Total mercury concentrations in whole blood of American alligators in South Carolina and Florida
This dataset is from a study that examined total mercury (THg) concentrations in whole blood of subadult and adult American alligators (Alligator mississippiensis) in South Carolina and Florida*: Title: Nonlinear patterns in mercury bioaccumulation in American alligators are a function of predicted age Authors: Lawson, A.J., C.T. Moore, T.R. Rainwater, F.M. Nilsen, P.M. Wilkinson, R.H. Lowers, L.J. Guillette Jr., K.W. McFadden, and P.G.R. Jodice Journal: Science of the Total Environment Issue: In press (accepted in October 2019) *Only the Florida data are provided here Study description: We investigated individual and temporal drivers of mercury bioaccumulation from two alligator populations: (1) the Tom Yawkey Wildlife Center (YWC; 2010-2017) located in South Carolina, USA (data available at: https://doi.org/10.5066/P98XHBCY) and (2) Merritt Island National Wildlife Refuge (MINWR; 2007-2014), located Florida, USA (this dataset). Data from the MINWR study were originally published by Nilsen et al. (2017), though in this analysis we introduced an additional covariate (predicted age; included in the .csv file) and evaluated previously-unexplored model structures (quadratic and random effects). Background: Mercury is capable of both bioaccumulation within individuals and biomagnification in wetland food webs. Previous studies in alligators indicated that relative trophic level generally increases with body size, though fine-scale age related patterns have not been evaluated, which is important given recently-described determinate (asymptotic) growth patterns. Using data from two marked alligator populations, we evaluated the relationship between THg in whole blood and age-indicators: predicted age derived from a growth model (Wilkinson et al. 2016) based on first capture, and snout-vent length (size). Findings: At both sites we identified consistent, strong support for a quadratic relationship between THg concentrations in whole blood and age-indicators in alligators (predicted age for YWC; snout-vent length for MINWR). For both sites, THg concentrations increased from early adulthood and peaked around the mean age or size, and then began to decline. Thus, the quadratic relationship between mercury concentrations and age/sizes contrasts with expected mercury bioaccumulation patterns. Here we describe potential mechanisms for the relationship we detected and provide suggestions for future studies to further evaluate these patterns. References: Nilsen, F.M., et al., 2017. Evaluating mercury concentrations and body condition in American alligators (Alligator mississippiensis) at Merritt Island National Wildlife Refuge (MINWR), Florida. Sci. Total Environ. 607–608, 1056–1064. https://doi.org/10.1016/j.scitotenv.2017.07.073 Wilkinson, P.M., et al., 2016. Determinate Growth and Reproductive Lifespan in the American Alligator (Alligator mississippiensis): Evidence from Long-term Recaptures. Copeia 104, 843–852. https://doi.org/10.1643/ch-16-430
Steps to reproduce
All data cleaning, analysis, and figures were produced in the R Statistical program using the following packages: arm, dplyr, MuMin, lme4, nlme, nortest, and ggplot2. Prior to analysis we removed all outliers . We analyzed the data from each site separately, and z-standardized continuous covariates (within each site). To meet the assumptions of normality required for linear regression, we applied a Box Cox transformation to each study population.