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CAITLIN R. KEATING-BITONTI AND JONATHAN L. PAYNE


while enhancing oxygen diffusion rates (Bernhard 1986). Contrary to expectations, our data indicate that changes in calcite saturation state and POC flux are unlikely to have important direct effects on the composition of foraminiferal communities, as the coefficients associated with these environ- mental parameters are relatively small compared with the coefficients associatedwith temperature and dissolved oxygen (Fig. 3).


Summary


physicochemical conditions exert an identifiable influence on the sizes and volume–to–surface area ratios of benthic foraminifera. Test volume and volume–to–surface area ratio of Rotallid foraminifers are inversely correlated with ambient water temperature across broad geographic scales and among species, consistent with a direct metabolic influence on cell size. These morphological parameters correlate positively with oxygen availability, which exerts a stronger control at lower oxygen concentrations typical of upwelling zones in the eastern Pacific, generally below 3 ml/liter. In contrast, the calcite saturation state of seawater and POC flux to the seafloor do not exert statistically identifiable controls on size and volume–to–surface area ratio beyond their correlations with temperature and oxygen. These findings highlight the role of energy metabolism in the biogeographic distribution of protistan species. To the extent that space can be substituted for time, they also suggest that high temperatures were at least as important as lower oxygen availability in driving test reductions during episodes of global warming and oxygen depletion in Earth’spast. They further suggest that warming waters and expansion of hypoxic zones during the coming millennium are more likely to impact benthic foraminiferal communities than declines in calcite saturation or changes in primary productivity.


Across the North American continental shelf, Acknowledgments


We thank S. J. Culver and M. A. Buzas for sharing their biogeographic benthic


foraminifera data set; D. T. Medeiros for ArcGIS assistance; C. R. McClain for providing POC flux data; R. M. Key and L. G. Anderson for assistance finding ALK and DIC data in the Caribbean and Arctic, respectively; N. A. Heim for helpful discussion and R assistance; E. A. Sperling for helpful comments on an early draft of the manuscript; and a number of summer high school student interns for data collection. C.R.K.-B. was supported by a National Science Foundation Graduate Research Fellowship. J.L.P. was supported by a CAREER grant from the U.S. National Science Foundation (EAR-1151022).


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