PHYSIOLOGICAL EFFECTS ON COMMUNITY-LEVEL SIZE DISTRIBUTIONS OF BENTHIC FORAMINIFERA AROUND THE NORTH AMERICAN CONTINENTAL MARGIN

Organism size often varies systematically across latitude and water depth, but the proximal environmental factors that directly mediate these size gradients remain incompletely understood. Benthic foraminifera, a diverse group of amoeboid protists that inhabit all marine environments, provide an unparalleled opportunity to test statistically among the various potential controls on size. Using a model-fitting approach, we seek to determine which oceanographic parameters (individually or in combination) exert the strongest influences on foraminiferal cell size across multiple environmental gradients. In this study, we use previously published biogeographic data from the North America continental margin and test size measurements of holotype specimens illustrated in the Ellis and Messina catalogue to map cell size over this area. For each of the 844 localities, we compiled environmental parameters hypothesized to influence the physiology of calcareous benthic foraminifera: temperature, dissolved oxygen concentration, calcite saturation state of seawater, flux of particulate organic carbon (POC) to the seafloor, and the seasonal range of temperature. Based on AIC and BIC model selection criteria, we find that temperature, dissolved oxygen concentration, and the calcite saturation state of seawater best predict test volumes for the North American dataset. The strength and direction of these environmental signals are consistent with a priori predictions from the first principles of physiology. When the Atlantic and Pacific are considered separately, we find that temperature and oxygen are the significant predictors of size in the Pacific; whereas, temperature, calcite saturation, and POC flux yield the best model for the Atlantic. Differences between the continental margins can be accounted for by the absence of low-oxygen waters in the Atlantic. In the Pacific data, the correlation between oxygen and size is driven almost entirely by localities with dissolved oxygen concentrations below 3 ml/L. These results argue that community-level size distributions are significantly influenced by environmental parameters on organism physiology, consistent with the metabolic theory of ecology.