Environmental controls on biogeographic patterns of modern benthic foraminiferal size and morphology from the North American continental margin [electronic resource]
Keywords: Geological Sciences.

Naturalists and scientists have long been fascinated by temporal and spatial variations in organism sizes and for good reason—size regulates most biological matters. Biogeographic patterns of size variation provide indirect evidence of how the physical environment influences organism physiology. These types of data are readily applied to specific groups of organisms (e.g., terrestrial mammals and marine mollusks) to better understand their physiological responses to environmental change. However, other groups, like marine protists, have been understudied. For example, the environmental constraints on the distribution of benthic foraminifera, a diverse and geographically widespread group of marine protists, throughout the global oceans remain incompletely understood. Thorough analysis of biogeographic patterns of size variation in benthic foraminifera can shed light on the physical and chemical oceanographic drivers of protist physiology and, thus, their evolutionary history. The goals of my doctoral thesis research are: first, to quantify the relative importance of oceanographic parameters on foraminiferal size distributions and, second, to determine if patterns of size variation, both within and among species, are consistent with predictions based on principles of cell physiology. Throughout my research I incorporate physiological models constrained by modern observations to both predict and interpret the morphological responses of foraminifera to environmental change. I use model-fitting approaches on among-species size distributions from across continental margins (Chapters 1 and 2) and specimen-based studies of within-species size variations from a local basin (Chapter 3) to quantify the relative importance of frequently hypothesized oceanographic parameters in structuring biogeographic patterns. The hypothesized oceanographic parameters that I consider include: temperature, dissolved oxygen concentration, calcite saturation state, and food availability. Across broad geographic scales and across species, I find consistent support for physicochemical controls on the distribution of modern benthic foraminiferal size and morphology. Temperature and dissolved oxygen concentrations are the dominant controls on adult foraminiferal size and the direction of their associations are consistent with predictions based on cell physiology (see Chapter 1). Although the distributions of embryonic and adult size are distinct across environmental habitats, the initial size of the cell is determined by parent size and not identifiably by the local physical environment. This finding is also consistent with the expectation that environmental constraints related to biovolume and surface area have a greater influence over organism physiology at larger sizes (see Chapter 2). However, my examination of within-species morphological response to local oxygen availability shows that species' responses do not necessarily follow predictions based on cell physiology. One species' morphological response cannot be extrapolated from the response of another species (see Chapter 3). Indeed, benthic foraminifera are highly sensitive to oceanographic change via physiological constraints imposed by the environment. Their responsiveness to variations in local temperature and oxygen availability lend to their favorability as a proxy used for ancient ocean conditions. However, caution should be exercised when using fossil protist size to reconstruct paleo-oxygen levels (see Chapter 3). Moreover, in light of anthropogenic climate change, both ocean warming and deoxygenation will have adverse effects on foraminiferal communities (see Chapter 1 and 2). Benthic foraminifera are important contributors to global carbonate production and to biogeochemical cycling within the sediments. Thus, physiological stress imposed on them by rapidly changing environmental conditions will have consequences for both the biological and chemical ocean systems. Through an examination of foraminiferal biogeographic patterns across different scales, this research lays the groundwork for better understanding how ocean conditions influence their physiology and, thus, drives macroevolutionary trends in marine protists.