Utilizing Multidisciplinary Methods to Understand Trace Element Accumulation in Northern Gulf of Mexico Odontocetes
Due to their long lifespan and top trophic position, odontocetes (toothed whales) can accumulate high concentrations of trace elements [e.g., cadmium (Cd), mercury (Hg)] in their tissues. In addition, for many species, their coastal distribution makes them appropriate sentinels for ecosystem and human health. Acquiring odontocete tissues for ecotoxicology studies is a challenge due to logistical and legal constraints. Although data is opportunistic, collecting tissues from deceased stranded individuals is a viable alternative to sampling free-ranging populations. In this dissertation, I focused on trace element accumulation in odontocetes, primarily bottlenose dolphins (Tursiops trunactus), that stranded along the northern Gulf of Mexico (nGOM) coast. Many samples were from dolphins that stranded during the nGOM Cetacean Unusual Morality Event (2010-2014), which provided access to an unusually large number of individuals (> 250). The primary objectives were to 1) measure the concentration of total Hg (THg) in blubber and skin from bottlenose dolphins that stranded along the Florida (FL) panhandle and Louisiana (LA) coasts and explore the relationship between total Hg (THg) concentration and sex, body length, age, stranding location, diet/relative tropic position, (δ13C and δ15N, respectively), and foraging habitat (δ34S); 2) assign bottlenose dolphins to predicted habits east and west of the Mississippi River Delta (MRD) using δ13C and δ34S values, and determine whether variation in THg concentrations could be explained by differences in trophic positions among dolphins; 3) explore the tissue-specific accumulation of Hg and selenium (Se) and the potential protective role of Se against Hg toxicity by measuring the concentrations of THg and Se in multiple tissues from 11 species of odontocetes that stranded along the FL panhandle and Louisiana coast and calculating the Se:Hg molar ratios; 4) determine the effects of long-term and short-term formalin fixation on the concentration of 14 trace elements in bottlenose dolphin tissues; and 5) utilize a scanning electron microscope (SEM) equipped with energy dispersive X-ray spectroscopy (EDS) to determine the distribution of major (e.g., Ca, P), minor (e.g., Cl, Mg, Na), and trace elements (e.g., Cd, Hg, Pb, Zn) in teeth from 12 bottlenose dolphins. To address objective 1, I measured THg concentrations in bottlenose dolphin skin and blubber using a direct mercury analyzer and stable isotope ratios were measured in dolphin skin. In both tissues, there was a positive relationship between THg concentration and body length/age (p < 0.001). Dolphins that stranded in FL had greater THg concentrations than those that stranded in LA (p < 0.001). Next, to address objective 2, I assigned dolphins to predicted habitats using δ13C and δ34S values and estimated trophic positions using δ15N values from stranded dolphin skin and primary consumers taken from the literature following a Bayesian framework. I found that dolphins assigned to estuarine habitats east of the MRD, particularly those associated with habitats along the FL panhandle, had greater THg concentrations and higher estimated trophic positions, suggesting that differences in trophic positions among bottlenose dolphins contribute to variation in Hg concentrations. To address objective 3, I measured the THg and Se concentrations across several tissues and species using inductively coupled plasma mass spectrometry (ICP-MS). The concentration of THg was greatest in the liver and lowest in the blubber, lung, or skin. Se:Hg molar ratios decreased with increasing THg concentration tissues following an exponential decay relationship. On average, in bottlenose dolphins, Se:Hg molar ratios were approximately 1:1 in the liver and > 1:1 in the other tissues, suggesting that Se likely protects against Hg toxicity. In objective 4, trace elements were measured in several bottlenose tissues using ICP-MS. Following both short-term (6 weeks) and long-term preservation (3-7 years), there were significant differences in tissue trace element concentrations between preservation methods. Evidence of both leaching of trace elements from tissues and contamination of tissues with trace elements, presumably from formalin, was observed; however, the results suggest that it may be possible to account for the effects of formalin fixation for some trace elements. Finally, to address objective 5, I used SEM-EDS to explore the distribution of major, minor, and trace elements within dolphin teeth. There was variation in the weight percentage (wt %) of major and minor elements between the enamel and pre-natal dentin. Except of Al, which may be a result of backscatter from the SEM stub, trace elements were not detected. However, trace elements may be present but at wt % values below the detection limit. Overall, this dissertation contributes to knowledge of several trace elements in odontocete tissues. In particular, this work contributes to the understanding of Hg and Se accumulation in inshore bottlenose dolphins from the nGoM. Further, I address methodological questions related to tissue preservation and the use of SEM-EDS to measure major, minor, and trace elements in dolphin teeth which will benefit future studies.
toxicology, dolphins, mercury, odontocetes
McCormack, M. (2021). Utilizing multidisciplinary methods to understand trace element accumulation in Northern Gulf of Mexico odontocetes (Unpublished dissertation). Texas State University, San Marcos, Texas.