Subsequently, the Ba/Ca at each site and at global (i.e. all sites) level was modeled using  Linear Mixed Effects (LME) models (Wood 2006) with random intercept. The fixed predictors were in-situ seawater variables which include “Sal” “Temp,” suspended solids (SS) (i.e. either “TSS”, “OSS” or “ISS”), and sedimentation rate (SD) (i.e. either “TSD”, “OSD” or “ISD”), while the random effect was "id" (individual colony). Due to the significant collinear relationship among the total, organic and inorganic components, only one of the component of SS and SD was used as predictor to avoid unstable parameter estimates (Pinheiro and Bates 2000). A continuous autoregressive(1) correlation structure was also incorporated into the model to account for temporal correlation between BA/CA readings, or mismatch of data due to the monthly spot measurements.
We heuristically tested all combinations of the predictors, including all possible two-way interaction terms, with a limit of five predictors per model. The best five models based on AICc were selected for each site, and at the global level (i.e. all sites). We then used the leave-one-out-cross-validation method (LOOCV, James et al 2013) to assess the predictive performance of the selected models. In brief, n-1 observations were used to estimate the parameters for each model, and to predict the mean response of the left-out observation. The square error was calculated and the previous step was repeated n times, i.e. until all observation were left-out once. The normalized cross-validated residual mean standard error (NRMSE) (or root mean square error?) for each model was calculated and compared: model with smaller NRMSE is better. All statistical analyses were performed using the statistical program R (version 3.0.3) (R Core Team 2014), using packages 'stats', 'nlme' (Pinheiro et al. 2017), and 'glmulti' (Calcagno & Mazancourt 2010).  
References
Calcagno V., & Mazancourt C. (2010). glmulti: An R Package for Easy Automated Model Selection with (Generalized) Linear Models. Journal of Statistical Software, 34(12): 1-29.
James, G., Witten, D., Hastie, T., & Tibshirani, R. (2013). An Introduction to Statistical Learning. New York, NY: Springer New York. https://doi.org/10.1007/978-1-4614-7138-7