule, i.e. increase in body-size towards high latitudes) and evolution (Cope’s law, i.e. increase in body-size along a lineage). Fewer studies have addressed the factors influencing size of marine organisms. Recent examples have identified oxygen availability as controlling size in amphipods (Chapelle and Peck, 1999, Peck, 2001), benthic foraminifers (Kaiho, 1998), and gastropods (McClain and Rex, 2001).
Planktic foraminifers, because of their wide geographical occurrence, allow global studies of ecological and evolutionary influences on size. Size changes in modern planktic foraminiferal species have been related to ecological factors such as temperature (Bé et al., 1973, Hecht, 1976) and upwelling intensity (Naidu and Malmgren, 1995). From an evolutionary point of view, foraminifers as a group have undergone at least three periods of diversification since their origin in the Mid-Jurassic (Loeblich and Tappan, 1985), each of which is thought to have involved a general increase in test size (Arnold et al., 1995).
Understanding the repetitive morphological evolutionary radiation of foraminifers in the Cenozoic demands a test of various ecological factors influencing test size. The goal of this study is to provide a calibration which will allow to quantify and attempt to understand the paleoecological, paleobiogeographical and evolutionary significance of size variability in planktic foraminiferal in the past.
Most studies of evolutionary size changes in planktic foraminifers focused on single species or lineages (e.g. Malmgren and Kennett, 1981, Arnold, 1983, Malmgren et al., 1983, Spencer-Cervato and Thierstein, 1997, Kucera and Malmgren, 1998, MacLeod et al., 2000). Single species or lineages have provided regional ecological and stratigraphic information on size change. Analyses of entire planktic foraminiferal assemblages, which integrate information of all individual species, hold the potential for giving insights into long-term macro-evolutionary processes or potential global environmental changes. The only attempt to study Cenozoic size variation of the entire group of planktic foraminifers (Arnold et al., 1995, Parker et al., 1999) was based on analyses of one specimen per species, restricting the reliability of the results.
In order to understand the ecological significance of size variability we have studied planktic foraminifer assemblages in a Holocene data set as an important step towards an analysis of size changes of planktic foraminifers in Quaternary (Schmidt et al., 2003) and the late Phanerozoic. We can expect test sizes of total planktic foraminiferal assemblages to be the result of various processes acting on at least three different scales. (1) They have been found to be influenced by the physical and chemical properties of the ambient sea water, such as temperature, salinity, nutrient availability, carbonate saturation, and oxygen availability (e.g. Berger, 1969, Bé and Tolderlund, 1971, Caron et al., 1981, Caron et al., 1987a, Bijma et al., 1992, Schiebel et al., 2001). If so, size ought to depend on these factors on a global scale. (2) Biogeographic differences in species composition and diversity have been well documented (Bé and Tolderlund, 1971, Hemleben et al., 1989). Since individual species show distinct size variability (Hecht, 1976), biogeographic changes in species composition may lead to size changes of the entire assemblages. (3) Test sizes of populations of individual species are known to vary with environmental factors (Hecht, 1976, Ortiz et al., 1995, Naidu and Malmgren, 1996) with size maxima at distinct environmental conditions. Size changes of the entire planktic foraminiferal assemblages may therefore represent a composite of size spectra of various species, which may have lived at or outside their environmental optima.
Section snippets
Materials
We have selected a set of 69 surface sediment samples (Fig. 1) covering all biogeographic zones which were defined based on the species composition of planktic foraminifers (Bé and Tolderlund, 1971, Hemleben et al., 1989). A large subset of these samples has previously been used for the Holocene calibration of the CLIMAP study (CLIMAP, 1981). The samples are from water depths of 808–4825 m (mean of 3000 m) and show no signs of dissolution and few non-foraminiferal particles. The samples cover
General size distribution
The size distributions of all analyzed planktic foraminiferal assemblages are strongly skewed towards larger sizes (Fig. 2). From the polar to the tropical zone, the sizeassemblage5 doubles, from 315 to 625 μm and the maximum test size almost triples from 512 to 1448 μm. The upwelling assemblages have size distributions most similar to the temperate and subtropical ones.
To analyze these patterns in more detail, the relationship of the sizeassemblage5 with the main environmental parameters is
Temperature
The most important trend emerging from our results is the general increase in planktic foraminiferal test size from the poles to the tropics (Fig. 4, Fig. 5). Besides the possible direct influence of water temperature, many other ecological parameters co-vary with temperature. Cell physiology in general is known to accelerate with temperature and enzymatic activity has been shown to approximately double when temperature increases by 10°C in different species of planktic foraminifers (Caron et
Conclusions
(1). On a global scale, planktic foraminifers as a group increase in size from the poles to the tropics. This pattern can be attributed to the co-variation of several temperature-related effects such as metabolic efficiency, carbonate supersaturation, niche richness, diversity, species replacement, and intraspecific size variation.
(2) The close global temperature correlation of maximum size and relative abundance of individual species allow definition of environmental optima, beyond which
Acknowledgements
We acknowledge the generosity of B. Donner, A. Mackensen, Ch. Hemleben, R. Spielhagen, and G. Wefer to share their material with us. We are grateful to U. Gerber, R. Hoffmann, and M. Mettler for their skillful technical help. We thank J. Bijma, H. Hilbrecht, and J. Ortiz for discussion during the various stages of the work, as well as Helen Williams for linguistic improvements. This paper has benefited from the comments of J. Cullen, M. Spindler, R. Thunell and A. Mackensen. The research was
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Present address: PaléoEnvironnements et PaléoBiosphère, UMR 5125 CNRS UCB Lyon 1, 69622 Villeurbanne Cedex, France.